Internet DRAFT - draft-housley-cms-chacha20-poly1305
draft-housley-cms-chacha20-poly1305
Internet-Draft R. Housley
Intended status: Standards Track Vigil Security
Expires: 4 October 2016 4 April 2016
Using ChaCha20-Poly1305 Authenticated Encryption
in the Cryptographic Message Syntax (CMS)
draft-housley-cms-chacha20-poly1305-00
Abstract
This document describes the conventions for using ChaCha20-Poly1305
Authenticated Encryption in the Cryptographic Message Syntax (CMS).
ChaCha20-Poly1305 is a construction of the ChaCha stream cipher and
Poly1305 authenticator.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 4 October 2016.
Copyright Notice
Copyright (c) 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/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.
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 1]
Internet-Draft 4 April 2016
1. Introduction
This document specifies the conventions for using the
ChaCha20-Poly1305 Authenticated Encryption as the content-
authenticated-encryption algorithm 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. ChaCha20 is described in [FORIETF].
Poly1305 [POLY1305] is a Wegman-Carter, one-time authenticator
designed by D. J. Bernstein. Poly1305 produces a 16-byte
authentication tag; it requires a a 256-bit, single-use key.
Poly1305 is also described in [FORIETF].
ChaCha20 and Poly1305 have been designed for high performance in
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). A nonce value cannot be used
securely more than once with the same key.
A high-level summary of AEAD_CHACHA20_POLY1305 authenticated
encryption processing is:
1) A Poly1305 one-time key is generated from the 256-bit key and
and nonce using the procedure described in Section 2.6 of
[FORIETF].
2) The ChaCha20 encryption function is used to encrypt the
plaintext, using the same key and nonce, and with the initial
counter set to 1.
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 2]
Internet-Draft 4 April 2016
3) The Poly1305 function is used with the Poly1305 key from
step 1, and a buffer constructed as a concatenation of the AAD,
padding1, the ciphertext, padding2, the length of the AAD in
octets, and the length of the ciphertext in octets. The
padding fields contain up to 15 octets, with all bits set to
zero, and the padding brings the total length of the buffer so
far to an integral multiple of 16. If the buffer length was
already an integral multiple of 16 octets, then the padding
field is zero octets. The length fields contain 64-bit little-
endian integers.
AEAD_CHACHA20_POLY1305 produces 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. 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].
1.3. ASN.1
CMS values are generated using ASN.1 [X680], which uses the Basic
Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
[X690].
2. Automated Key Management
The reuse of an ChaCha20 and Poly1305 AEAD nonce/key combination
destroys the security guarantees. As a result, it can be extremely
difficult to use the ChaCha20 and Poly1305 AEAD securely when using
statically configured keys. For safety's sake, implementations MUST
use an automated key management system [KEYMGMT].
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 3]
Internet-Draft 4 April 2016
The CMS authenticated-enveloped-data content type supports four
general key management techniques:
Key Transport: the 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, then
the content-authenticated-encryption key is encrypted in the
pairwise symmetric key;
Symmetric Key-Encryption Keys: the content-authenticated-
encryption key is encrypted in a previously distributed
symmetric key-encryption key; and
Passwords: the content-authenticated-encryption key is encrypted
in a key-encryption key that is derived from a password or
other shared secret value.
All of these key management techniques meet the automated key
management system requirement as long as a fresh content-
authenticated-encryption key is generated for the protection of each
content. Note that some of these key management techniques use one
key-encryption key to encrypt more than one content-authenticated-
encryption key during the system life cycle. As long as fresh
content-authenticated-encryption key is used each time,
AEAD_CHACHA20_POLY1305 can be used safely with the CMS authenticated-
enveloped-data content type.
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 can be made about whether these key management
techniques meet the automated key management system requirement.
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 content authenticated encryption using
the AEAD_CHACHA20_POLY1305 algorithm.
Content authenticated encryption algorithm identifiers are located in
the AuthEnvelopedData EncryptedContentInfo contentEncryptionAlgorithm
field.
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 4]
Internet-Draft 4 April 2016
Content authenticated encryption algorithms are used to encipher the
content located in the AuthEnvelopedData EncryptedContentInfo
encryptedContent field and to provide the message authentication code
for the AuthEnvelopedData mac field. Note that the message
authentication code 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 identifiers 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) TBD1 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field must contain a 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. IANA Considerations
IANA is requested to add the following entry in the SMI Security for
S/MIME Algorithms (1.2.840.113549.1.9.16.3) registry:
TBD1 id-alg-AEADChaCha20Poly1305 [This Document]
IANA is requested to add the following entry in the SMI Security for
S/MIME Module Identifier (1.2.840.113549.1.9.16.0) registry:
TBD2 id-mod-CMS-AEADChaCha20Poly1305 [This Document]
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 5]
Internet-Draft 4 April 2016
5. Security Considerations
The CMS AuthEnvelopedData provides all of the tools needed to avoid
reuse of the same nonce value under the same key. Automated key
management is discussed in Section 2.
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 likely ot be
sufficient to handle the size of any CMS content type. Note that
ciphertext length field in the authentication buffer will accomodate
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 pseudo-random 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.
6. Acknowledgements
Thanks to Jim Schaad for his review and insightful comments.
7. Normative References
[AUTHENV] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
November 2007.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
5652, September 2009.
[FORIETF] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, May 2015.
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 6]
Internet-Draft 4 April 2016
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, 2002.
[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, 2002.
8. Informative References
[AEAD] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[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>.
[KEYMGMT] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[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, D., Schiller, J., and S. Crocker, "Randomness
Recommendations for Security", BCP 106, RFC 4086, June
2005.
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 7]
Internet-Draft 4 April 2016
Appendix: ASN.1 Module
CMS-AEADChaCha20Poly1305
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) TBD2 }
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) TBD1 }
AEADChaCha20Poly1305Nonce ::= OCTET STRING (SIZE(12))
END
Author's Addresses
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Housley Using AEAD_CHACHA20_POLY1305 with CMS [Page 8]