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Internet Engineering Task Force (IETF)
Request for Comments: 8103
Category: Standards Track
ISSN: 2070-1721
R. Housley
Vigil Security
February 2017

Using ChaCha20-Poly1305 Authenticated Encryption

in the Cryptographic Message Syntax (CMS)

Abstract

This document describes the conventions for using ChaCha20-Poly1305 Authenticated Encryption in the Cryptographic Message Syntax (CMS). ChaCha20-Poly1305 is an authenticated encryption algorithm constructed of the ChaCha stream cipher and Poly1305 authenticator.

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.rfc-editor.org/info/rfc8103.

Copyright Notice

Copyright © 2017 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/license-info) 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 ....................................................2
      1.1. The ChaCha20 and Poly1305 AEAD Construction ................3
      1.2. ASN.1 ......................................................3
      1.3. Terminology ................................................3
   2. Key Management ..................................................4
   3. Using the AEAD_CHACHA20_POLY1305 Algorithm with
      AuthEnvelopedData ...............................................4
   4. S/MIME Capabilities Attribute ...................................5
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................8
   Appendix A. ASN.1 Module ...........................................9
   Acknowledgements ...................................................9
   Author's Address ...................................................9

1. Introduction

This document specifies the conventions for using ChaCha20-Poly1305 Authenticated Encryption with the Cryptographic Message Syntax (CMS) [CMS] authenticated-enveloped-data content type [AUTHENV].

ChaCha [CHACHA] is a stream cipher developed by D. J. Bernstein in 2008. It is a refinement of Salsa20, which is one of the ciphers in the eSTREAM portfolio [ESTREAM].

ChaCha20 is the 20-round variant of ChaCha; it requires a 256-bit key and a 96-bit nonce. [FORIETF] provides a detailed algorithm description, examples, and test vectors of ChaCha20.

Poly1305 [POLY1305] is a Wegman-Carter, one-time authenticator designed by D. J. Bernstein. Poly1305 produces a 16-byte authentication tag; it requires a 256-bit, single-use key. [FORIETF] also provides a detailed algorithm description, examples, and test vectors of Poly1305.

ChaCha20 and Poly1305 have been designed for high-performance software implementations. They can typically be implemented with few resources and inexpensive operations, making them suitable on a wide range of systems. They have also been designed to minimize leakage of information through side channels.

1.1. The ChaCha20 and Poly1305 AEAD Construction

ChaCha20 and Poly1305 have been combined to create an Authenticated Encryption with Associated Data (AEAD) algorithm [AEAD]. This AEAD algorithm is often referred to as AEAD_CHACHA20_POLY1305, and it is described in [FORIETF].

AEAD_CHACHA20_POLY1305 accepts four inputs: a 256-bit key, a 96-bit nonce, an arbitrary-length plaintext, and an arbitrary-length additional authenticated data (AAD). As the name implies, a nonce value cannot be used securely more than once with the same key.

AEAD_CHACHA20_POLY1305 produces two outputs: ciphertext of the same length as the plaintext and a 128-bit authentication tag.

AEAD_CHACHA20_POLY1305 authenticated decryption processing is similar to the encryption processing. Of course, the roles of ciphertext and plaintext are reversed, so the ChaCha20 encryption function is applied to the ciphertext, producing the plaintext. The Poly1305 function is run over the AAD and the ciphertext, not the plaintext, and the resulting authentication tag is bitwise compared to the received authentication tag. The message is authenticated if and only if the calculated and received authentication tags match.

1.2. ASN.1

CMS values are generated using ASN.1 [X680], which uses the Basic Encoding Rules (BER) and the Distinguished Encoding Rules (DER) [X690].

1.3. Terminology

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 RFC 2119 [STDWORDS].

2. Key Management

The reuse of an AEAD_CHACHA20_POLY1305 nonce value with the same key destroys the security guarantees. It can be extremely difficult to use a statically configured AEAD_CHACHA20_POLY1305 key and never repeat a nonce value; however, the CMS authenticated-enveloped-data content type supports four key management techniques that allow a fresh AEAD_CHACHA20_POLY1305 key to be used as the content-authenticated-encryption key for a single protected content: 

      Key Transport: the fresh content-authenticated-encryption key is
         encrypted in the recipient's public key;

Key Agreement: the recipient's public key and the sender's private

key are used to generate a pairwise symmetric key-encryption key, then the fresh content-authenticated-encryption key is encrypted in the pairwise symmetric key; 

      Symmetric Key-Encryption Keys: the fresh content-authenticated-
         encryption key is encrypted in a previously distributed
         symmetric key-encryption key; and
      
      Passwords: the fresh content-authenticated-encryption key is
         encrypted in a key-encryption key that is derived from a
         password or other shared secret value.

