Request for Comments: 8017
Obsoletes: 3447
Category: Informational
ISSN: 20701721
EMC Corporation
B. Kaliski
Verisign
J. Jonsson
Subset AB
A. Rusch
RSA
November 2016
PKCS #1: RSA Cryptography Specifications Version 2.2
Abstract

This document provides recommendations for the implementation of publickey cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes.
This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' PublicKey Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF.
This document also obsoletes RFC 3447.
Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfceditor.org/info/rfc8017.
Copyright Notice

Copyright © 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/licenseinfo) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Key Types . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. RSA Public Key . . . . . . . . . . . . . . . . . . . . . 8 3.2. RSA Private Key . . . . . . . . . . . . . . . . . . . . . 9 4. Data Conversion Primitives . . . . . . . . . . . . . . . . . 11 4.1. I2OSP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2. OS2IP . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. Cryptographic Primitives . . . . . . . . . . . . . . . . . . 12 5.1. Encryption and Decryption Primitives . . . . . . . . . . 12 5.1.1. RSAEP . . . . . . . . . . . . . . . . . . . . . . . . 13 5.1.2. RSADP . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2. Signature and Verification Primitives . . . . . . . . . . 15 5.2.1. RSASP1 . . . . . . . . . . . . . . . . . . . . . . . 15 5.2.2. RSAVP1 . . . . . . . . . . . . . . . . . . . . . . . 16 6. Overview of Schemes . . . . . . . . . . . . . . . . . . . . . 17 7. Encryption Schemes . . . . . . . . . . . . . . . . . . . . . 18 7.1. RSAESOAEP . . . . . . . . . . . . . . . . . . . . . . . 19 7.1.1. Encryption Operation . . . . . . . . . . . . . . . . 22 7.1.2. Decryption Operation . . . . . . . . . . . . . . . . 25 7.2. RSAESPKCS1v1_5 . . . . . . . . . . . . . . . . . . . . 27 7.2.1. Encryption Operation . . . . . . . . . . . . . . . . 28 7.2.2. Decryption Operation . . . . . . . . . . . . . . . . 29 8. Signature Scheme with Appendix . . . . . . . . . . . . . . . 31 8.1. RSASSAPSS . . . . . . . . . . . . . . . . . . . . . . . 32 8.1.1. Signature Generation Operation . . . . . . . . . . . 33 8.1.2. Signature Verification Operation . . . . . . . . . . 34 8.2. RSASSAPKCS1v1_5 . . . . . . . . . . . . . . . . . . . . 35 8.2.1. Signature Generation Operation . . . . . . . . . . . 36 8.2.2. Signature Verification Operation . . . . . . . . . . 37 9. Encoding Methods for Signatures with Appendix . . . . . . . . 39 9.1. EMSAPSS . . . . . . . . . . . . . . . . . . . . . . . . 40 9.1.1. Encoding Operation . . . . . . . . . . . . . . . . . 42 9.1.2. Verification Operation . . . . . . . . . . . . . . . 44 9.2. EMSAPKCS1v1_5 . . . . . . . . . . . . . . . . . . . . . 45 10. Security Considerations . . . . . . . . . . . . . . . . . . . 47 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 48 11.1. Normative References . . . . . . . . . . . . . . . . . . 48 11.2. Informative References . . . . . . . . . . . . . . . . . 48 Appendix A. ASN.1 Syntax . . . . . . . . . . . . . . . . . . . . 54 A.1. RSA Key Representation . . . . . . . . . . . . . . . . . 54 A.1.1. RSA Public Key Syntax . . . . . . . . . . . . . . . . 54 A.1.2. RSA Private Key Syntax . . . . . . . . . . . . . . . 55 A.2. Scheme Identification . . . . . . . . . . . . . . . . . . 57 A.2.1. RSAESOAEP . . . . . . . . . . . . . . . . . . . . . 57 A.2.2. RSAESPKCSv1_5 . . . . . . . . . . . . . . . . . . . 60 A.2.3. RSASSAPSS . . . . . . . . . . . . . . . . . . . . . 60 A.2.4. RSASSAPKCSv1_5 . . . . . . . . . . . . . . . . . . 62 Appendix B. Supporting Techniques . . . . . . . . . . . . . . . 63 B.1. Hash Functions . . . . . . . . . . . . . . . . . . . . . 63 B.2. Mask Generation Functions . . . . . . . . . . . . . . . . 66 B.2.1. MGF1 . . . . . . . . . . . . . . . . . . . . . . . . 67 Appendix C. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 68 Appendix D. Revision History of PKCS #1 . . . . . . . . . . . . 76 Appendix E. About PKCS . . . . . . . . . . . . . . . . . . . . . 77 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 78
1. Introduction

This document provides recommendations for the implementation of publickey cryptography based on the RSA algorithm [RSA], covering the following aspects:
 Cryptographic primitives
 Encryption schemes
 Signature schemes with appendix
 ASN.1 syntax for representing keys and for identifying the schemes
The recommendations are intended for general application within computer and communications systems and as such include a fair amount of flexibility. It is expected that application standards based on these specifications may include additional constraints. The recommendations are intended to be compatible with the standards IEEE 1363 [IEEE1363], IEEE 1363a [IEEE1363A], and ANSI X9.44 [ANSIX944].
This document supersedes PKCS #1 version 2.1 [RFC3447] but includes compatible techniques.
The organization of this document is as follows:
 Section 1 is an introduction.
 Section 2 defines some notation used in this document.
 Section 3 defines the RSA public and private key types.
 Sections 4 and 5 define several primitives, or basic mathematical operations. Data conversion primitives are in Section 4, and cryptographic primitives (encryptiondecryption and signature verification) are in Section 5.
 Sections 6, 7, and 8 deal with the encryption and signature schemes in this document. Section 6 gives an overview. Along with the methods found in PKCS #1 v1.5, Section 7 defines an encryption scheme based on Optimal Asymmetric Encryption Padding (OAEP) [OAEP], and Section 8 defines a signature scheme with appendix based on the Probabilistic Signature Scheme (PSS) [RSARABIN] [PSS].
 Section 9 defines the encoding methods for the signature schemes in Section 8.
 Appendix A defines the ASN.1 syntax for the keys defined in Section 3 and the schemes in Sections 7 and 8.
 Appendix B defines the hash functions and the mask generation function (MGF) used in this document, including ASN.1 syntax for the techniques.
 Appendix C gives an ASN.1 module.
 Appendices D and E outline the revision history of PKCS #1 and provide general information about the PublicKey Cryptography Standards.
This document represents a republication of PKCS #1 v2.2 [PKCS1_22] from RSA Laboratories' PublicKey Cryptography Standards (PKCS) series.
1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
2. Notation

The notation in this document includes:
c ciphertext representative, an integer between 0 and n1 C ciphertext, an octet string d RSA private exponent d_i additional factor r_i's CRT exponent, a positive integer such that e * d_i == 1 (mod (r_i1)), i = 3, ..., u dP p's CRT exponent, a positive integer such that
e * dP == 1 (mod (p1))

dQ q's CRT exponent, a positive integer such that
e * dQ == 1 (mod (q1))

e RSA public exponent EM encoded message, an octet string emBits (intended) length in bits of an encoded message EM emLen (intended) length in octets of an encoded message EM GCD(. , .) greatest common divisor of two nonnegative integers Hash hash function hLen output length in octets of hash function Hash k length in octets of the RSA modulus n K RSA private key L optional RSAESOAEP label, an octet string LCM(., ..., .) least common multiple of a list of nonnegative integers m message representative, an integer between 0 and n1 M message, an octet string mask MGF output, an octet string maskLen (intended) length of the octet string mask MGF mask generation function mgfSeed seed from which mask is generated, an octet string mLen length in octets of a message M n RSA modulus, n = r_1 * r_2 * ... * r_u , u >= 2 (n, e) RSA public key p, q first two prime factors of the RSA modulus n qInv CRT coefficient, a positive integer less than p such that q * qInv == 1 (mod p) r_i prime factors of the RSA modulus n, including r_1 = p, r_2 = q, and additional factors if any s signature representative, an integer between 0 and n1 S signature, an octet string sLen length in octets of the EMSAPSS salt t_i additional prime factor r_i's CRT coefficient, a positive integer less than r_i such that r_1 * r_2 * ... * r_(i1) * t_i == 1 (mod r_i) , i = 3, ... , u u number of prime factors of the RSA modulus, u >= 2 x a nonnegative integer X an octet string corresponding to x xLen (intended) length of the octet string X 0x indicator of hexadecimal representation of an octet or an octet string: "0x48" denotes the octet with hexadecimal value 48; "(0x)48 09 0e" denotes the string of three consecutive octets with hexadecimal values 48, 09, and 0e, respectively \lambda(n) LCM(r_11, r_21, ... , r_u1) \xor bitwise exclusiveor of two octet strings \ceil(.) ceiling function; \ceil(x) is the smallest integer larger than or equal to the real number x  concatenation operator == congruence symbol; a == b (mod n) means that the integer n divides the integer a  b
Note: The Chinese Remainder Theorem (CRT) can be applied in a non recursive as well as a recursive way. In this document, a recursive approach following Garner's algorithm [GARNER] is used. See also Note 1 in Section 3.2.
3. Key Types

Two key types are employed in the primitives and schemes defined in this document: RSA public key and RSA private key. Together, an RSA public key and an RSA private key form an RSA key pair.
This specification supports socalled "multiprime" RSA where the modulus may have more than two prime factors. The benefit of multi prime RSA is lower computational cost for the decryption and signature primitives, provided that the CRT is used. Better performance can be achieved on single processor platforms, but to a greater extent on multiprocessor platforms, where the modular exponentiations involved can be done in parallel.
For a discussion on how multiprime affects the security of the RSA cryptosystem, the reader is referred to [SILVERMAN].
3.1. RSA Public Key

For the purposes of this document, an RSA public key consists of two components:
n the RSA modulus, a positive integer e the RSA public exponent, a positive integer
In a valid RSA public key, the RSA modulus n is a product of u distinct odd primes r_i, i = 1, 2, ..., u, where u >= 2, and the RSA public exponent e is an integer between 3 and n  1 satisfying GCD(e,\lambda(n)) = 1, where \lambda(n) = LCM(r_1  1, ..., r_u  1). By convention, the first two primes r_1 and r_2 may also be denoted p and q, respectively.
A recommended syntax for interchanging RSA public keys between implementations is given in Appendix A.1.1; an implementation's internal representation may differ.
3.2. RSA Private Key

For the purposes of this document, an RSA private key may have either of two representations.
 The first representation consists of the pair (n, d), where the components have the following meanings:
n the RSA modulus, a positive integer
d the RSA private exponent, a positive integer

 The second representation consists of a quintuple (p, q, dP, dQ, qInv) and a (possibly empty) sequence of triplets (r_i, d_i, t_i), i = 3, ..., u, one for each prime not in the quintuple, where the components have the following meanings:
p the first factor, a positive integer q the second factor, a positive integer dP the first factor's CRT exponent, a positive integer dQ the second factor's CRT exponent, a positive integer qInv the (first) CRT coefficient, a positive integer r_i the ith factor, a positive integer d_i the ith factor's CRT exponent, a positive integer t_i the ith factor's CRT coefficient, a positive integer
In a valid RSA private key with the first representation, the RSA modulus n is the same as in the corresponding RSA public key and is the product of u distinct odd primes r_i, i = 1, 2, ..., u, where u >= 2. The RSA private exponent d is a positive integer less than n satisfying

e * d == 1 (mod \lambda(n)),
where e is the corresponding RSA public exponent and \lambda(n) is defined as in Section 3.1.
In a valid RSA private key with the second representation, the two factors p and q are the first two prime factors of the RSA modulus n (i.e., r_1 and r_2); the CRT exponents dP and dQ are positive integers less than p and q, respectively, satisfying
e * dP == 1 (mod (p1)) e * dQ == 1 (mod (q1)) ,
and the CRT coefficient qInv is a positive integer less than p satisfying

q * qInv == 1 (mod p).
If u > 2, the representation will include one or more triplets (r_i, d_i, t_i), i = 3, ..., u. The factors r_i are the additional prime factors of the RSA modulus n. Each CRT exponent d_i (i = 3, ..., u) satisfies

e * d_i == 1 (mod (r_i  1)).
Each CRT coefficient t_i (i = 3, ..., u) is a positive integer less than r_i satisfying
R_i * t_i == 1 (mod r_i) ,
where R_i = r_1 * r_2 * ... * r_(i1).
A recommended syntax for interchanging RSA private keys between implementations, which includes components from both representations, is given in Appendix A.1.2; an implementation's internal representation may differ.
Notes:
 The definition of the CRT coefficients here and the formulas that use them in the primitives in Section 5 generally follow Garner's algorithm [GARNER] (see also Algorithm 14.71 in [HANDBOOK]). However, for compatibility with the representations of RSA private keys in PKCS #1 v2.0 and previous versions, the roles of p and q are reversed compared to the rest of the primes. Thus, the first CRT coefficient, qInv, is defined as the inverse of q mod p, rather than as the inverse of R_1 mod r_2, i.e., of p mod q.
 Quisquater and Couvreur [FASTDEC] observed the benefit of applying the CRT to RSA operations.
4. Data Conversion Primitives

Two data conversion primitives are employed in the schemes defined in this document:
o I2OSP  IntegertoOctetString primitive o OS2IP  OctetStringtoInteger primitive
For the purposes of this document, and consistent with ASN.1 syntax, an octet string is an ordered sequence of octets (eightbit bytes). The sequence is indexed from first (conventionally, leftmost) to last (rightmost). For purposes of conversion to and from integers, the first octet is considered the most significant in the following conversion primitives.
4.1. I2OSP

I2OSP converts a nonnegative integer to an octet string of a specified length.
I2OSP (x, xLen)
Input:
x nonnegative integer to be converted xLen intended length of the resulting octet string
Output:


X corresponding octet string of length xLen

Error: "integer too large"
Steps:

 If x >= 256^xLen, output "integer too large" and stop.
 Write the integer x in its unique xLendigit representation in base 256:
x = x_(xLen1) 256^(xLen1) + x_(xLen2) 256^(xLen2) + ... + x_1 256 + x_0,


where 0 <= x_i < 256 (note that one or more leading digits will be zero if x is less than 256^(xLen1)).
 Let the octet X_i have the integer value x_(xLeni) for 1 <= i <= xLen. Output the octet string


X = X_1 X_2 ... X_xLen.



