Request for Comments: 8230
Category: Standards Track
ISSN: 20701721
Microsoft
September 2017
Using RSA Algorithms with
CBOR Object Signing and Encryption (COSE) Messages
Abstract

The CBOR Object Signing and Encryption (COSE) specification defines cryptographic message encodings using Concise Binary Object Representation (CBOR). This specification defines algorithm encodings and representations enabling RSA algorithms to be used for COSE messages. Encodings are specified for the use of RSA Probabilistic Signature Scheme (RSASSAPSS) signatures, RSA Encryption Scheme  Optimal Asymmetric Encryption Padding (RSAES OAEP) encryption, and RSA keys.
Status of This Memo

This is an Internet Standards Track document.
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). Further information on Internet Standards is available in 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/rfc8230.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Notation and Conventions . . . . . . . . . . 3 2. RSASSAPSS Signature Algorithm . . . . . . . . . . . . . . . 3 3. RSAESOAEP Key Encryption Algorithm . . . . . . . . . . . . . 4 4. RSA Keys . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 5.1. COSE Algorithms Registrations . . . . . . . . . . . . . . 6 5.2. COSE Key Type Registrations . . . . . . . . . . . . . . . 7 5.3. COSE Key Type Parameters Registrations . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6.1. Key Size Security Considerations . . . . . . . . . . . . 9 6.2. RSASSAPSS Security Considerations . . . . . . . . . . . 10 6.3. RSAESOAEP Security Considerations . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 11 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction

The CBOR Object Signing and Encryption (COSE) [RFC8152] specification defines cryptographic message encodings using Concise Binary Object Representation (CBOR) [RFC7049]. This specification defines algorithm encodings and representations enabling RSA algorithms to be used for COSE messages.
1.1. Requirements Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
2. RSASSAPSS Signature Algorithm

The RSASSAPSS signature algorithm is defined in [RFC8017].
The RSASSAPSS signature algorithm is parameterized with a hash function (h), a mask generation function (mgf), and a salt length (sLen). For this specification, the mask generation function is fixed to be MGF1 as defined in [RFC8017]. It has been recommended that the same hash function be used for hashing the data as well as in the mask generation function. This specification follows this recommendation. The salt length is the same length as the hash function output.
Implementations need to check that the key type is 'RSA' when creating or verifying a signature.
The RSASSAPSS algorithms specified in this document are in the following table.
++++++  Name  Value  Hash  Salt Length  Description  ++++++  PS256  37  SHA256  32  RSASSAPSS w/ SHA256   PS384  38  SHA384  48  RSASSAPSS w/ SHA384   PS512  39  SHA512  64  RSASSAPSS w/ SHA512  ++++++
Table 1: RSASSAPSS Algorithm Values
3. RSAESOAEP Key Encryption Algorithm

RSAESOAEP is an asymmetric key encryption algorithm. The definition of RSAEAOAEP can be found in Section 7.1 of [RFC8017]. The algorithm is parameterized using a mask generation function (mgf), a hash function (h), and encoding parameters (P). For the algorithm identifiers defined in this section:
 mgf is always set to MGF1 as defined in [RFC8017] and uses the same hash function as h.
 P is always set to the empty octet string.
The following table summarizes the rest of the values.
+++++  Name  Value  Hash  Description  +++++  RSAESOAEP w/ RFC 8017  40  SHA1  RSAESOAEP w/   default parameters    SHA1   RSAESOAEP w/ SHA256  41  SHA256  RSAESOAEP w/      SHA256   RSAESOAEP w/ SHA512  42  SHA512  RSAESOAEP w/      SHA512  +++++
Table 2: RSAESOAEP Algorithm Values

The key type MUST be 'RSA'.
4. RSA Keys

Key types are identified by the 'kty' member of the COSE_Key object. This specification defines one value for this member in the following table.
++++  Name  Value  Description  ++++  RSA  3  RSA Key  ++++
Table 3: Key Type Values