In addition to these four general key management techniques, CMS supports other key management techniques. See Section 6.2.5 of [CMS]. Since the properties of these key management techniques are unknown, no statement about their support of fresh content-authenticated-encryption keys can be made. Designers and implementers must perform their own analysis if one of these other key management techniques is supported.

3. Using the AEAD_CHACHA20_POLY1305 Algorithm with AuthEnvelopedData

This section specifies the conventions employed by CMS implementations that support the authenticated-enveloped-data content type and the AEAD_CHACHA20_POLY1305 algorithm.

The AEAD_CHACHA20_POLY1305 algorithm identifier is located in the AuthEnvelopedData EncryptedContentInfo contentEncryptionAlgorithm field.

The AEAD_CHACHA20_POLY1305 algorithm is used to (1) authenticate the attributes located in the AuthEnvelopedData authAttrs field, if any are present, (2) encipher the content located in the AuthEnvelopedData EncryptedContentInfo encryptedContent field, and (3) provide the message authentication code (MAC) located in the AuthEnvelopedData mac field. The authenticated attributes are DER encoded to produce the AAD input value to the AEAD_CHACHA20_POLY1305 algorithm. The ciphertext and the MAC are the two outputs of the AEAD_CHACHA20_POLY1305 algorithm. Note that the MAC, which is called the authentication tag in [FORIETF], provides integrity protection for both the AuthEnvelopedData authAttrs and the AuthEnvelopedData EncryptedContentInfo encryptedContent.

Neither the plaintext content nor the optional AAD inputs need to be padded prior to invoking the AEAD_CHACHA20_POLY1305 algorithm.

There is one algorithm identifier for the AEAD_CHACHA20_POLY1305 algorithm: 

      id-alg-AEADChaCha20Poly1305 OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
            pkcs9(9) smime(16) alg(3) 18 }

The AlgorithmIdentifier parameters field MUST be present, and the parameters field MUST contain an AEADChaCha20Poly1305Nonce: 

      AEADChaCha20Poly1305Nonce ::= OCTET STRING (SIZE(12))

The AEADChaCha20Poly1305Nonce contains a 12-octet nonce. With the CMS, the content-authenticated-encryption key is normally used for a single content. Within the scope of any content-authenticated- encryption key, the nonce value MUST be unique. That is, the set of nonce values used with any given key MUST NOT contain any duplicate values.

4. S/MIME Capabilities Attribute

Section 2.5.2 of RFC 5751 [MSG] defines the SMIMECapabilities attribute, which is used to specify a partial list of algorithms that the software announcing the SMIMECapabilities can support. When constructing a CMS signed-data content type, compliant software MAY include the SMIMECapabilities signed attribute to announce support for the AEAD_CHACHA20_POLY1305 algorithm.

The SMIMECapability SEQUENCE representing the AEAD_CHACHA20_POLY1305 algorithm MUST include the id-alg-AEADChaCha20Poly1305 object identifier in the capabilityID field and MUST omit the parameters field.

The DER encoding of an SMIMECapability SEQUENCE is the same as the DER encoding of an AlgorithmIdentifier. The DER encoding for the AEAD_CHACHA20_POLY1305 algorithm in the SMIMECapability SEQUENCE (in hexadecimal) is: 

      30 0d 06 0b 2a 86 48 86 f7 0d 01 09 10 03 12

5. IANA Considerations

IANA has added the following entry in the "SMI Security for S/MIME Algorithms (1.2.840.113549.1.9.16.3)" registry: 

      18   id-alg-AEADChaCha20Poly1305         RFC 8103

IANA has added the following entry in the "SMI Security for S/MIME Module Identifier (1.2.840.113549.1.9.16.0)" registry: 

      66   id-mod-CMS-AEADChaCha20Poly1305     RFC 8103

6. Security Considerations

The CMS AuthEnvelopedData provides all of the tools needed to avoid reuse of the same nonce value under the same key. See the discussion in Section 2 of this document. RFC 7539 [FORIETF] describes the consequences of using a nonce value more than once:

Consequences of repeating a nonce: If a nonce is repeated, then both the one-time Poly1305 key and the keystream are identical between the messages. This reveals the XOR of the plaintexts, because the XOR of the plaintexts is equal to the XOR of the ciphertexts.