4.2. OS2IP

OS2IP converts an octet string to a nonnegative integer.
OS2IP (X) Input: X octet string to be converted Output: x corresponding nonnegative integer
Steps:

 Let X_1 X_2 ... X_xLen be the octets of X from first to last, and let x_(xLeni) be the integer value of the octet X_i for 1 <= i <= xLen.
2. Let x = x_(xLen1) 256^(xLen1) + x_(xLen2) 256^(xLen2) + ... + x_1 256 + x_0.

 Output x.

5. Cryptographic Primitives

Cryptographic primitives are basic mathematical operations on which cryptographic schemes can be built. They are intended for implementation in hardware or as software modules and are not intended to provide security apart from a scheme.
Four types of primitive are specified in this document, organized in pairs: encryption and decryption; and signature and verification.
The specifications of the primitives assume that certain conditions are met by the inputs, in particular that RSA public and private keys are valid.
5.1. Encryption and Decryption Primitives

An encryption primitive produces a ciphertext representative from a message representative under the control of a public key, and a decryption primitive recovers the message representative from the ciphertext representative under the control of the corresponding private key.
One pair of encryption and decryption primitives is employed in the encryption schemes defined in this document and is specified here: RSA Encryption Primitive (RSAEP) / RSA Decryption Primitive (RSADP). RSAEP and RSADP involve the same mathematical operation, with different keys as input. The primitives defined here are the same as Integer Factorization Encryption Primitive using RSA (IFEPRSA) / Integer Factorization Decryption Primitive using RSA (IFDPRSA) in IEEE 1363 [IEEE1363] (except that support for multiprime RSA has been added) and are compatible with PKCS #1 v1.5.
The main mathematical operation in each primitive is exponentiation.
5.1.1. RSAEP

RSAEP ((n, e), m)
Input:
(n, e) RSA public key
m message representative, an integer between 0 and n  1

Output: c ciphertext representative, an integer between 0 and n  1 Error: "message representative out of range" Assumption: RSA public key (n, e) is valid
Steps:

 If the message representative m is not between 0 and n  1, output "message representative out of range" and stop.
 Let c = m^e mod n.
 Output c.

5.1.2. RSADP

RSADP (K, c)
Input:
K RSA private key, where K has one of the following forms:

+ a pair (n, d)
+ a quintuple (p, q, dP, dQ, qInv) and a possibly empty
sequence of triplets (r_i, d_i, t_i), i = 3, ..., u



c ciphertext representative, an integer between 0 and n  1

Output: m message representative, an integer between 0 and n  1 Error: "ciphertext representative out of range" Assumption: RSA private key K is valid
Steps:

 If the ciphertext representative c is not between 0 and n  1, output "ciphertext representative out of range" and stop.
 The message representative m is computed as follows.

a. If the first form (n, d) of K is used, let m = c^d mod n.
b. If the second form (p, q, dP, dQ, qInv) and (r_i, d_i, t_i) of K is used, proceed as follows:

i. Let m_1 = c^dP mod p and m_2 = c^dQ mod q.




ii. If u > 2, let m_i = c^(d_i) mod r_i, i = 3, ..., u.



iii. Let h = (m_1  m_2) * qInv mod p.




iv. Let m = m_2 + q * h.



v. If u > 2, let R = r_1 and for i = 3 to u do





 Let R = R * r_(i1).




2. Let h = (m_i  m) * t_i mod r_i.





 Let m = m + R * h.


 Output m.

Note: Step 2.b can be rewritten as a single loop, provided that one reverses the order of p and q. For consistency with PKCS #1 v2.0, however, the first two primes p and q are treated separately from the additional primes.

5.2. Signature and Verification Primitives

A signature primitive produces a signature representative from a message representative under the control of a private key, and a verification primitive recovers the message representative from the signature representative under the control of the corresponding public key. One pair of signature and verification primitives is employed in the signature schemes defined in this document and is specified here: RSA Signature Primitive, version 1 (RSASP1) / RSA Verification Primitive, version 1 (RSAVP1).
The primitives defined here are the same as Integer Factorization Signature Primitive using RSA, version 1 (IFSPRSA1) / Integer Factorization Verification Primitive using RSA, version 1 (IFVPRSA1) in IEEE 1363 [IEEE1363] (except that support for multiprime RSA has been added) and are compatible with PKCS #1 v1.5.
The main mathematical operation in each primitive is exponentiation, as in the encryption and decryption primitives of Section 5.1. RSASP1 and RSAVP1 are the same as RSADP and RSAEP except for the names of their input and output arguments; they are distinguished as they are intended for different purposes.
5.2.1. RSASP1

RSASP1 (K, m)
Input:
K RSA private key, where K has one of the following forms:  a pair (n, d)  a quintuple (p, q, dP, dQ, qInv) and a (possibly empty) sequence of triplets (r_i, d_i, t_i), i = 3, ..., u m message representative, an integer between 0 and n  1
Output:
s signature representative, an integer between 0 and n  1 Error: "message representative out of range" Assumption: RSA private key K is valid
Steps:

 If the message representative m is not between 0 and n  1, output "message representative out of range" and stop.
 The signature representative s is computed as follows.

a. If the first form (n, d) of K is used, let s = m^d mod n.
b. If the second form (p, q, dP, dQ, qInv) and (r_i, d_i, t_i) of K is used, proceed as follows:

1. Let s_1 = m^dP mod p and s_2 = m^dQ mod q.




 If u > 2, let s_i = m^(d_i) mod r_i, i = 3, ..., u.
 Let h = (s_1  s_2) * qInv mod p.
 Let s = s_2 + q * h.



5. If u > 2, let R = r_1 and for i = 3 to u do





a. Let R = R * r_(i1).




b. Let h = (s_i  s) * t_i mod r_i.





c. Let s = s + R * h.


 Output s.

Note: Step 2.b can be rewritten as a single loop, provided that one reverses the order of p and q. For consistency with PKCS #1 v2.0, however, the first two primes p and q are treated separately from the additional primes.

5.2.2. RSAVP1

RSAVP1 ((n, e), s)
Input:
(n, e) RSA public key
s signature representative, an integer between 0 and n  1

Output:
m message representative, an integer between 0 and n  1

Error: "signature representative out of range" Assumption: RSA public key (n, e) is valid
Steps:

 If the signature representative s is not between 0 and n  1, output "signature representative out of range" and stop.
 Let m = s^e mod n.
 Output m.

6. Overview of Schemes

A scheme combines cryptographic primitives and other techniques to achieve a particular security goal. Two types of scheme are specified in this document: encryption schemes and signature schemes with appendix.
The schemes specified in this document are limited in scope in that their operations consist only of steps to process data with an RSA public or private key, and they do not include steps for obtaining or validating the key. Thus, in addition to the scheme operations, an application will typically include key management operations by which parties may select RSA public and private keys for a scheme operation. The specific additional operations and other details are outside the scope of this document.
As was the case for the cryptographic primitives (Section 5), the specifications of scheme operations assume that certain conditions are met by the inputs, in particular that RSA public and private keys are valid. The behavior of an implementation is thus unspecified when a key is invalid. The impact of such unspecified behavior depends on the application. Possible means of addressing key validation include explicit key validation by the application; key validation within the publickey infrastructure; and assignment of liability for operations performed with an invalid key to the party who generated the key.
A generally good cryptographic practice is to employ a given RSA key pair in only one scheme. This avoids the risk that vulnerability in one scheme may compromise the security of the other and may be essential to maintain provable security. While RSAESPKCS1v1_5
(Section 7.2) and RSASSAPKCS1v1_5 (Section 8.2) have traditionally been employed together without any known bad interactions (indeed, this is the model introduced by PKCS #1 v1.5), such a combined use of an RSA key pair is NOT RECOMMENDED for new applications.
To illustrate the risks related to the employment of an RSA key pair in more than one scheme, suppose an RSA key pair is employed in both RSAESOAEP (Section 7.1) and RSAESPKCS1v1_5. Although RSAESOAEP by itself would resist attack, an opponent might be able to exploit a weakness in the implementation of RSAESPKCS1v1_5 to recover messages encrypted with either scheme. As another example, suppose an RSA key pair is employed in both RSASSAPSS (Section 8.1) and RSASSAPKCS1v1_5. Then the security proof for RSASSAPSS would no longer be sufficient since the proof does not account for the possibility that signatures might be generated with a second scheme. Similar considerations may apply if an RSA key pair is employed in one of the schemes defined here and in a variant defined elsewhere.
7. Encryption Schemes

For the purposes of this document, an encryption scheme consists of an encryption operation and a decryption operation, where the encryption operation produces a ciphertext from a message with a recipient's RSA public key, and the decryption operation recovers the message from the ciphertext with the recipient's corresponding RSA private key.
An encryption scheme can be employed in a variety of applications. A typical application is a key establishment protocol, where the message contains key material to be delivered confidentially from one party to another. For instance, PKCS #7 [RFC2315] employs such a protocol to deliver a contentencryption key from a sender to a recipient; the encryption schemes defined here would be suitable key encryption algorithms in that context.
Two encryption schemes are specified in this document: RSAESOAEP and RSAESPKCS1v1_5. RSAESOAEP is REQUIRED to be supported for new applications; RSAESPKCS1v1_5 is included only for compatibility with existing applications.
The encryption schemes given here follow a general model similar to that employed in IEEE 1363 [IEEE1363], combining encryption and decryption primitives with an encoding method for encryption. The encryption operations apply a message encoding operation to a message to produce an encoded message, which is then converted to an integer message representative. An encryption primitive is applied to the message representative to produce the ciphertext. Reversing this, the decryption operations apply a decryption primitive to the ciphertext to recover a message representative, which is then converted to an octetstringencoded message. A message decoding operation is applied to the encoded message to recover the message and verify the correctness of the decryption.
To avoid implementation weaknesses related to the way errors are handled within the decoding operation (see [BLEICHENBACHER] and [MANGER]), the encoding and decoding operations for RSAESOAEP and RSAESPKCS1v1_5 are embedded in the specifications of the respective encryption schemes rather than defined in separate specifications. Both encryption schemes are compatible with the corresponding schemes in PKCS #1 v2.1.
7.1. RSAESOAEP

RSAESOAEP combines the RSAEP and RSADP primitives (Sections 5.1.1 and 5.1.2) with the EMEOAEP encoding method (Step 2 in Section 7.1.1, and Step 3 in Section 7.1.2). EMEOAEP is based on Bellare and Rogaway's Optimal Asymmetric Encryption scheme [OAEP]. It is compatible with the Integer Factorization Encryption Scheme (IFES) defined in IEEE 1363 [IEEE1363], where the encryption and decryption primitives are IFEPRSA and IFDPRSA and the message encoding method is EMEOAEP. RSAESOAEP can operate on messages of length up to k  2hLen 2 octets, where hLen is the length of the output from the underlying hash function and k is the length in octets of the recipient's RSA modulus.
Assuming that computing eth roots modulo n is infeasible and the mask generation function in RSAESOAEP has appropriate properties, RSAESOAEP is semantically secure against adaptive chosenciphertext attacks. This assurance is provable in the sense that the difficulty of breaking RSAESOAEP can be directly related to the difficulty of inverting the RSA function, provided that the mask generation function is viewed as a black box or random oracle; see [FOPS] and the note below for further discussion.
Both the encryption and the decryption operations of RSAESOAEP take the value of a label L as input. In this version of PKCS #1, L is the empty string; other uses of the label are outside the scope of this document. See Appendix A.2.1 for the relevant ASN.1 syntax.
RSAESOAEP is parameterized by the choice of hash function and mask generation function. This choice should be fixed for a given RSA key. Suggested hash and mask generation functions are given in Appendix B.
Note: Past results have helpfully clarified the security properties of the OAEP encoding method [OAEP] (roughly the procedure described in Step 2 in Section 7.1.1). The background is as follows. In 1994, Bellare and Rogaway [OAEP] introduced a security concept that they denoted plaintext awareness (PA94). They proved that if a deterministic publickey encryption primitive (e.g., RSAEP) is hard to invert without the private key, then the corresponding OAEPbased encryption scheme is plaintext aware (in the random oracle model), meaning roughly that an adversary cannot produce a valid ciphertext without actually "knowing" the underlying plaintext. Plaintext awareness of an encryption scheme is closely related to the resistance of the scheme against chosenciphertext attacks. In such attacks, an adversary is given the opportunity to send queries to an oracle simulating the decryption primitive. Using the results of these queries, the adversary attempts to decrypt a challenge ciphertext.
However, there are two flavors of chosenciphertext attacks, and PA94 implies security against only one of them. The difference relies on what the adversary is allowed to do after she is given the challenge ciphertext. The indifferent attack scenario (denoted CCA1) does not admit any queries to the decryption oracle after the adversary is given the challenge ciphertext, whereas the adaptive scenario (denoted CCA2) does (except that the decryption oracle refuses to decrypt the challenge ciphertext once it is published). In 1998, Bellare and Rogaway, together with Desai and Pointcheval [PA98], came up with a new, stronger notion of plaintext awareness (PA98) that does imply security against CCA2.
To summarize, there have been two potential sources for misconception: that PA94 and PA98 are equivalent concepts, or that CCA1 and CCA2 are equivalent concepts. Either assumption leads to the conclusion that the BellareRogaway paper implies security of OAEP against CCA2, which it does not.
(Footnote: It might be fair to mention that PKCS #1 v2.0 cites [OAEP] and claims that "a chosen ciphertext attack is ineffective against a plaintextaware encryption scheme such as RSAESOAEP" without specifying the kind of plaintext awareness or chosen ciphertext attack considered.)
OAEP has never been proven secure against CCA2; in fact, Victor Shoup [SHOUP] has demonstrated that such a proof does not exist in the general case. Put briefly, Shoup showed that an adversary in the CCA2 scenario who knows how to partially invert the encryption primitive but does not know how to invert it completely may well be able to break the scheme. For example, one may imagine an attacker who is able to break RSAESOAEP if she knows how to recover all but the first 20 bytes of a random integer encrypted with RSAEP. Such an attacker does not need to be able to fully invert RSAEP, because she does not use the first 20 octets in her attack.
Still, RSAESOAEP is secure against CCA2, which was proved by Fujisaki, Okamoto, Pointcheval, and Stern [FOPS] shortly after the announcement of Shoup's result. Using clever lattice reduction techniques, they managed to show how to invert RSAEP completely given a sufficiently large part of the preimage. This observation, combined with a proof that OAEP is secure against CCA2 if the underlying encryption primitive is hard to partially invert, fills the gap between what Bellare and Rogaway proved about RSAESOAEP and what some may have believed that they proved. Somewhat paradoxically, we are hence saved by an ostensible weakness in RSAEP (i.e., the whole inverse can be deduced from parts of it).
Unfortunately, however, the security reduction is not efficient for concrete parameters. While the proof successfully relates an adversary A against the CCA2 security of RSAESOAEP to an algorithm I inverting RSA, the probability of success for I is only approximately \epsilon^2 / 2^18, where \epsilon is the probability of success for A.
(Footnote: In [FOPS], the probability of success for the inverter was \epsilon^2 / 4. The additional factor 1 / 2^16 is due to the eight fixed zero bits at the beginning of the encoded message EM, which are not present in the variant of OAEP considered in [FOPS]. (A must be applied twice to invert RSA, and each application corresponds to a factor 1 / 2^8.))
In addition, the running time for I is approximately t^2, where t is the running time of the adversary. The consequence is that we cannot exclude the possibility that attacking RSAESOAEP is considerably easier than inverting RSA for concrete parameters. Still, the existence of a security proof provides some assurance that the RSAESOAEP construction is sounder than ad hoc constructions such as RSAESPKCS1v1_5.
Hybrid encryption schemes based on the RSA Key Encapsulation Mechanism (RSAKEM) paradigm offer tight proofs of security directly applicable to concrete parameters; see [ISO18033] for discussion. Future versions of PKCS #1 may specify schemes based on this paradigm.
7.1.1. Encryption Operation