This document defines a key structure for both the public and private parts of RSA keys. Together, an RSA public key and an RSA private key form an RSA key pair.
The document also provides support for the socalled "multiprime" RSA keys, in which the modulus may have more than two prime factors. The benefit of multiprime RSA is lower computational cost for the decryption and signature primitives. For a discussion on how multi prime affects the security of RSA cryptosystems, the reader is referred to [MultiPrimeRSA].
This document follows the naming convention of [RFC8017] for the naming of the fields of an RSA public or private key, and the corresponding fields have identical semantics. The requirements for fields for RSA keys are as follows:
 For all keys, 'kty' MUST be present and MUST have a value of 3.
 For public keys, the fields 'n' and 'e' MUST be present. All other fields defined in the following table below MUST be absent.
 For private keys with two primes, the fields 'other', 'r_i', 'd_i', and 't_i' MUST be absent; all other fields MUST be present.
 For private keys with more than two primes, all fields MUST be present. For the third to nth primes, each of the primes is represented as a map containing the fields 'r_i', 'd_i', and 't_i'. The field 'other' is an array of those maps.
 All numeric key parameters are encoded in an unsigned bigendian representation as an octet sequence using the CBOR byte string type (major type 2). The octet sequence MUST utilize the minimum number of octets needed to represent the value. For instance, the value 32,768 is represented as the CBOR byte sequence 0b010_00010, 0x80 0x00 (major type 2, additional information 2 for the length).
The following table provides a summary of the label values and the types associated with each of those labels.
++++++  Key  Name  Label  CBOR  Description   Type    Type   ++++++  3  n  1  bstr  the RSA modulus n   3  e  2  bstr  the RSA public exponent e   3  d  3  bstr  the RSA private exponent d   3  p  4  bstr  the prime factor p of n   3  q  5  bstr  the prime factor q of n   3  dP  6  bstr  dP is d mod (p  1)   3  dQ  7  bstr  dQ is d mod (q  1)   3  qInv  8  bstr  qInv is the CRT coefficient       q^(1) mod p   3  other  9  array  other prime infos, an array   3  r_i  10  bstr  a prime factor r_i of n, where i       >= 3   3  d_i  11  bstr  d_i = d mod (r_i  1)   3  t_i  12  bstr  the CRT coefficient t_i = (r_1 *       r_2 * ... * r_(i1))^(1) mod r_i  ++++++
Table 4: RSA Key Parameters
5. IANA Considerations
5.1. COSE Algorithms Registrations

IANA has registered the following values in the IANA "COSE Algorithms" registry [IANA.COSE].
o Name: PS256
o Value: 37
o Description: RSASSAPSS w/ SHA256
o Reference: Section 2 of this document
o Recommended: Yeso Name: PS384
o Value: 38
o Description: RSASSAPSS w/ SHA384
o Reference: Section 2 of this document
o Recommended: Yes
o Name: PS512
o Value: 39
o Description: RSASSAPSS w/ SHA512
o Reference: Section 2 of this document
o Recommended: Yeso Name: RSAESOAEP w/ RFC 8017 default parameters
o Value: 40
o Description: RSAESOAEP w/ SHA1
o Reference: Section 3 of this document
o Recommended: Yeso Name: RSAESOAEP w/ SHA256 o Value: 41 o Description: RSAESOAEP w/ SHA256 o Reference: Section 3 of this document o Recommended: Yes o Name: RSAESOAEP w/ SHA512 o Value: 42 o Description: RSAESOAEP w/ SHA512 o Reference: Section 3 of this document o Recommended: Yes
5.2. COSE Key Type Registrations

IANA has registered the following value in the IANA "COSE Key Types" registry [IANA.COSE].
o Name: RSA
o Value: 3
o Description: RSA Key
o Reference: Section 4 of this document
5.3. COSE Key Type Parameters Registrations

IANA has registered the following values in the IANA "COSE Key Type Parameters" registry [IANA.COSE].
o Key Type: 3
o Name: n
o Label: 1
o CBOR Type: bstr
o Description: the RSA modulus n
o Reference: Section 4 of this document
o Key Type: 3
o Name: e
o Label: 2
o CBOR Type: bstr
o Description: the RSA public exponent e
o Reference: Section 4 of this documento Key Type: 3
o Name: d
o Label: 3
o CBOR Type: bstr
o Description: the RSA private exponent d
o Reference: Section 4 of this documento Key Type: 3
o Name: p
o Label: 4
o CBOR Type: bstr
o Description: the prime factor p of n
o Reference: Section 4 of this documento Key Type: 3
o Name: q
o Label: 5
o CBOR Type: bstr
o Description: the prime factor q of n
o Reference: Section 4 of this documento Key Type: 3
o Name: dP
o Label: 6
o CBOR Type: bstr
o Description: dP is d mod (p  1)
o Reference: Section 4 of this documento Key Type: 3
o Name: dQ
o Label: 7
o CBOR Type: bstr
o Description: dQ is d mod (q  1)
o Reference: Section 4 of this documento Key Type: 3
o Name: qInv
o Label: 8
o CBOR Type: bstr
o Description: qInv is the CRT coefficient q^(1) mod p
o Reference: Section 4 of this document
o Key Type: 3
o Name: other
o Label: 9
o CBOR Type: array
o Description: other prime infos, an array
o Reference: Section 4 of this documento Key Type: 3
o Name: r_i
o Label: 10
o CBOR Type: bstr
o Description: a prime factor r_i of n, where i >= 3
o Reference: Section 4 of this documento Key Type: 3
o Name: d_i
o Label: 11
o CBOR Type: bstr
o Description: d_i = d mod (r_i  1)
o Reference: Section 4 of this documento Key Type: 3 o Name: t_i o Label: 12 o CBOR Type: bstr o Description: the CRT coefficient t_i = (r_1 * r_2 * ... * r_(i1))^(1) mod r_i o Reference: Section 4 of this document
6. Security Considerations
6.1. Key Size Security Considerations

