Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 8103 Vigil Security
Category: Standards Track February 2017
ISSN: 2070-1721
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 (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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.
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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].
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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.
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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.
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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].
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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>.
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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>.
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RFC 8103 Using AEAD_CHACHA20_POLY1305 with CMS February 2017
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
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