RSAESOAEPENCRYPT ((n, e), M, L)
Options:
Hash hash function (hLen denotes the length in octets of the hash function output) MGF mask generation function
Input:
(n, e) recipient's RSA public key (k denotes the length in octets of the RSA modulus n) M message to be encrypted, an octet string of length mLen, where mLen <= k  2hLen  2 L optional label to be associated with the message; the default value for L, if L is not provided, is the empty string
Output:
C ciphertext, an octet string of length k Errors: "message too long"; "label too long" Assumption: RSA public key (n, e) is valid
Steps:

 Length checking:

a. If the length of L is greater than the input limitation

for the hash function (2^61  1 octets for SHA1), output "label too long" and stop.
b. If mLen > k  2hLen  2, output "message too long" and

stop.
 EMEOAEP encoding (see Figure 1 below):

a. If the label L is not provided, let L be the empty string.

Let lHash = Hash(L), an octet string of length hLen (see the note below).
b. Generate a padding string PS consisting of k  mLen 

2hLen  2 zero octets. The length of PS may be zero.
c. Concatenate lHash, PS, a single octet with hexadecimal

value 0x01, and the message M to form a data block DB of length k  hLen  1 octets as

DB = lHash  PS  0x01  M.


d. Generate a random octet string seed of length hLen.



e. Let dbMask = MGF(seed, k  hLen  1).
f. Let maskedDB = DB \xor dbMask.
g. Let seedMask = MGF(maskedDB, hLen).
h. Let maskedSeed = seed \xor seedMask.
i. Concatenate a single octet with hexadecimal value 0x00,

maskedSeed, and maskedDB to form an encoded message EM of length k octets as


EM = 0x00  maskedSeed  maskedDB.


 RSA encryption:

a. Convert the encoded message EM to an integer message

representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSAEP encryption primitive (Section 5.1.1) to

the RSA public key (n, e) and the message representative m to produce an integer ciphertext representative c:

c = RSAEP ((n, e), m).

c. Convert the ciphertext representative c to a ciphertext C

of length k octets (see Section 4.1):

C = I2OSP (c, k).

 Output the ciphertext C.
_________________________________________________________________ +++++ DB =  lHash  PS 01 M  +++++  ++   seed   ++    > MGF > xor   ++ V  00 xor < MGF < ++      V V V ++++ EM = 00maskedSeed maskedDB  ++++ _________________________________________________________________

Figure 1: EMEOAEP Encoding Operation

Notes:
 lHash is the hash of the optional label L.
 The decoding operation follows reverse steps to recover M and verify lHash and PS.
 If L is the empty string, the corresponding hash value lHash has the following hexadecimal representation for different choices of Hash:
SHA1: (0x)da39a3ee 5e6b4b0d 3255bfef 95601890 afd80709 SHA256: (0x)e3b0c442 98fc1c14 9afbf4c8 996fb924 27ae41e4 649b934c a495991b 7852b855 SHA384: (0x)38b060a7 51ac9638 4cd9327e b1b1e36a 21fdb711 14be0743 4c0cc7bf 63f6e1da 274edebf e76f65fb d51ad2f1 4898b95b SHA512: (0x)cf83e135 7eefb8bd f1542850 d66d8007 d620e405 0b5715dc 83f4a921 d36ce9ce 47d0d13c 5d85f2b0 ff8318d2 877eec2f 63b931bd 47417a81 a538327a f927da3e
7.1.2. Decryption Operation

RSAESOAEPDECRYPT (K, C, L)
Options:
Hash hash function (hLen denotes the length in octets of the hash function output) MGF mask generation function
Input:
K recipient's RSA private key (k denotes the length in octets of the RSA modulus n), where k >= 2hLen + 2 C ciphertext to be decrypted, an octet string of length k L optional label whose association with the message is to be verified; the default value for L, if L is not provided, is the empty string
Output:
M message, an octet string of length mLen, where mLen <= k  2hLen  2 Error: "decryption error"
Steps:

 Length checking:

a. If the length of L is greater than the input limitation

for the hash function (2^61  1 octets for SHA1), output "decryption error" and stop.
b. If the length of the ciphertext C is not k octets, output

"decryption error" and stop.
c. If k < 2hLen + 2, output "decryption error" and stop.
 RSA decryption:

a. Convert the ciphertext C to an integer ciphertext

representative c (see Section 4.2):

c = OS2IP (C).

b. Apply the RSADP decryption primitive (Section 5.1.2) to

the RSA private key K and the ciphertext representative c to produce an integer message representative m:

m = RSADP (K, c).
If RSADP outputs "ciphertext representative out of range" (meaning that c >= n), output "decryption error" and stop.

c. Convert the message representative m to an encoded message

EM of length k octets (see Section 4.1):





EM = I2OSP (m, k).


 EMEOAEP decoding:

a. If the label L is not provided, let L be the empty string.

Let lHash = Hash(L), an octet string of length hLen (see the note in Section 7.1.1).
b. Separate the encoded message EM into a single octet Y, an

octet string maskedSeed of length hLen, and an octet string maskedDB of length k  hLen  1 as


EM = Y  maskedSeed  maskedDB.



c. Let seedMask = MGF(maskedDB, hLen).
d. Let seed = maskedSeed \xor seedMask.
e. Let dbMask = MGF(seed, k  hLen  1).
f. Let DB = maskedDB \xor dbMask.
g. Separate DB into an octet string lHash' of length hLen, a

(possibly empty) padding string PS consisting of octets with hexadecimal value 0x00, and a message M as

DB = lHash'  PS  0x01  M.
If there is no octet with hexadecimal value 0x01 to separate PS from M, if lHash does not equal lHash', or if Y is nonzero, output "decryption error" and stop. (See the note below.)

 Output the message M.
Note: Care must be taken to ensure that an opponent cannot distinguish the different error conditions in Step 3.g, whether by error message or timing, and, more generally, that an opponent cannot learn partial information about the encoded message EM. Otherwise, an opponent may be able to obtain useful information about the decryption of the ciphertext C, leading to a chosen ciphertext attack such as the one observed by Manger [MANGER].


7.2. RSAESPKCS1v1_5

RSAESPKCS1v1_5 combines the RSAEP and RSADP primitives (Sections 5.1.1 and 5.1.2) with the EMEPKCS1v1_5 encoding method (Step 2 in Section 7.2.1, and Step 3 in Section 7.2.2). It is mathematically equivalent to the encryption scheme in PKCS #1 v1.5. RSAESPKCS1v1_5 can operate on messages of length up to k  11 octets (k is the octet length of the RSA modulus), although care should be taken to avoid certain attacks on lowexponent RSA due to Coppersmith, Franklin, Patarin, and Reiter when long messages are encrypted (see the third bullet in the notes below and [LOWEXP]; [NEWATTACK] contains an improved attack). As a general rule, the use of this scheme for encrypting an arbitrary message, as opposed to a randomly generated key, is NOT RECOMMENDED.
It is possible to generate valid RSAESPKCS1v1_5 ciphertexts without knowing the corresponding plaintexts, with a reasonable probability of success. This ability can be exploited in a chosenciphertext attack as shown in [BLEICHENBACHER]. Therefore, if RSAESPKCS1v1_5 is to be used, certain easily implemented countermeasures should be taken to thwart the attack found in [BLEICHENBACHER]. Typical examples include the addition of structure to the data to be encoded, rigorous checking of PKCS #1 v1.5 conformance (and other redundancy) in decrypted messages, and the consolidation of error messages in a clientserver protocol based on PKCS #1 v1.5. These can all be effective countermeasures and do not involve changes to a protocol based on PKCS #1 v1.5. See [BKS] for a further discussion of these and other countermeasures. It has recently been shown that the security of the SSL/TLS handshake protocol [RFC5246], which uses RSAESPKCS1v1_5 and certain countermeasures, can be related to a variant of the RSA problem; see [RSATLS] for discussion.
Note: The following passages describe some security recommendations pertaining to the use of RSAESPKCS1v1_5. Recommendations from PKCS #1 v1.5 are included as well as new recommendations motivated by cryptanalytic advances made in the intervening years.
 It is RECOMMENDED that the pseudorandom octets in Step 2 in Section 7.2.1 be generated independently for each encryption process, especially if the same data is input to more than one encryption process. Haastad's results [HAASTAD] are one motivation for this recommendation.
 The padding string PS in Step 2 in Section 7.2.1 is at least eight octets long, which is a security condition for publickey operations that makes it difficult for an attacker to recover data by trying all possible encryption blocks.
 The pseudorandom octets can also help thwart an attack due to Coppersmith et al. [LOWEXP] (see [NEWATTACK] for an improvement of the attack) when the size of the message to be encrypted is kept small. The attack works on lowexponent RSA when similar messages are encrypted with the same RSA public key. More specifically, in one flavor of the attack, when two inputs to RSAEP agree on a large fraction of bits (8/9) and lowexponent RSA (e = 3) is used to encrypt both of them, it may be possible to recover both inputs with the attack. Another flavor of the attack is successful in decrypting a single ciphertext when a large fraction (2/3) of the input to RSAEP is already known. For typical applications, the message to be encrypted is short (e.g., a 128bit symmetric key), so not enough information will be known or common between two messages to enable the attack. However, if a long message is encrypted, or if part of a message is known, then the attack may be a concern. In any case, the RSAESOAEP scheme overcomes the attack.
7.2.1. Encryption Operation

RSAESPKCS1V1_5ENCRYPT ((n, e), M)
Input:
(n, e) recipient's RSA public key (k denotes the length in octets of the modulus n) M message to be encrypted, an octet string of length mLen, where mLen <= k  11
Output:
C ciphertext, an octet string of length k Error: "message too long"
Steps:

 Length checking: If mLen > k  11, output "message too long" and stop.
 EMEPKCS1v1_5 encoding:

a. Generate an octet string PS of length k  mLen  3
consisting of pseudorandomly generated nonzero octets.
The length of PS will be at least eight octets.



b. Concatenate PS, the message M, and other padding to form


an encoded message EM of length k octets as
EM = 0x00  0x02  PS  0x00  M.


 RSA encryption:

a. Convert the encoded message EM to an integer message

representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSAEP encryption primitive (Section 5.1.1) to

the RSA public key (n, e) and the message representative m to produce an integer ciphertext representative c:

c = RSAEP ((n, e), m).

c. Convert the ciphertext representative c to a ciphertext C

of length k octets (see Section 4.1):

C = I2OSP (c, k).

 Output the ciphertext C.

7.2.2. Decryption Operation

RSAESPKCS1V1_5DECRYPT (K, C)
Input:
K recipient's RSA private key C ciphertext to be decrypted, an octet string of length k, where k is the length in octets of the RSA modulus n
Output:
M message, an octet string of length at most k  11 Error: "decryption error"
Steps:

 Length checking: If the length of the ciphertext C is not k octets (or if k < 11), output "decryption error" and stop.
 RSA decryption:

a. Convert the ciphertext C to an integer ciphertext

representative c (see Section 4.2):

c = OS2IP (C).

b. Apply the RSADP decryption primitive (Section 5.1.2) to

the RSA private key (n, d) and the ciphertext
representative c to produce an integer message
representative m:





m = RSADP ((n, d), c).
If RSADP outputs "ciphertext representative out of range" (meaning that c >= n), output "decryption error" and stop.

c. Convert the message representative m to an encoded message



EM of length k octets (see Section 4.1):





EM = I2OSP (m, k).



3. EMEPKCS1v1_5 decoding: Separate the encoded message EM into an octet string PS consisting of nonzero octets and a message M as



EM = 0x00  0x02  PS  0x00  M.
If the first octet of EM does not have hexadecimal value 0x00, if the second octet of EM does not have hexadecimal value 0x02, if there is no octet with hexadecimal value 0x00 to separate PS from M, or if the length of PS is less than 8 octets, output "decryption error" and stop. (See the note below.)

 Output M.
Note: Care shall be taken to ensure that an opponent cannot distinguish the different error conditions in Step 3, whether by error message or timing. Otherwise, an opponent may be able to obtain useful information about the decryption of the ciphertext C, leading to a strengthened version of Bleichenbacher's attack [BLEICHENBACHER]; compare to Manger's attack [MANGER].