A key size of 2048 bits or larger MUST be used with these algorithms. This key size corresponds roughly to the same strength as provided by a 128bit symmetric encryption algorithm. Implementations SHOULD be able to encrypt and decrypt with modulus between 2048 and 16K bits in length. Applications can impose additional restrictions on the length of the modulus.
In addition to needing to worry about keys that are too small to provide the required security, there are issues with keys that are too large. Denialofservice attacks have been mounted with overly large keys or oddly sized keys. This has the potential to consume resources with these keys. It is highly recommended that checks on the key length be done before starting a cryptographic operation.
There are two reasonable ways to address this attack. First, a key should not be used for a cryptographic operation until it has been verified that it is controlled by a party trusted by the recipient. This approach means that no cryptography will be done until a trust decision about the key has been made, a process described in Appendix D, Item 4 of [RFC7515]. Second, applications can impose maximum as well as minimumlength requirements on keys. This limits the resources that would otherwise be consumed by the use of overly large keys.
6.2. RSASSAPSS Security Considerations

There is a theoretical hash substitution attack that can be mounted against RSASSAPSS [HASHID]. However, the requirement that the same hash function be used consistently for all operations is an effective mitigation against it. Unlike an Elliptic Curve Digital Signature Algorithm (ECDSA), hash function outputs are not truncated so that the full hash value is always signed. The internal padding structure of RSASSAPSS means that one needs to have multiple collisions between the two hash functions to be successful in producing a forgery based on changing the hash function. This is highly unlikely.
6.3. RSAESOAEP Security Considerations

A version of RSAESOAEP using the default parameters specified in Appendix A.2.1 of [RFC8017] is included because this is the most widely implemented set of OAEP parameter choices. (Those default parameters are the SHA1 hash function and the MGF1 with SHA1 mask generation function.)
Keys used with RSAESOAEP MUST follow the constraints in Section 7.1 of [RFC8017]. Also, keys with a low private key exponent value, as described in Section 3 of "Twenty Years of Attacks on the RSA Cryptosystem" [Boneh99], MUST NOT be used.
7. References
7.1. Normative References

[Boneh99] Boneh, D., "Twenty Years of Attacks on the RSA Cryptosystem", Notices of the American Mathematical Society (AMS), Vol. 46, No. 2, pp. 203213, 1999, <http://www.ams.org/notices/199902/boneh.pdf>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfceditor.org/info/rfc2119>. [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, <https://www.rfceditor.org/info/rfc7049>. [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015, <https://www.rfceditor.org/info/rfc7515>. [RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016, <https://www.rfceditor.org/info/rfc8017>. [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, July 2017, <https://www.rfceditor.org/info/rfc8152>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfceditor.org/info/rfc8174>.
7.2. Informative References

[HASHID] Kaliski, B., "On Hash Function Firewalls in Signature Schemes", Lecture Notes in Computer Science (LNCS), Volume 2271, pp. 116, DOI 10.1007/3540457607_1, February 2002, <https://rd.springer.com/chapter/ 10.1007/3540457607_1>.
[IANA.COSE] IANA, "CBOR Object Signing and Encryption (COSE)",
<http://www.iana.org/assignments/cose>.

[MultiPrimeRSA]
Hinek, M. and D. Cheriton, "On the Security of Multiprime RSA", June 2006, <http://cacr.uwaterloo.ca/techreports/ 2006/cacr200616.pdf>.
Acknowledgements

This specification incorporates text from "CBOR Encoded Message Syntax" (September 2015) authored by Jim Schaad and Brian Campbell. Thanks are due to Ben Campbell, Roni Even, Steve Kent, Kathleen Moriarty, Eric Rescorla, Adam Roach, Rich Salz, and Jim Schaad for their reviews of the specification.
Author's Address

Michael B. Jones
MicrosoftEmail: mbj@microsoft.com URI: http://selfissued.info/