When using AEAD_CHACHA20_POLY1305, the resulting ciphertext is always the same size as the original plaintext. Some other mechanism needs to be used in conjunction with AEAD_CHACHA20_POLY1305 if disclosure of the size of the plaintext is a concern.

The amount of encrypted data possible in a single invocation of AEAD_CHACHA20_POLY1305 is 2^32-1 blocks of 64 octets each, because of the size of the block counter field in the ChaCha20 block function. This gives a total of 247,877,906,880 octets, which is likely to be sufficient to handle the size of any CMS content type. Note that the ciphertext length field in the authentication buffer will accommodate 2^64 octets, which is much larger than necessary.

The AEAD_CHACHA20_POLY1305 construction is a novel composition of ChaCha20 and Poly1305. A security analysis of this composition is given in [PROCTER].

Implementations must randomly generate content-authenticated- encryption keys. The use of inadequate pseudorandom number generators (PRNGs) to generate cryptographic keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys, searching the resulting small set of possibilities rather than "brute force" searching the whole key space. The generation of quality random numbers is difficult. RFC 4086 [RANDOM] offers important guidance in this area.

7. References

7.1. Normative References

   [AUTHENV]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              DOI 10.17487/RFC5083, November 2007,
              <http://www.rfc-editor.org/info/rfc5083>.
   
   [CMS]      Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.
   
   [FORIETF]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
              <http://www.rfc-editor.org/info/rfc7539>.
   
   [MSG]      Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751,
              January 2010, <http://www.rfc-editor.org/info/rfc5751>.
   
   [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [X680]     ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC 8824-1, August 2015,
              <https://www.itu.int/rec/T-REC-X.680/en>.
   
   [X690]     ITU-T, "Information technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1,
              August 2015, <https://www.itu.int/rec/T-REC-X.690/en>.

7.2. Informative References

   [AEAD]     McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.
   
   [CHACHA]   Bernstein, D., "ChaCha, a variant of Salsa20",
              January 2008,
              <http://cr.yp.to/chacha/chacha-20080128.pdf>.
   
   [ESTREAM]  Babbage, S., DeCanniere, C., Cantenaut, A., Cid, C.,
              Gilbert, H., Johansson, T., Parker, M., Preneel, B.,
              Rijmen, V., and M. Robshaw, "The eSTREAM Portfolio
              (rev. 1)", September 2008,
              <http://www.ecrypt.eu.org/stream/finallist.html>.
   
   [POLY1305] Bernstein, D., "The Poly1305-AES message-authentication
              code", March 2005,
              <http://cr.yp.to/mac/poly1305-20050329.pdf>.
   
   [PROCTER]  Procter, G., "A Security Analysis of the Composition of
              ChaCha20 and Poly1305", August 2014,
              <http://eprint.iacr.org/2014/613.pdf>.
   
   [RANDOM]   Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

Appendix A. ASN.1 Module

CMS-AEADChaCha20Poly1305 

       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
       
         pkcs-9(9) smime(16) modules(0) 66 }
   
   DEFINITIONS IMPLICIT TAGS ::= BEGIN

IMPORTS

CONTENT-ENCRYPTION
FROM AlgorithmInformation-2009

{ iso(1) identified-organization(3) dod(6) internet(1)

security(5) mechanisms(5) pkix(7) id-mod(0)

id-mod-algorithmInformation-02(58) };

   -- EXPORTS All

AEADContentEncryptionAlgs CONTENT-ENCRYPTION ::= 

       { cea-AEADChaCha20Poly1305, ... }
   
   cea-AEADChaCha20Poly1305 CONTENT-ENCRYPTION ::= {
       IDENTIFIER id-alg-AEADChaCha20Poly1305
       PARAMS TYPE AEADChaCha20Poly1305Nonce ARE required
       SMIME-CAPS { IDENTIFIED BY id-alg-AEADChaCha20Poly1305 } }
   
   id-alg-AEADChaCha20Poly1305 OBJECT IDENTIFIER ::=
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs9(9) smime(16) alg(3) 18 }
   
   AEADChaCha20Poly1305Nonce ::= OCTET STRING (SIZE(12))
   
   END

Acknowledgements

Thanks to Jim Schaad, Daniel Migault, Stephen Farrell, Yoav Nir, and Niclas Comstedt for their review and insightful comments.

Author's Address

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA  20170
   United States of America

Email: 

          housley@vigilsec.com