8. Signature Scheme with Appendix

For the purposes of this document, a signature scheme with appendix consists of a signature generation operation and a signature verification operation, where the signature generation operation produces a signature from a message with a signer's RSA private key, and the signature verification operation verifies the signature on the message with the signer's corresponding RSA public key. To verify a signature constructed with this type of scheme, it is necessary to have the message itself. In this way, signature schemes with appendix are distinguished from signature schemes with message recovery, which are not supported in this document.
A signature scheme with appendix can be employed in a variety of applications. For instance, the signature schemes with appendix defined here would be suitable signature algorithms for X.509 certificates [ISO9594]. Related signature schemes could be employed in PKCS #7 [RFC2315], although for technical reasons the current version of PKCS #7 separates a hash function from a signature scheme, which is different than what is done here; see the note in Appendix A.2.3 for more discussion.
Two signature schemes with appendix are specified in this document: RSASSAPSS and RSASSAPKCS1v1_5. Although no attacks are known against RSASSAPKCS1v1_5, in the interest of increased robustness, RSASSAPSS is REQUIRED in new applications. RSASSAPKCS1v1_5 is included only for compatibility with existing applications.
The signature schemes with appendix given here follow a general model similar to that employed in IEEE 1363 [IEEE1363], combining signature and verification primitives with an encoding method for signatures. The signature generation operations apply a message encoding operation to a message to produce an encoded message, which is then converted to an integer message representative. A signature primitive is applied to the message representative to produce the signature. Reversing this, the signature verification operations apply a signature verification primitive to the signature to recover a message representative, which is then converted to an octetstring encoded message. A verification operation is applied to the message and the encoded message to determine whether they are consistent.
If the encoding method is deterministic (e.g., EMSAPKCS1v1_5), the verification operation may apply the message encoding operation to the message and compare the resulting encoded message to the previously derived encoded message. If there is a match, the signature is considered valid. If the method is randomized (e.g., EMSAPSS), the verification operation is typically more complicated. For example, the verification operation in EMSAPSS extracts the random salt and a hash output from the encoded message and checks whether the hash output, the salt, and the message are consistent; the hash output is a deterministic function in terms of the message and the salt. For both signature schemes with appendix defined in this document, the signature generation and signature verification operations are readily implemented as "singlepass" operations if the signature is placed after the message. See PKCS #7 [RFC2315] for an example format in the case of RSASSAPKCS1v1_5.
8.1. RSASSAPSS

RSASSAPSS combines the RSASP1 and RSAVP1 primitives with the EMSAPSS encoding method. It is compatible with the Integer Factorization Signature Scheme with Appendix (IFSSA) as amended in IEEE 1363a [IEEE1363A], where the signature and verification primitives are IFSPRSA1 and IFVPRSA1 as defined in IEEE 1363 [IEEE1363], and the message encoding method is EMSA4. EMSA4 is slightly more general than EMSAPSS as it acts on bit strings rather than on octet strings. EMSAPSS is equivalent to EMSA4 restricted to the case that the operands as well as the hash and salt values are octet strings.
The length of messages on which RSASSAPSS can operate is either unrestricted or constrained by a very large number, depending on the hash function underlying the EMSAPSS encoding method.
Assuming that computing eth roots modulo n is infeasible and the hash and mask generation functions in EMSAPSS have appropriate properties, RSASSAPSS provides secure signatures. This assurance is provable in the sense that the difficulty of forging signatures can be directly related to the difficulty of inverting the RSA function, provided that the hash and mask generation functions are viewed as black boxes or random oracles. The bounds in the security proof are essentially "tight", meaning that the success probability and running time for the best forger against RSASSAPSS are very close to the corresponding parameters for the best RSA inversion algorithm; see [RSARABIN] [PSSPROOF] [JONSSON] for further discussion.
In contrast to the RSASSAPKCS1v1_5 signature scheme, a hash function identifier is not embedded in the EMSAPSS encoded message, so in theory it is possible for an adversary to substitute a different (and potentially weaker) hash function than the one selected by the signer. Therefore, it is RECOMMENDED that the EMSAPSS mask generation function be based on the same hash function. In this manner, the entire encoded message will be dependent on the hash function, and it will be difficult for an opponent to substitute a different hash function than the one intended by the signer. This matching of hash functions is only for the purpose of preventing hash function substitution and is not necessary if hash function substitution is addressed by other means (e.g., the verifier accepts only a designated hash function). See [HASHID] for further discussion of these points. The provable security of RSASSAPSS does not rely on the hash function in the mask generation function being the same as the hash function applied to the message.
RSASSAPSS is different from other RSAbased signature schemes in that it is probabilistic rather than deterministic, incorporating a randomly generated salt value. The salt value enhances the security of the scheme by affording a "tighter" security proof than deterministic alternatives such as Full Domain Hashing (FDH); see [RSARABIN] for discussion. However, the randomness is not critical to security. In situations where random generation is not possible, a fixed value or a sequence number could be employed instead, with the resulting provable security similar to that of FDH [FDH].
8.1.1. Signature Generation Operation

RSASSAPSSSIGN (K, M)
Input:
K signer's RSA private key M message to be signed, an octet string
Output:
S signature, an octet string of length k, where k is the length in octets of the RSA modulus n Errors: "message too long;" "encoding error"
Steps:
1. EMSAPSS encoding: Apply the EMSAPSS encoding operation (Section 9.1.1) to the message M to produce an encoded message EM of length \ceil ((modBits  1)/8) octets such that the bit length of the integer OS2IP (EM) (see Section 4.2) is at most modBits  1, where modBits is the length in bits of the RSA modulus n:



EM = EMSAPSSENCODE (M, modBits  1).
Note that the octet length of EM will be one less than k if modBits  1 is divisible by 8 and equal to k otherwise. If the encoding operation outputs "message too long", output "message too long" and stop. If the encoding operation outputs "encoding error", output "encoding error" and stop.

 RSA signature:

a. Convert the encoded message EM to an integer message

representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSASP1 signature primitive (Section 5.2.1) to

the RSA private key K and the message representative m to produce an integer signature representative s:

s = RSASP1 (K, m).

c. Convert the signature representative s to a signature S of

length k octets (see Section 4.1):

S = I2OSP (s, k).

 Output the signature S.


8.1.2. Signature Verification Operation

RSASSAPSSVERIFY ((n, e), M, S)
Input:
(n, e) signer's RSA public key M message whose signature is to be verified, an octet string S signature to be verified, an octet string of length k, where k is the length in octets of the RSA modulus n Output: "valid signature" or "invalid signature"
Steps:

 Length checking: If the length of the signature S is not k octets, output "invalid signature" and stop.
 RSA verification:

a. Convert the signature S to an integer signature

representative s (see Section 4.2):

s = OS2IP (S).

b. Apply the RSAVP1 verification primitive (Section 5.2.2) to

the RSA public key (n, e) and the signature representative s to produce an integer message representative m:

m = RSAVP1 ((n, e), s).
If RSAVP1 output "signature representative out of range", output "invalid signature" and stop.

c. Convert the message representative m to an encoded message

EM of length emLen = \ceil ((modBits  1)/8) octets, where modBits is the length in bits of the RSA modulus n (see Section 4.1):

EM = I2OSP (m, emLen).
Note that emLen will be one less than k if modBits  1 is divisible by 8 and equal to k otherwise. If I2OSP outputs "integer too large", output "invalid signature" and stop.

3. EMSAPSS verification: Apply the EMSAPSS verification operation (Section 9.1.2) to the message M and the encoded message EM to determine whether they are consistent:

Result = EMSAPSSVERIFY (M, EM, modBits  1).


 If Result = "consistent", output "valid signature". Otherwise, output "invalid signature".

8.2. RSASSAPKCS1v1_5

RSASSAPKCS1v1_5 combines the RSASP1 and RSAVP1 primitives with the EMSAPKCS1v1_5 encoding method. It is compatible with the IFSSA scheme defined in IEEE 1363 [IEEE1363], where the signature and verification primitives are IFSPRSA1 and IFVPRSA1, and the message encoding method is EMSAPKCS1v1_5 (which is not defined in IEEE 1363 but is in IEEE 1363a [IEEE1363A]).
The length of messages on which RSASSAPKCS1v1_5 can operate is either unrestricted or constrained by a very large number, depending on the hash function underlying the EMSAPKCS1v1_5 method.
Assuming that computing eth roots modulo n is infeasible and the hash function in EMSAPKCS1v1_5 has appropriate properties, RSASSAPKCS1v1_5 is conjectured to provide secure signatures. More precisely, forging signatures without knowing the RSA private key is conjectured to be computationally infeasible. Also, in the encoding method EMSAPKCS1v1_5, a hash function identifier is embedded in the encoding. Because of this feature, an adversary trying to find a message with the same signature as a previously signed message must find collisions of the particular hash function being used; attacking a different hash function than the one selected by the signer is not useful to the adversary. See [HASHID] for further discussion.
Note: As noted in PKCS #1 v1.5, the EMSAPKCS1v1_5 encoding method has the property that the encoded message, converted to an integer message representative, is guaranteed to be large and at least somewhat "random". This prevents attacks of the kind proposed by Desmedt and Odlyzko [CHOSEN] where multiplicative relationships between message representatives are developed by factoring the message representatives into a set of small values (e.g., a set of small primes). Coron, Naccache, and Stern [PADDING] showed that a stronger form of this type of attack could be quite effective against some instances of the ISO/IEC 97962 signature scheme. They also analyzed the complexity of this type of attack against the EMSAPKCS1v1_5 encoding method and concluded that an attack would be impractical, requiring more operations than a collision search on the underlying hash function (i.e., more than 2^80 operations). Coppersmith, Halevi, and Jutla [FORGERY] subsequently extended Coron et al.'s attack to break the ISO/IEC 97961 signature scheme with message recovery. The various attacks illustrate the importance of carefully constructing the input to the RSA signature primitive, particularly in a signature scheme with message recovery. Accordingly, the EMSAPKCSv1_5 encoding method explicitly includes a hash operation and is not intended for signature schemes with message recovery. Moreover, while no attack is known against the EMSAPKCSv1_5 encoding method, a gradual transition to EMSAPSS is recommended as a precaution against future developments.
8.2.1. Signature Generation Operation

RSASSAPKCS1V1_5SIGN (K, M)
Input:
K signer's RSA private key M message to be signed, an octet string
Output:
S signature, an octet string of length k, where k is the length in octets of the RSA modulus n Errors: "message too long"; "RSA modulus too short"
Steps:
1. EMSAPKCS1v1_5 encoding: Apply the EMSAPKCS1v1_5 encoding operation (Section 9.2) to the message M to produce an encoded message EM of length k octets:



EM = EMSAPKCS1V1_5ENCODE (M, k).
If the encoding operation outputs "message too long", output "message too long" and stop. If the encoding operation outputs "intended encoded message length too short", output "RSA modulus too short" and stop.

 RSA signature:

a. Convert the encoded message EM to an integer message

representative m (see Section 4.2):

m = OS2IP (EM).

b. Apply the RSASP1 signature primitive (Section 5.2.1) to

the RSA private key K and the message representative m to produce an integer signature representative s:

s = RSASP1 (K, m).

c. Convert the signature representative s to a signature S of

length k octets (see Section 4.1):

S = I2OSP (s, k).

 Output the signature S.


8.2.2. Signature Verification Operation

RSASSAPKCS1V1_5VERIFY ((n, e), M, S)
Input:
(n, e) signer's RSA public key M message whose signature is to be verified, an octet string S signature to be verified, an octet string of length k, where k is the length in octets of the RSA modulus n Output "valid signature" or "invalid signature" Errors: "message too long"; "RSA modulus too short"
Steps:

 Length checking: If the length of the signature S is not k octets, output "invalid signature" and stop.
 RSA verification:

a. Convert the signature S to an integer signature

representative s (see Section 4.2):

s = OS2IP (S).

b. Apply the RSAVP1 verification primitive (Section 5.2.2) to

the RSA public key (n, e) and the signature representative s to produce an integer message representative m:

m = RSAVP1 ((n, e), s).
If RSAVP1 outputs "signature representative out of range", output "invalid signature" and stop.

c. Convert the message representative m to an encoded message

EM of length k octets (see Section 4.1):





EM = I2OSP (m, k).
If I2OSP outputs "integer too large", output "invalid signature" and stop.



3. EMSAPKCS1v1_5 encoding: Apply the EMSAPKCS1v1_5 encoding operation (Section 9.2) to the message M to produce a second encoded message EM' of length k octets:



EM' = EMSAPKCS1V1_5ENCODE (M, k).
If the encoding operation outputs "message too long", output "message too long" and stop. If the encoding operation outputs "intended encoded message length too short", output "RSA modulus too short" and stop.

 Compare the encoded message EM and the second encoded message EM'. If they are the same, output "valid signature"; otherwise, output "invalid signature".
Note: Another way to implement the signature verification operation is to apply a "decoding" operation (not specified in this document) to the encoded message to recover the underlying hash value, and then compare it to a newly computed hash value. This has the advantage that it requires less intermediate storage (two hash values rather than two encoded messages), but the disadvantage that it requires additional code.


9. Encoding Methods for Signatures with Appendix

Encoding methods consist of operations that map between octet string messages and octetstringencoded messages, which are converted to and from integer message representatives in the schemes. The integer message representatives are processed via the primitives. The encoding methods thus provide the connection between the schemes, which process messages, and the primitives.
An encoding method for signatures with appendix, for the purposes of this document, consists of an encoding operation and optionally a verification operation. An encoding operation maps a message M to an encoded message EM of a specified length. A verification operation determines whether a message M and an encoded message EM are consistent, i.e., whether the encoded message EM is a valid encoding of the message M.
The encoding operation may introduce some randomness, so that different applications of the encoding operation to the same message will produce different encoded messages, which has benefits for provable security. For such an encoding method, both an encoding and a verification operation are needed unless the verifier can reproduce the randomness (e.g., by obtaining the salt value from the signer). For a deterministic encoding method, only an encoding operation is needed.
Two encoding methods for signatures with appendix are employed in the signature schemes and are specified here: EMSAPSS and EMSAPKCS1v1_5.
9.1. EMSAPSS

This encoding method is parameterized by the choice of hash function, mask generation function, and salt length. These options should be fixed for a given RSA key, except that the salt length can be variable (see [JONSSON] for discussion). Suggested hash and mask generation functions are given in Appendix B. The encoding method is based on Bellare and Rogaway's Probabilistic Signature Scheme (PSS) [RSARABIN][PSS]. It is randomized and has an encoding operation and a verification operation.
Figure 2 illustrates the encoding operation.
__________________________________________________________________ ++  M  ++  V Hash  V ++++ M' = Padding1 mHash  salt  ++++  +++ V DB = Padding2 salt  Hash +++    V  xor < MGF <     V V ++++ EM =  maskedDB  H bc ++++ __________________________________________________________________ Figure 2: EMSAPSS Encoding Operation
Note that the verification operation follows reverse steps to recover salt and then forward steps to recompute and compare H.
Notes:
 The encoding method defined here differs from the one in Bellare and Rogaway's submission to IEEE 1363a [PSS] in three respects:

 It applies a hash function rather than a mask generation function to the message. Even though the mask generation function is based on a hash function, it seems more natural to apply a hash function directly.
 The value that is hashed together with the salt value is the string (0x)00 00 00 00 00 00 00 00  mHash rather than the message M itself. Here, mHash is the hash of M. Note that the hash function is the same in both steps. See Note 3 below for further discussion. (Also, the name "salt" is used instead of "seed", as it is more reflective of the value's role.)
 The encoded message in EMSAPSS has nine fixed bits; the first bit is 0 and the last eight bits form a "trailer field", the octet 0xbc. In the original scheme, only the first bit is fixed. The rationale for the trailer field is for compatibility with the Integer Factorization Signature Primitive using RabinWilliams (IFSPRW) in IEEE 1363 [IEEE1363] and the corresponding primitive in ISO/IEC 97962:2010 [ISO9796].
 Assuming that the mask generation function is based on a hash function, it is RECOMMENDED that the hash function be the same as the one that is applied to the message; see Section 8.1 for further discussion.
 Without compromising the security proof for RSASSAPSS, one may perform Steps 1 and 2 of EMSAPSSENCODE and EMSAPSSVERIFY (the application of the hash function to the message) outside the module that computes the rest of the signature operation, so that mHash rather than the message M itself is input to the module. In other words, the security proof for RSASSAPSS still holds even if an opponent can control the value of mHash. This is convenient if the module has limited I/O bandwidth, e.g., a smart card. Note that previous versions of PSS [RSARABIN][PSS] did not have this property. Of course, it may be desirable for other security reasons to have the module process the full message. For instance, the module may need to "see" what it is signing if it does not trust the component that computes the hash value.
 Typical salt lengths in octets are hLen (the length of the output of the hash function Hash) and 0. In both cases, the security of RSASSAPSS can be closely related to the hardness of inverting RSAVP1. Bellare and Rogaway [RSARABIN] give a tight lower bound for the security of the original RSAPSS scheme, which corresponds roughly to the former case, while Coron [FDH] gives a lower bound for the related Full Domain Hashing scheme, which corresponds roughly to the latter case. In [PSSPROOF], Coron provides a general treatment with various salt lengths ranging from 0 to hLen; see [IEEE1363A] for discussion. See also [JONSSON], which adapts the security proofs in [RSARABIN] [PSSPROOF] to address the differences between the original and the present version of RSAPSS as listed in Note 1 above.
 As noted in IEEE 1363a [IEEE1363A], the use of randomization in signature schemes  such as the salt value in EMSAPSS  may provide a "covert channel" for transmitting information other than the message being signed. For more on covert channels, see [SIMMONS].
9.1.1. Encoding Operation

EMSAPSSENCODE (M, emBits)
Options:
Hash hash function (hLen denotes the length in octets of the hash function output) MGF mask generation function sLen intended length in octets of the salt
Input:
M message to be encoded, an octet string emBits maximal bit length of the integer OS2IP (EM) (see Section 4.2), at least 8hLen + 8sLen + 9
Output:
EM encoded message, an octet string of length emLen = \ceil (emBits/8) Errors: "Encoding error"; "message too long"
Steps:
1. If the length of M is greater than the input limitation for the hash function (2^61  1 octets for SHA1), output "message too long" and stop. 2. Let mHash = Hash(M), an octet string of length hLen. 3. If emLen < hLen + sLen + 2, output "encoding error" and stop. 4. Generate a random octet string salt of length sLen; if sLen = 0, then salt is the empty string. 5. Let M' = (0x)00 00 00 00 00 00 00 00  mHash  salt;


M' is an octet string of length 8 + hLen + sLen with eight initial zero octets.

6. Let H = Hash(M'), an octet string of length hLen. 7. Generate an octet string PS consisting of emLen  sLen  hLen  2 zero octets. The length of PS may be 0. 8. Let DB = PS  0x01  salt; DB is an octet string of length emLen  hLen  1. 9. Let dbMask = MGF(H, emLen  hLen  1).

 Let maskedDB = DB \xor dbMask.
 Set the leftmost 8emLen  emBits bits of the leftmost octet in maskedDB to zero.
 Let EM = maskedDB  H  0xbc.
 Output EM.

9.1.2. Verification Operation

EMSAPSSVERIFY (M, EM, emBits)
Options:
Hash hash function (hLen denotes the length in octets of the hash function output) MGF mask generation function sLen intended length in octets of the salt
Input:
M message to be verified, an octet string EM encoded message, an octet string of length emLen = \ceil (emBits/8) emBits maximal bit length of the integer OS2IP (EM) (see Section 4.2), at least 8hLen + 8sLen + 9 Output: "consistent" or "inconsistent"
Steps:
1. If the length of M is greater than the input limitation for the hash function (2^61  1 octets for SHA1), output "inconsistent" and stop. 2. Let mHash = Hash(M), an octet string of length hLen. 3. If emLen < hLen + sLen + 2, output "inconsistent" and stop. 4. If the rightmost octet of EM does not have hexadecimal value 0xbc, output "inconsistent" and stop. 5. Let maskedDB be the leftmost emLen  hLen  1 octets of EM, and let H be the next hLen octets. 6. If the leftmost 8emLen  emBits bits of the leftmost octet in maskedDB are not all equal to zero, output "inconsistent" and stop. 7. Let dbMask = MGF(H, emLen  hLen  1). 8. Let DB = maskedDB \xor dbMask. 9. Set the leftmost 8emLen  emBits bits of the leftmost octet in DB to zero.

 If the emLen  hLen  sLen  2 leftmost octets of DB are not zero or if the octet at position emLen  hLen  sLen  1 (the leftmost position is "position 1") does not have hexadecimal value 0x01, output "inconsistent" and stop.
 Let salt be the last sLen octets of DB.
 Let
M' = (0x)00 00 00 00 00 00 00 00  mHash  salt ;


M' is an octet string of length 8 + hLen + sLen with eight initial zero octets.
 Let H' = Hash(M'), an octet string of length hLen.

14. If H = H', output "consistent". Otherwise, output "inconsistent".

9.2. EMSAPKCS1v1_5

This encoding method is deterministic and only has an encoding operation.
EMSAPKCS1v1_5ENCODE (M, emLen)
Option:
Hash hash function (hLen denotes the length in octets of the hash function output)
Input:
M message to be encoded emLen intended length in octets of the encoded message, at least tLen + 11, where tLen is the octet length of the Distinguished Encoding Rules (DER) encoding T of a certain value computed during the encoding operation
Output:
EM encoded message, an octet string of length emLen Errors: "message too long"; "intended encoded message length too short"
Steps:

 Apply the hash function to the message M to produce a hash value H:


H = Hash(M).
If the hash function outputs "message too long", output "message too long" and stop.

 Encode the algorithm ID for the hash function and the hash value into an ASN.1 value of type DigestInfo (see Appendix A.2.4) with the DER, where the type DigestInfo has the syntax
DigestInfo ::= SEQUENCE { digestAlgorithm AlgorithmIdentifier, digest OCTET STRING }


The first field identifies the hash function and the second contains the hash value. Let T be the DER encoding of the DigestInfo value (see the notes below), and let tLen be the length in octets of T.
 If emLen < tLen + 11, output "intended encoded message length too short" and stop.
 Generate an octet string PS consisting of emLen  tLen  3 octets with hexadecimal value 0xff. The length of PS will be at least 8 octets.
 Concatenate PS, the DER encoding T, and other padding to form the encoded message EM as


EM = 0x00  0x01  PS  0x00  T.

 Output EM.

Notes:
 For the nine hash functions mentioned in Appendix B.1, the DER encoding T of the DigestInfo value is equal to the following:
MD2: (0x)30 20 30 0c 06 08 2a 86 48 86 f7 0d 02 02 05 00 04 10  H. MD5: (0x)30 20 30 0c 06 08 2a 86 48 86 f7 0d 02 05 05 00 04 10  H. SHA1: (0x)30 21 30 09 06 05 2b 0e 03 02 1a 05 00 04 14  H. SHA224: (0x)30 2d 30 0d 06 09 60 86 48 01 65 03 04 02 04 05 00 04 1c  H. SHA256: (0x)30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20  H. SHA384: (0x)30 41 30 0d 06 09 60 86 48 01 65 03 04 02 02 05 00 04 30  H. SHA512: (0x)30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40  H. SHA512/224: (0x)30 2d 30 0d 06 09 60 86 48 01 65 03 04 02 05 05 00 04 1c  H. SHA512/256: (0x)30 31 30 0d 06 09 60 86 48 01 65 03 04 02 06 05 00 04 20  H.
 In version 1.5 of this document, T was defined as the BER encoding, rather than the DER encoding, of the DigestInfo value. In particular, it is possible  at least in theory  that the verification operation defined in this document (as well as in version 2.0) rejects a signature that is valid with respect to the specification given in PKCS #1 v1.5. This occurs if other rules than DER are applied to DigestInfo (e.g., an indefinite length encoding of the underlying SEQUENCE type). While this is unlikely to be a concern in practice, a cautious implementor may choose to employ a verification operation based on a BER decoding operation as specified in PKCS #1 v1.5. In this manner, compatibility with any valid implementation based on PKCS #1 v1.5 is obtained. Such a verification operation should indicate whether the underlying BER encoding is a DER encoding and hence whether the signature is valid with respect to the specification given in this document.

10. Security Considerations

Security considerations are discussed throughout this memo.
11. References
11.1. Normative References

[GARNER] Garner, H., "The Residue Number System", IRE Transactions on Electronic Computers, Volume EC8, Issue 2, pp. 140147, DOI 10.1109/TEC.1959.5219515, June 1959. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfceditor.org/info/rfc2119>. [RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for Obtaining Digital Signatures and PublicKey Cryptosystems", Communications of the ACM, Volume 21, Issue 2, pp. 120126, DOI 10.1145/359340.359342, February 1978.
11.2. Informative References

[ANSIX944] ANSI, "Key Establishment Using Integer Factorization
Cryptography", ANSI X9.442007, August 2007.

[BKS] Bleichenbacher, D., Kaliski, B., and J. Staddon, "Recent Results on PKCS #1: RSA Encryption Standard", RSA Laboratories, Bulletin No. 7, June 1998.
[BLEICHENBACHER]
Bleichenbacher, D., "Chosen Ciphertext Attacks Against Protocols Based on the RSA Encryption Standard PKCS #1", Lecture Notes in Computer Science, Volume 1462, pp. 112, 1998. [CHOSEN] Desmedt, Y. and A. Odlyzko, "A Chosen Text Attack on the RSA Cryptosystem and Some Discrete Logarithm Schemes", Lecture Notes in Computer Science, Volume 218, pp. 516522, 1985. [COCHRAN] Cochran, M., "Notes on the Wang et al. 2^63 SHA1 Differential Path", Cryptology ePrint Archive: Report 2007/474, August 2008, <http://eprint.iacr.org/2007/474>. [FASTDEC] Quisquater, J. and C. Couvreur, "Fast Decipherment Algorithm for RSA PublicKey Cryptosystem", Electronic Letters, Volume 18, Issue 21, pp. 905907, DOI 10.1049/el:19820617, October 1982. [FDH] Coron, J., "On the Exact Security of Full Domain Hash", Lecture Notes in Computer Science, Volume 1880, pp. 229235, 2000. [FOPS] Fujisaki, E., Okamoto, T., Pointcheval, D., and J. Stern, "RSAOAEP is Secure under the RSA Assumption", Lecture Notes in Computer Science, Volume 2139, pp. 260274, August 2001. [FORGERY] Coppersmith, D., Halevi, S., and C. Jutla, "ISO 97961 and the new forgery strategy", rump session of Crypto, August 1999. [HAASTAD] Haastad, J., "Solving Simultaneous Modular Equations of Low Degree", SIAM Journal on Computing, Volume 17, Issue 2, pp. 336341, DOI 10.1137/0217019, April 1988. [HANDBOOK] Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook of Applied Cryptography", CRC Press, ISBN: 0849385237, 1996. [HASHID] Kaliski, B., "On Hash Function Firewalls in Signature Schemes", Lecture Notes in Computer Science, Volume 2271, pp. 116, DOI 10.1007/3540457607_1, February 2002.
[IEEE1363] IEEE, "Standard Specifications for Public Key
Cryptography", IEEE Std 13632000,
DOI 10.1109/IEEESTD.2000.92292, August 2000,
<http://ieeexplore.ieee.org/document/891000/>.

[IEEE1363A]
IEEE, "Standard Specifications for Public Key Cryptography  Amendment 1: Additional Techniques", IEEE Std 1363a 2004, DOI 10.1109/IEEESTD.2004.94612, September 2004, <http://ieeexplore.ieee.org/document/1335427/>.
[ISO18033] International Organization for Standardization,
"Information technology  Security techniques  Encryption algorithms  Part 2: Asymmetric ciphers", ISO/ IEC 180332:2006, May 2006. [ISO9594] International Organization for Standardization, "Information technology  Open Systems Interconnection  The Directory: Publickey and attribute certificate frameworks", ISO/IEC 95948:2008, December 2008. [ISO9796] International Organization for Standardization, "Information technology  Security techniques  Digital signature schemes giving message recovery  Part 2: Integer factorization based mechanisms", ISO/IEC 97962:2010, December 2010. [JONSSON] Jonsson, J., "Security Proofs for the RSAPSS Signature Scheme and Its Variants", Cryptology ePrint Archive: Report 2001/053, March 2002, <http://eprint.iacr.org/2001/053>. [LOWEXP] Coppersmith, D., Franklin, M., Patarin, J., and M. Reiter, "LowExponent RSA with Related Messages", Lecture Notes in Computer Science, Volume 1070, pp. 19, 1996. [MANGER] Manger, J., "A Chosen Ciphertext Attack on RSA Optimal Asymmetric Encryption Padding (OAEP) as Standardized in PKCS #1 v2.0", Lecture Notes in Computer Science, Volume 2139, pp. 230238, DOI 10.1007/3540446478_14, 2001. [MD4] Dobbertin, H., "Cryptanalysis of MD4", Lecture Notes in Computer Science, Volume 1039, pp. 5369, DOI 10.1007/3540608656_43, 1996. [MD4FIRST] Dobbertin, H., "The First Two Rounds of MD4 are Not One Way", Lecture Notes in Computer Science, Volume 1372, pp. 284292, DOI 10.1007/3540697101_19, March 1998. [MD4LAST] den Boer, B. and A. Bosselaers, "An Attack on the Last Two Rounds of MD4", Lecture Notes in Computer Science, Volume 576, pp. 194203, DOI 10.1007/3540467661_14, 1992.
[NEWATTACK]
Coron, J., Joye, M., Naccache, D., and P. Paillier, "New Attacks on PKCS #1 v1.5 Encryption", Lecture Notes in Computer Science, Volume 1807, pp. 369381, DOI 10.1007/3540455396_25, May 2000. [OAEP] Bellare, M. and P. Rogaway, "Optimal Asymmetric Encryption  How to Encrypt with RSA", Lecture Notes in Computer Science, Volume 950, pp. 92111, November 1995. [PA98] Bellare, M., Desai, A., Pointcheval, D., and P. Rogaway, "Relations Among Notions of Security for PublicKey Encryption Schemes", Lecture Notes in Computer Science, Volume 1462, pp. 2645, DOI 10.1007/BFb0055718, 1998. [PADDING] Coron, J., Naccache, D., and J. Stern, "On the Security of RSA Padding", Lecture Notes in Computer Science, Volume 1666, pp. 118, DOI 10.1007/3540484051_1, December 1999.
[PKCS1_22] RSA Laboratories, "PKCS #1: RSA Cryptography Standard



Version 2.2", October 2012.


[PREFIX] Stevens, M., Lenstra, A., and B. de Weger, "Chosenprefix collisions for MD5 and applications", International Journal of Applied Cryptography, Volume 2, No. 4, pp. 322359, July 2012. [PSS] Bellare, M. and P. Rogaway, "PSS: Provably Secure Encoding Method for Digital Signatures", Submission to IEEE P1363a, August 1998, <http://grouper.ieee.org/groups/1363/ P1363a/contributions/psssubmission.pdf>. [PSSPROOF] Coron, J., "Optimal Security Proofs for PSS and Other Signature Schemes", Lecture Notes in Computer Science, Volume 2332, pp. 272287, DOI 10.1007/3540460357_18, 2002. [RFC1319] Kaliski, B., "The MD2 MessageDigest Algorithm", RFC 1319, DOI 10.17487/RFC1319, April 1992, <http://www.rfceditor.org/info/rfc1319>. [RFC1321] Rivest, R., "The MD5 MessageDigest Algorithm", RFC 1321, DOI 10.17487/RFC1321, April 1992, <http://www.rfceditor.org/info/rfc1321>. [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC 2313, DOI 10.17487/RFC2313, March 1998, <http://www.rfceditor.org/info/rfc2313>. [RFC2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998, <http://www.rfceditor.org/info/rfc2315>. [RFC2437] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography Specifications Version 2.0", RFC 2437, DOI 10.17487/RFC2437, October 1998, <http://www.rfceditor.org/info/rfc2437>. [RFC3447] Jonsson, J. and B. Kaliski, "PublicKey Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003, <http://www.rfceditor.org/info/rfc3447>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfceditor.org/info/rfc5246>. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, <http://www.rfceditor.org/info/rfc5652>. [RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, DOI 10.17487/RFC5958, August 2010, <http://www.rfceditor.org/info/rfc5958>. [RFC6149] Turner, S. and L. Chen, "MD2 to Historic Status", RFC 6149, DOI 10.17487/RFC6149, March 2011, <http://www.rfceditor.org/info/rfc6149>. [RFC7292] Moriarty, K., Ed., Nystrom, M., Parkinson, S., Rusch, A., and M. Scott, "PKCS #12: Personal Information Exchange Syntax v1.1", RFC 7292, DOI 10.17487/RFC7292, July 2014, <http://www.rfceditor.org/info/rfc7292>. [RSARABIN] Bellare, M. and P. Rogaway, "The Exact Security of Digital Signatures  How to Sign with RSA and Rabin", Lecture Notes in Computer Science, Volume 1070, pp. 399416, DOI 10.1007/3540683399_34, 1996. [RSATLS] Jonsson, J. and B. Kaliski, "On the Security of RSA Encryption in TLS", Lecture Notes in Computer Science, Volume 2442, pp. 127142, DOI 10.1007/3540457089_9, 2002.
[SHA1CRYPT]
Wang, X., Yao, A., and F. Yao, "Cryptanalysis on SHA1", Lecture Notes in Computer Science, Volume 2442, pp. 127142, February 2005, <http://csrc.nist.gov/groups/ST/hash/documents/ Wang_SHA1NewResult.pdf>. [SHOUP] Shoup, V., "OAEP Reconsidered (Extended Abstract)", Lecture Notes in Computer Science, Volume 2139, pp. 239259, DOI 10.1007/3540446478_15, 2001. [SHS] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 1804, August 2015, <http://dx.doi.org/10.6028/NIST.FIPS.1804>.
[SILVERMAN]
Silverman, R., "A CostBased Security Analysis of Symmetric and Asymmetric Key Lengths", RSA Laboratories, Bulletin No. 13, 2000. [SIMMONS] Simmons, G., "Subliminal Communication is Easy Using the DSA", Lecture Notes in Computer Science, Volume 765, pp. 218232, DOI 10.1007/3540482857_18, 1994.
Appendix A. ASN.1 Syntax
A.1. RSA Key Representation

This section defines ASN.1 object identifiers for RSA public and private keys and defines the types RSAPublicKey and RSAPrivateKey. The intended application of these definitions includes X.509 certificates, PKCS #8 [RFC5958], and PKCS #12 [RFC7292].
The object identifier rsaEncryption identifies RSA public and private keys as defined in Appendices A.1.1 and A.1.2. The parameters field has associated with this OID in a value of type AlgorithmIdentifier SHALL have a value of type NULL.
rsaEncryption OBJECT IDENTIFIER ::= { pkcs1 1 }
The definitions in this section have been extended to support multi prime RSA, but they are backward compatible with previous versions.
A.1.1. RSA Public Key Syntax

An RSA public key should be represented with the ASN.1 type RSAPublicKey:
RSAPublicKey ::= SEQUENCE { modulus INTEGER,  n publicExponent INTEGER  e }
The fields of type RSAPublicKey have the following meanings:
 modulus is the RSA modulus n.
 publicExponent is the RSA public exponent e.
A.1.2. RSA Private Key Syntax

An RSA private key should be represented with the ASN.1 type RSAPrivateKey:
RSAPrivateKey ::= SEQUENCE { version Version, modulus INTEGER,  n publicExponent INTEGER,  e privateExponent INTEGER,  d prime1 INTEGER,  p prime2 INTEGER,  q exponent1 INTEGER,  d mod (p1) exponent2 INTEGER,  d mod (q1) coefficient INTEGER,  (inverse of q) mod p otherPrimeInfos OtherPrimeInfos OPTIONAL }
The fields of type RSAPrivateKey have the following meanings:
 version is the version number, for compatibility with future revisions of this document. It SHALL be 0 for this version of the document, unless multiprime is used; in which case, it SHALL be 1.
Version ::= INTEGER { twoprime(0), multi(1) } (CONSTRAINED BY { version must be multi if otherPrimeInfos present })
 modulus is the RSA modulus n.
 publicExponent is the RSA public exponent e.
 privateExponent is the RSA private exponent d.
 prime1 is the prime factor p of n.
 prime2 is the prime factor q of n.
 exponent1 is d mod (p  1).
 exponent2 is d mod (q  1).
 coefficient is the CRT coefficient q^(1) mod p.
 otherPrimeInfos contains the information for the additional primes r_3, ..., r_u, in order. It SHALL be omitted if version is 0 and SHALL contain at least one instance of OtherPrimeInfo if version is 1.
OtherPrimeInfos ::= SEQUENCE SIZE(1..MAX) OF OtherPrimeInfo OtherPrimeInfo ::= SEQUENCE { prime INTEGER,  ri exponent INTEGER,  di coefficient INTEGER  ti }
The fields of type OtherPrimeInfo have the following meanings:
 prime is a prime factor r_i of n, where i >= 3.
 exponent is d_i = d mod (r_i  1).
o coefficient is the CRT coefficient t_i = (r_1 * r_2 * ... * r_(i1))^(1) mod r_i.
Note: It is important to protect the RSA private key against both disclosure and modification. Techniques for such protection are outside the scope of this document. Methods for storing and distributing private keys and other cryptographic data are described in PKCS #12 and #15.
A.2. Scheme Identification

This section defines object identifiers for the encryption and signature schemes. The schemes compatible with PKCS #1 v1.5 have the same definitions as in PKCS #1 v1.5. The intended application of these definitions includes X.509 certificates and PKCS #7.
Here are type identifier definitions for the PKCS #1 OIDs:
PKCS1Algorithms ALGORITHMIDENTIFIER ::= { { OID rsaEncryption PARAMETERS NULL }  { OID md2WithRSAEncryption PARAMETERS NULL }  { OID md5WithRSAEncryption PARAMETERS NULL }  { OID sha1WithRSAEncryption PARAMETERS NULL }  { OID sha224WithRSAEncryption PARAMETERS NULL }  { OID sha256WithRSAEncryption PARAMETERS NULL }  { OID sha384WithRSAEncryption PARAMETERS NULL }  { OID sha512WithRSAEncryption PARAMETERS NULL }  { OID sha512224WithRSAEncryption PARAMETERS NULL }  { OID sha512256WithRSAEncryption PARAMETERS NULL }  { OID idRSAESOAEP PARAMETERS RSAESOAEPparams }  PKCS1PSourceAlgorithms  { OID idRSASSAPSS PARAMETERS RSASSAPSSparams }, ...  Allows for future expansion  }
A.2.1. RSAESOAEP

The object identifier idRSAESOAEP identifies the RSAESOAEP encryption scheme.
idRSAESOAEP OBJECT IDENTIFIER ::= { pkcs1 7 }
The parameters field associated with this OID in a value of type AlgorithmIdentifier SHALL have a value of type RSAESOAEPparams:
RSAESOAEPparams ::= SEQUENCE { hashAlgorithm [0] HashAlgorithm DEFAULT sha1, maskGenAlgorithm [1] MaskGenAlgorithm DEFAULT mgf1SHA1, pSourceAlgorithm [2] PSourceAlgorithm DEFAULT pSpecifiedEmpty }
The fields of type RSAESOAEPparams have the following meanings:
 hashAlgorithm identifies the hash function. It SHALL be an algorithm ID with an OID in the set OAEPPSSDigestAlgorithms. For a discussion of supported hash functions, see Appendix B.1.
HashAlgorithm ::= AlgorithmIdentifier { {OAEPPSSDigestAlgorithms} } OAEPPSSDigestAlgorithms ALGORITHMIDENTIFIER ::= { { OID idsha1 PARAMETERS NULL } { OID idsha224 PARAMETERS NULL } { OID idsha256 PARAMETERS NULL } { OID idsha384 PARAMETERS NULL } { OID idsha512 PARAMETERS NULL } { OID idsha512224 PARAMETERS NULL } { OID idsha512256 PARAMETERS NULL }, ...  Allows for future expansion  }
The default hash function is SHA1:
sha1 HashAlgorithm ::= { algorithm idsha1, parameters SHA1Parameters : NULL } SHA1Parameters ::= NULL
 maskGenAlgorithm identifies the mask generation function. It SHALL be an algorithm ID with an OID in the set PKCS1MGFAlgorithms, which for this version SHALL consist of idmgf1, identifying the MGF1 mask generation function (see Appendix B.2.1). The parameters field associated with idmgf1 SHALL be an algorithm ID with an OID in the set OAEPPSSDigestAlgorithms, identifying the hash function on which MGF1 is based.
MaskGenAlgorithm ::= AlgorithmIdentifier { {PKCS1MGFAlgorithms} } PKCS1MGFAlgorithms ALGORITHMIDENTIFIER ::= { { OID idmgf1 PARAMETERS HashAlgorithm }, ...  Allows for future expansion  }
 The default mask generation function is MGF1 with SHA1:
mgf1SHA1 MaskGenAlgorithm ::= { algorithm idmgf1, parameters HashAlgorithm : sha1 }
 pSourceAlgorithm identifies the source (and possibly the value) of the label L. It SHALL be an algorithm ID with an OID in the set PKCS1PSourceAlgorithms, which for this version SHALL consist of idpSpecified, indicating that the label is specified explicitly. The parameters field associated with idpSpecified SHALL have a value of type OCTET STRING, containing the label. In previous versions of this specification, the term "encoding parameters" was used rather than "label", hence the name of the type below.
PSourceAlgorithm ::= AlgorithmIdentifier { {PKCS1PSourceAlgorithms} } PKCS1PSourceAlgorithms ALGORITHMIDENTIFIER ::= { { OID idpSpecified PARAMETERS EncodingParameters }, ...  Allows for future expansion  } idpSpecified OBJECT IDENTIFIER ::= { pkcs1 9 } EncodingParameters ::= OCTET STRING(SIZE(0..MAX))
 The default label is an empty string (so that lHash will contain the hash of the empty string):
pSpecifiedEmpty PSourceAlgorithm ::= { algorithm idpSpecified, parameters EncodingParameters : emptyString } emptyString EncodingParameters ::= ''H
If all of the default values of the fields in RSAESOAEPparams are used, then the algorithm identifier will have the following value:
rSAESOAEPDefaultIdentifier RSAESAlgorithmIdentifier ::= { algorithm idRSAESOAEP, parameters RSAESOAEPparams : { hashAlgorithm sha1, maskGenAlgorithm mgf1SHA1, pSourceAlgorithm pSpecifiedEmpty } } RSAESAlgorithmIdentifier ::= AlgorithmIdentifier { {PKCS1Algorithms} }
A.2.2. RSAESPKCSv1_5

The object identifier rsaEncryption (see Appendix A.1) identifies the RSAESPKCS1v1_5 encryption scheme. The parameters field associated with this OID in a value of type AlgorithmIdentifier SHALL have a value of type NULL. This is the same as in PKCS #1 v1.5.
rsaEncryption OBJECT IDENTIFIER ::= { pkcs1 1 }
A.2.3. RSASSAPSS

The object identifier idRSASSAPSS identifies the RSASSAPSS encryption scheme.
idRSASSAPSS OBJECT IDENTIFIER ::= { pkcs1 10 }
The parameters field associated with this OID in a value of type AlgorithmIdentifier SHALL have a value of type RSASSAPSSparams:
RSASSAPSSparams ::= SEQUENCE { hashAlgorithm [0] HashAlgorithm DEFAULT sha1, maskGenAlgorithm [1] MaskGenAlgorithm DEFAULT mgf1SHA1, saltLength [2] INTEGER DEFAULT 20, trailerField [3] TrailerField DEFAULT trailerFieldBC }
The fields of type RSASSAPSSparams have the following meanings:
 hashAlgorithm identifies the hash function. It SHALL be an algorithm ID with an OID in the set OAEPPSSDigestAlgorithms (see Appendix A.2.1). The default hash function is SHA1.
 maskGenAlgorithm identifies the mask generation function. It SHALL be an algorithm ID with an OID in the set PKCS1MGFAlgorithms (see Appendix A.2.1). The default mask generation function is MGF1 with SHA1. For MGF1 (and more generally, for other mask generation functions based on a hash function), it is RECOMMENDED that the underlying hash function be the same as the one identified by hashAlgorithm; see Note 2 in Section 9.1 for further comments.
 saltLength is the octet length of the salt. It SHALL be an integer. For a given hashAlgorithm, the default value of saltLength is the octet length of the hash value. Unlike the other fields of type RSASSAPSSparams, saltLength does not need to be fixed for a given RSA key pair.
 trailerField is the trailer field number, for compatibility with IEEE 1363a [IEEE1363A]. It SHALL be 1 for this version of the document, which represents the trailer field with hexadecimal value 0xbc. Other trailer fields (including the trailer field HashID  0xcc in IEEE 1363a) are not supported in this document.
TrailerField ::= INTEGER { trailerFieldBC(1) }
If the default values of the hashAlgorithm, maskGenAlgorithm, and trailerField fields of RSASSAPSSparams are used, then the algorithm identifier will have the following value:
rSASSAPSSDefaultIdentifier RSASSAAlgorithmIdentifier ::= { algorithm idRSASSAPSS, parameters RSASSAPSSparams : { hashAlgorithm sha1, maskGenAlgorithm mgf1SHA1, saltLength 20, trailerField trailerFieldBC } } RSASSAAlgorithmIdentifier ::= AlgorithmIdentifier { {PKCS1Algorithms} }
Note: In some applications, the hash function underlying a signature scheme is identified separately from the rest of the operations in the signature scheme. For instance, in PKCS #7 [RFC2315], a hash function identifier is placed before the message and a "digest encryption" algorithm identifier (indicating the rest of the operations) is carried with the signature. In order for PKCS #7 to support the RSASSAPSS signature scheme, an object identifier would need to be defined for the operations in RSASSAPSS after the hash function (analogous to the RSAEncryption OID for the RSASSAPKCS1v1_5 scheme). S/MIME Cryptographic Message Syntax (CMS) [RFC5652] takes a different approach. Although a hash function identifier is placed before the message, an algorithm identifier for the full signature scheme may be carried with a CMS signature (this is done for DSA signatures). Following this convention, the idRSASSAPSS OID can be used to identify RSASSAPSS signatures in CMS. Since CMS is considered the successor to PKCS #7 and new developments such as the addition of support for RSASSAPSS will be pursued with respect to CMS rather than PKCS #7, an OID for the "rest of" RSASSAPSS is not defined in this version of PKCS #1.
A.2.4. RSASSAPKCSv1_5

The object identifier for RSASSAPKCS1v1_5 SHALL be one of the following. The choice of OID depends on the choice of hash algorithm: MD2, MD5, SHA1, SHA224, SHA256, SHA384, SHA512, SHA512/224, or SHA512/256. Note that if either MD2 or MD5 is used, then the OID is just as in PKCS #1 v1.5. For each OID, the parameters field associated with this OID in a value of type AlgorithmIdentifier SHALL have a value of type NULL. The OID should be chosen in accordance with the following table:
Hash algorithm OID  MD2 md2WithRSAEncryption ::= {pkcs1 2} MD5 md5WithRSAEncryption ::= {pkcs1 4} SHA1 sha1WithRSAEncryption ::= {pkcs1 5} SHA256 sha224WithRSAEncryption ::= {pkcs1 14} SHA256 sha256WithRSAEncryption ::= {pkcs1 11} SHA384 sha384WithRSAEncryption ::= {pkcs1 12} SHA512 sha512WithRSAEncryption ::= {pkcs1 13} SHA512/224 sha512224WithRSAEncryption ::= {pkcs1 15} SHA512/256 sha512256WithRSAEncryption ::= {pkcs1 16}
The EMSAPKCS1v1_5 encoding method includes an ASN.1 value of type DigestInfo, where the type DigestInfo has the syntax
DigestInfo ::= SEQUENCE { digestAlgorithm DigestAlgorithm, digest OCTET STRING }
digestAlgorithm identifies the hash function and SHALL be an algorithm ID with an OID in the set PKCS1v15DigestAlgorithms. For a discussion of supported hash functions, see Appendix B.1.
DigestAlgorithm ::= AlgorithmIdentifier { {PKCS1v15DigestAlgorithms} } PKCS1v15DigestAlgorithms ALGORITHMIDENTIFIER ::= { { OID idmd2 PARAMETERS NULL } { OID idmd5 PARAMETERS NULL } { OID idsha1 PARAMETERS NULL } { OID idsha224 PARAMETERS NULL } { OID idsha256 PARAMETERS NULL } { OID idsha384 PARAMETERS NULL } { OID idsha512 PARAMETERS NULL } { OID idsha512224 PARAMETERS NULL } { OID idsha512256 PARAMETERS NULL } }
Appendix B. Supporting Techniques

This section gives several examples of underlying functions supporting the encryption schemes in Section 7 and the encoding methods in Section 9. A range of techniques is given here to allow compatibility with existing applications as well as migration to new techniques. While these supporting techniques are appropriate for applications to implement, none of them is required to be implemented. It is expected that profiles for PKCS #1 v2.2 will be developed that specify particular supporting techniques.
This section also gives object identifiers for the supporting techniques.
B.1. Hash Functions

Hash functions are used in the operations contained in Sections 7 and 9. Hash functions are deterministic, meaning that the output is completely determined by the input. Hash functions take octet strings of variable length and generate fixedlength octet strings. The hash functions used in the operations contained in Sections 7 and 9 should generally be collisionresistant. This means that it is infeasible to find two distinct inputs to the hash function that produce the same output. A collisionresistant hash function also has the desirable property of being oneway; this means that given an output, it is infeasible to find an input whose hash is the specified output. In addition to the requirements, the hash function should yield a mask generation function (Appendix B.2) with pseudorandom output.
Nine hash functions are given as examples for the encoding methods in this document: MD2 [RFC1319] (which was retired by [RFC6149]), MD5 [RFC1321], SHA1, SHA224, SHA256, SHA384, SHA512, SHA512/224, and SHA512/256 [SHS]. For the RSAESOAEP encryption scheme and EMSAPSS encoding method, only SHA1, SHA224, SHA256, SHA384, SHA 512, SHA512/224, and SHA512/256 are RECOMMENDED. For the EMSA PKCS1v1_5 encoding method, SHA224, SHA256, SHA384, SHA512, SHA 512/224, and SHA512/256 are RECOMMENDED for new applications. MD2, MD5, and SHA1 are recommended only for compatibility with existing applications based on PKCS #1 v1.5. The object identifiers idmd2, idmd5, idsha1, idsha224, idsha256, idsha384, idsha512, idsha512/224, and idsha512/256 identify the respective hash functions: idmd2 OBJECT IDENTIFIER ::= { iso (1) memberbody (2) us (840) rsadsi (113549) digestAlgorithm (2) 2 } idmd5 OBJECT IDENTIFIER ::= { iso (1) memberbody (2) us (840) rsadsi (113549) digestAlgorithm (2) 5 } idsha1 OBJECT IDENTIFIER ::= { iso(1) identifiedorganization(3) oiw(14) secsig(3) algorithms(2) 26 } idsha224 OBJECT IDENTIFIER ::= { jointisoitut (2) country (16) us (840) organization (1) gov (101) csor (3) nistalgorithm (4) hashalgs (2) 4 } idsha256 OBJECT IDENTIFIER ::= { jointisoitut (2) country (16) us (840) organization (1) gov (101) csor (3) nistalgorithm (4) hashalgs (2) 1 } idsha384 OBJECT IDENTIFIER ::= { jointisoitut (2) country (16) us (840) organization (1) gov (101) csor (3) nistalgorithm (4) hashalgs (2) 2 } idsha512 OBJECT IDENTIFIER ::= { jointisoitut (2) country (16) us (840) organization (1) gov (101) csor (3) nistalgorithm (4) hashalgs (2) 3 } idsha512224 OBJECT IDENTIFIER ::= { jointisoitut (2) country (16) us (840) organization (1) gov (101) csor (3) nistalgorithm (4) hashalgs (2) 5 } idsha512256 OBJECT IDENTIFIER ::= { jointisoitut (2) country (16) us (840) organization (1) gov (101) csor (3) nistalgorithm (4) hashalgs (2) 6 }
The parameters field associated with these OIDs in a value of type AlgorithmIdentifier SHALL have a value of type NULL.
The parameters field associated with idmd2 and idmd5 in a value of type AlgorithmIdentifier shall have a value of type NULL.
The parameters field associated with idsha1, idsha224, idsha256, idsha384, idsha512, idsha512/224, and idsha512/256 should generally be omitted, but if present, it shall have a value of type NULL.
This is to align with the definitions originally promulgated by NIST. For the SHA algorithms, implementations MUST accept AlgorithmIdentifier values both without parameters and with NULL parameters.
Exception: When formatting the DigestInfoValue in EMSAPKCS1v1_5 (see Section 9.2), the parameters field associated with idsha1, idsha224, idsha256, idsha384, idsha512, idsha512/224, and idsha512/256 shall have a value of type NULL. This is to maintain compatibility with existing implementations and with the numeric information values already published for EMSAPKCS1v1_5, which are also reflected in IEEE 1363a [IEEE1363A].
Note: Version 1.5 of PKCS #1 also allowed for the use of MD4 in signature schemes. The cryptanalysis of MD4 has progressed significantly in the intervening years. For example, Dobbertin [MD4] demonstrated how to find collisions for MD4 and that the first two rounds of MD4 are not oneway [MD4FIRST]. Because of these results and others (e.g., [MD4LAST]), MD4 is NOT RECOMMENDED.
Further advances have been made in the cryptanalysis of MD2 and MD5, especially after the findings of Stevens et al. [PREFIX] on chosen prefix collisions on MD5. MD2 and MD5 should be considered cryptographically broken and removed from existing applications. This version of the standard supports MD2 and MD5 just for backwards compatibility reasons.
There have also been advances in the cryptanalysis of SHA1. Particularly, the results of Wang et al. [SHA1CRYPT] (which have been independently verified by M. Cochran in his analysis [COCHRAN]) on using a differential path to find collisions in SHA1, which conclude that the security strength of the SHA1 hashing algorithm is significantly reduced. However, this reduction is not significant enough to warrant the removal of SHA1 from existing applications, but its usage is only recommended for backwardscompatibility reasons.
To address these concerns, only SHA224, SHA256, SHA384, SHA512, SHA512/224, and SHA512/256 are RECOMMENDED for new applications. As of today, the best (known) collision attacks against these hash functions are generic attacks with complexity 2L/2, where L is the bit length of the hash output. For the signature schemes in this document, a collision attack is easily translated into a signature forgery. Therefore, the value L / 2 should be at least equal to the desired security level in bits of the signature scheme (a security level of B bits means that the best attack has complexity 2B). The same rule of thumb can be applied to RSAESOAEP; it is RECOMMENDED that the bit length of the seed (which is equal to the bit length of the hash output) be twice the desired security level in bits.
B.2. Mask Generation Functions

A mask generation function takes an octet string of variable length and a desired output length as input and outputs an octet string of the desired length. There may be restrictions on the length of the input and output octet strings, but such bounds are generally very large. Mask generation functions are deterministic; the octet string output is completely determined by the input octet string. The output of a mask generation function should be pseudorandom: Given one part of the output but not the input, it should be infeasible to predict another part of the output. The provable security of RSAESOAEP and RSASSAPSS relies on the random nature of the output of the mask generation function, which in turn relies on the random nature of the underlying hash.
One mask generation function is given here: MGF1, which is based on a hash function. MGF1 coincides with the mask generation functions defined in IEEE 1363 [IEEE1363] and ANSI X9.44 [ANSIX944]. Future versions of this document may define other mask generation functions.
B.2.1. MGF1

MGF1 is a mask generation function based on a hash function.
MGF1 (mgfSeed, maskLen)
Options:
Hash hash function (hLen denotes the length in octets of the hash function output)
Input:
mgfSeed seed from which mask is generated, an octet string maskLen intended length in octets of the mask, at most 2^32 hLen
Output:
mask mask, an octet string of length maskLen
Error:
"mask too long"
Steps:
 If maskLen > 2^32 hLen, output "mask too long" and stop.
 Let T be the empty octet string.
 For counter from 0 to \ceil (maskLen / hLen)  1, do the following:

A. Convert counter to an octet string C of length 4 octets (see

Section 4.1):

C = I2OSP (counter, 4) .

B. Concatenate the hash of the seed mgfSeed and C to the octet

string T:

T = T  Hash(mgfSeed  C) .

 Output the leading maskLen octets of T as the octet string mask.
The object identifier idmgf1 identifies the MGF1 mask generation function:
idmgf1 OBJECT IDENTIFIER ::= { pkcs1 8 }
The parameters field associated with this OID in a value of type AlgorithmIdentifier shall have a value of type hashAlgorithm, identifying the hash function on which MGF1 is based.
Appendix C. ASN.1 Module

 PKCS #1 v2.2 ASN.1 Module  Revised October 27, 2012
 This module has been checked for conformance with the  ASN.1 standard by the OSS ASN.1 Tools
PKCS1 { iso(1) memberbody(2) us(840) rsadsi(113549) pkcs(1) pkcs1(1) modules(0) pkcs1(1) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
 EXPORTS ALL
 All types and values defined in this module are exported for use  in other ASN.1 modules.IMPORTS idsha224, idsha256, idsha384, idsha512, idsha512224, idsha512256 FROM NISTSHA2 { jointisoitut(2) country(16) us(840) organization(1) gov(101) csor(3) nistalgorithm(4) hashAlgs(2) };  ============================  Basic object identifiers  ============================  The DER encoding of this in hexadecimal is:  (0x)06 08  2A 86 48 86 F7 0D 01 01  pkcs1 OBJECT IDENTIFIER ::= { iso(1) memberbody(2) us(840) rsadsi(113549) pkcs(1) 1 }

 When rsaEncryption is used in an AlgorithmIdentifier, the parameters MUST be present and MUST be NULL.  rsaEncryption OBJECT IDENTIFIER ::= { pkcs1 1 }   When idRSAESOAEP is used in an AlgorithmIdentifier, the  parameters MUST be present and MUST be RSAESOAEPparams.  idRSAESOAEP OBJECT IDENTIFIER ::= { pkcs1 7 }   When idpSpecified is used in an AlgorithmIdentifier, the  parameters MUST be an OCTET STRING.  idpSpecified OBJECT IDENTIFIER ::= { pkcs1 9 }   When idRSASSAPSS is used in an AlgorithmIdentifier, the  parameters MUST be present and MUST be RSASSAPSSparams.  idRSASSAPSS OBJECT IDENTIFIER ::= { pkcs1 10 }   When the following OIDs are used in an AlgorithmIdentifier,  the parameters MUST be present and MUST be NULL.  md2WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 2 } md5WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 4 } sha1WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 5 } sha224WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 14 } sha256WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 11 } sha384WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 12 } sha512WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 13 } sha512224WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 15 } sha512256WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs1 16 }   This OID really belongs in a module with the secsig OIDs.  idsha1 OBJECT IDENTIFIER ::= { iso(1) identifiedorganization(3) oiw(14) secsig(3) algorithms(2) 26 }   OIDs for MD2 and MD5, allowed only in EMSAPKCS1v1_5.  idmd2 OBJECT IDENTIFIER ::= { iso(1) memberbody(2) us(840) rsadsi(113549) digestAlgorithm(2) 2 } idmd5 OBJECT IDENTIFIER ::= { iso(1) memberbody(2) us(840) rsadsi(113549) digestAlgorithm(2) 5 }   When idmgf1 is used in an AlgorithmIdentifier, the parameters  MUST be present and MUST be a HashAlgorithm, for example, sha1.  idmgf1 OBJECT IDENTIFIER ::= { pkcs1 8 }  ================  Useful types  ================ ALGORITHMIDENTIFIER ::= CLASS { &id OBJECT IDENTIFIER UNIQUE, &Type OPTIONAL } WITH SYNTAX { OID &id [PARAMETERS &Type] }  Note: the parameter InfoObjectSet in the following definitions  allows a distinct information object set to be specified for sets  of algorithms such as:  DigestAlgorithms ALGORITHMIDENTIFIER ::= {  { OID idmd2 PARAMETERS NULL }  { OID idmd5 PARAMETERS NULL }  { OID idsha1 PARAMETERS NULL }  }  AlgorithmIdentifier { ALGORITHMIDENTIFIER:InfoObjectSet } ::= SEQUENCE { algorithm ALGORITHMIDENTIFIER.&id({InfoObjectSet}), parameters ALGORITHMIDENTIFIER.&Type({InfoObjectSet}{@.algorithm}) OPTIONAL }  ==============  Algorithms  ==============   Allowed EMEOAEP and EMSAPSS digest algorithms.  OAEPPSSDigestAlgorithms ALGORITHMIDENTIFIER ::= { { OID idsha1 PARAMETERS NULL } { OID idsha224 PARAMETERS NULL } { OID idsha256 PARAMETERS NULL } { OID idsha384 PARAMETERS NULL } { OID idsha512 PARAMETERS NULL } { OID idsha512224 PARAMETERS NULL } { OID idsha512256 PARAMETERS NULL }, ...  Allows for future expansion  }   Allowed EMSAPKCS1v1_5 digest algorithms.  PKCS1v15DigestAlgorithms ALGORITHMIDENTIFIER ::= { { OID idmd2 PARAMETERS NULL } { OID idmd5 PARAMETERS NULL } { OID idsha1 PARAMETERS NULL } { OID idsha224 PARAMETERS NULL } { OID idsha256 PARAMETERS NULL } { OID idsha384 PARAMETERS NULL } { OID idsha512 PARAMETERS NULL } { OID idsha512224 PARAMETERS NULL } { OID idsha512256 PARAMETERS NULL } }
 When idmd2 and idmd5 are used in an AlgorithmIdentifier, the  parameters field shall have a value of type NULL.
 When idsha1, idsha224, idsha256, idsha384, idsha512,  idsha512224, and idsha512256 are used in an  AlgorithmIdentifier, the parameters (which are optional) SHOULD be  omitted, but if present, they SHALL have a value of type NULL.  However, implementations MUST accept AlgorithmIdentifier values  both without parameters and with NULL parameters.  Exception: When formatting the DigestInfoValue in EMSAPKCS1v1_5  (see Section 9.2), the parameters field associated with idsha1,  idsha224, idsha256, idsha384, idsha512, idsha512224, and  idsha512256 SHALL have a value of type NULL. This is to  maintain compatibility with existing implementations and with the  numeric information values already published for EMSAPKCS1v1_5,  which are also reflected in IEEE 1363a. sha1 HashAlgorithm ::= { algorithm idsha1, parameters SHA1Parameters : NULL } HashAlgorithm ::= AlgorithmIdentifier { {OAEPPSSDigestAlgorithms} } SHA1Parameters ::= NULL   Allowed mask generation function algorithms.  If the identifier is idmgf1, the parameters are a HashAlgorithm.  PKCS1MGFAlgorithms ALGORITHMIDENTIFIER ::= { { OID idmgf1 PARAMETERS HashAlgorithm }, ...  Allows for future expansion  }   Default AlgorithmIdentifier for idRSAESOAEP.maskGenAlgorithm and  idRSASSAPSS.maskGenAlgorithm.  mgf1SHA1 MaskGenAlgorithm ::= { algorithm idmgf1, parameters HashAlgorithm : sha1 } MaskGenAlgorithm ::= AlgorithmIdentifier { {PKCS1MGFAlgorithms} }   Allowed algorithms for pSourceAlgorithm.  PKCS1PSourceAlgorithms ALGORITHMIDENTIFIER ::= { { OID idpSpecified PARAMETERS EncodingParameters }, ...  Allows for future expansion  } EncodingParameters ::= OCTET STRING(SIZE(0..MAX))

 This identifier means that the label L is an empty string, so the  digest of the empty string appears in the RSA block before  masking.
pSpecifiedEmpty PSourceAlgorithm ::= { algorithm idpSpecified, parameters EncodingParameters : emptyString } PSourceAlgorithm ::= AlgorithmIdentifier { {PKCS1PSourceAlgorithms} } emptyString EncodingParameters ::= ''H   Type identifier definitions for the PKCS #1 OIDs.  PKCS1Algorithms ALGORITHMIDENTIFIER ::= { { OID rsaEncryption PARAMETERS NULL }  { OID md2WithRSAEncryption PARAMETERS NULL }  { OID md5WithRSAEncryption PARAMETERS NULL }  { OID sha1WithRSAEncryption PARAMETERS NULL }  { OID sha224WithRSAEncryption PARAMETERS NULL }  { OID sha256WithRSAEncryption PARAMETERS NULL }  { OID sha384WithRSAEncryption PARAMETERS NULL }  { OID sha512WithRSAEncryption PARAMETERS NULL }  { OID sha512224WithRSAEncryption PARAMETERS NULL }  { OID sha512256WithRSAEncryption PARAMETERS NULL }  { OID idRSAESOAEP PARAMETERS RSAESOAEPparams }  PKCS1PSourceAlgorithms  { OID idRSASSAPSS PARAMETERS RSASSAPSSparams }, ...  Allows for future expansion  }  ===================  Main structures  =================== RSAPublicKey ::= SEQUENCE { modulus INTEGER,  n publicExponent INTEGER  e }   Representation of RSA private key with information for the CRT  algorithm.  RSAPrivateKey ::= SEQUENCE { version Version, modulus INTEGER,  n publicExponent INTEGER,  e privateExponent INTEGER,  d prime1 INTEGER,  p prime2 INTEGER,  q exponent1 INTEGER,  d mod (p1) exponent2 INTEGER,  d mod (q1) coefficient INTEGER,  (inverse of q) mod p otherPrimeInfos OtherPrimeInfos OPTIONAL } Version ::= INTEGER { twoprime(0), multi(1) } (CONSTRAINED BY { version MUST be multi if otherPrimeInfos present }) OtherPrimeInfos ::= SEQUENCE SIZE(1..MAX) OF OtherPrimeInfo OtherPrimeInfo ::= SEQUENCE { prime INTEGER,  ri exponent INTEGER,  di coefficient INTEGER  ti }   AlgorithmIdentifier.parameters for idRSAESOAEP.  Note that the tags in this Sequence are explicit.  RSAESOAEPparams ::= SEQUENCE { hashAlgorithm [0] HashAlgorithm DEFAULT sha1, maskGenAlgorithm [1] MaskGenAlgorithm DEFAULT mgf1SHA1, pSourceAlgorithm [2] PSourceAlgorithm DEFAULT pSpecifiedEmpty }   Identifier for default RSAESOAEP algorithm identifier.  The DER encoding of this is in hexadecimal:  (0x)30 0D  06 09  2A 86 48 86 F7 0D 01 01 07  30 00  Notice that the DER encoding of default values is "empty".  rSAESOAEPDefaultIdentifier RSAESAlgorithmIdentifier ::= { algorithm idRSAESOAEP, parameters RSAESOAEPparams : { hashAlgorithm sha1, maskGenAlgorithm mgf1SHA1, pSourceAlgorithm pSpecifiedEmpty } } RSAESAlgorithmIdentifier ::= AlgorithmIdentifier { {PKCS1Algorithms} }   AlgorithmIdentifier.parameters for idRSASSAPSS.  Note that the tags in this Sequence are explicit.  RSASSAPSSparams ::= SEQUENCE { hashAlgorithm [0] HashAlgorithm DEFAULT sha1, maskGenAlgorithm [1] MaskGenAlgorithm DEFAULT mgf1SHA1, saltLength [2] INTEGER DEFAULT 20, trailerField [3] TrailerField DEFAULT trailerFieldBC } TrailerField ::= INTEGER { trailerFieldBC(1) }   Identifier for default RSASSAPSS algorithm identifier  The DER encoding of this is in hexadecimal:  (0x)30 0D  06 09  2A 86 48 86 F7 0D 01 01 0A  30 00  Notice that the DER encoding of default values is "empty".  rSASSAPSSDefaultIdentifier RSASSAAlgorithmIdentifier ::= { algorithm idRSASSAPSS, parameters RSASSAPSSparams : { hashAlgorithm sha1, maskGenAlgorithm mgf1SHA1, saltLength 20, trailerField trailerFieldBC } } RSASSAAlgorithmIdentifier ::= AlgorithmIdentifier { {PKCS1Algorithms} }   Syntax for the EMSAPKCS1v1_5 hash identifier.  DigestInfo ::= SEQUENCE { digestAlgorithm DigestAlgorithm, digest OCTET STRING } DigestAlgorithm ::= AlgorithmIdentifier { {PKCS1v15DigestAlgorithms} } END
Appendix D. Revision History of PKCS #1

Versions 1.0  1.5:

Versions 1.0  1.3 were distributed to participants in RSA Data Security, Inc.'s PublicKey Cryptography Standards meetings in February and March 1991.
Version 1.4 was part of the June 3, 1991 initial public release of PKCS. Version 1.4 was published as NIST/OSI Implementors' Workshop document SECSIG9118.
Version 1.5 incorporated several editorial changes, including updates to the references and the addition of a revision history. The following substantive changes were made:
 Section 10: "MD4 with RSA" signature and verification processes were added.
 Section 11: md4WithRSAEncryption object identifier was added.
Version 1.5 was republished as [RFC2313] (which was later obsoleted by [RFC2437]).
Version 2.0:

Version 2.0 incorporated major editorial changes in terms of the document structure and introduced the RSAESOAEP encryption scheme. This version continued to support the encryption and signature processes in version 1.5, although the hash algorithm MD4 was no longer allowed due to cryptanalytic advances in the intervening years. Version 2.0 was republished as [RFC2437] (which was later obsoleted by [RFC3447]).
Version 2.1:

Version 2.1 introduced multiprime RSA and the RSASSAPSS signature scheme with appendix along with several editorial improvements. This version continued to support the schemes in version 2.0. Version 2.1 was republished as [RFC3447].
Version 2.2:

Version 2.2 updates the list of allowed hashing algorithms to align them with FIPS 1804 [SHS], therefore adding SHA224, SHA512/224, and SHA512/256. The following substantive changes were made:
 Object identifiers for sha224WithRSAEncryption, sha512224WithRSAEncryption, and sha512256WithRSAEncryption were added.
 This version continues to support the schemes in version 2.1.

Appendix E. About PKCS

The PublicKey Cryptography Standards are specifications produced by RSA Laboratories in cooperation with secure systems developers worldwide for the purpose of accelerating the deployment of public key cryptography. First published in 1991 as a result of meetings with a small group of early adopters of publickey technology, the PKCS documents have become widely referenced and implemented. Contributions from the PKCS series have become part of many formal and de facto standards, including ANSI X9 and IEEE P1363 documents, PKIX, Secure Electronic Transaction (SET), S/MIME, SSL/TLS, and Wireless Application Protocol (WAP) / WAP Transport Layer Security (WTLS).
Further development of most PKCS documents occurs through the IETF. Suggestions for improvement are welcome.
Acknowledgements

This document is based on a contribution of RSA Laboratories, the research center of RSA Security Inc.
Authors' Addresses

Kathleen M. Moriarty (editor) EMC Corporation 176 South Street Hopkinton, MA 01748 United States of America Email: kathleen.moriarty@emc.com Burt Kaliski Verisign 12061 Bluemont Way Reston, VA 20190 United States of America Email: bkaliski@verisign.com URI: http://verisignlabs.com Jakob Jonsson Subset AB Munkbrogtan 4 Stockholm SE11127 Sweden Phone: +46 8 428 687 43 Email: jakob.jonsson@subset.se Andreas Rusch RSA 345 Queen Street Brisbane, QLD 4000 Australia Email: andreas.rusch@rsa.com