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IETF RFC 7251
Last modified on Friday, June 27th, 2014
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Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 7251 Cisco Systems
Category: Informational D. Bailey
ISSN: 2070-1721 Ruhr-University Bochum
M. Campagna
R. Dugal
Certicom Corp.
June 2014
AES-CCM Elliptic Curve Cryptography (ECC) Cipher Suites for TLS
 Abstract
This memo describes the use of the Advanced Encryption Standard (AES)
in the Counter and CBC-MAC Mode (CCM) of operation within Transport
Layer Security (TLS) to provide confidentiality and data-origin
authentication. The AES-CCM algorithm is amenable to compact
implementations, making it suitable for constrained environments,
while at the same time providing a high level of security. The
cipher suites defined in this document use Elliptic Curve
Cryptography (ECC) and are advantageous in networks with limited
bandwidth.
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 5741.
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/RFC 7251.
McGrew, et al. Informational PAGE 1 
RFC 7251 AES-CCM ECC TLS June 2014
Copyright Notice
Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. ECC-Based AES-CCM Cipher Suites . . . . . . . . . . . . . . . 3
2.1. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . 5
2.2. Requirements on Curves and Hashes . . . . . . . . . . . . 5
3. TLS Versions . . . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5.1. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 6
5.2. Counter Reuse . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Hardware Security Modules . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Recommended Curves and Algorithms . . . . . . . . . 9
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RFC 7251 AES-CCM ECC TLS June 2014
1. Introduction
This document describes the use of Advanced Encryption Standard (AES)
[AES] in Counter with CBC-MAC Mode (CCM) [CCM] in several TLS cipher
suites. AES-CCM provides both authentication and confidentiality
(encryption and decryption) and uses as its only primitive the AES
encrypt block cipher operation. This makes it amenable to compact
implementations, which are advantageous in constrained environments.
Of course, adoption outside of constrained environments is necessary
to enable interoperability, such as that between web clients and
embedded servers, or between embedded clients and web servers. The
use of AES-CCM has been specified for the IPsec Encapsulating
Security Payload (ESP) [RFC 4309] and 802.15.4 wireless networks
[IEEE802154].
Authenticated encryption, in addition to providing confidentiality
for the plaintext that is encrypted, provides a way to check its
integrity and authenticity. Authenticated Encryption with Associated
Data, or AEAD [RFC 5116], adds the ability to check the integrity and
authenticity of some associated data that is not encrypted. This
memo utilizes the AEAD facility within TLS 1.2 [RFC 5246] and the AES-
CCM-based AEAD algorithms defined in [RFC 5116] and [RFC 6655]. All of
these algorithms use AES-CCM; some have shorter authentication tags
and are therefore more suitable for use across networks in which
bandwidth is constrained and message sizes may be small.
The cipher suites defined in this document use Ephemeral Elliptic
Curve Diffie-Hellman (ECDHE) as their key establishment mechanism;
these cipher suites can be used with DTLS [RFC 6347].
1.1. Conventions Used in This Document
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].
2. ECC-Based AES-CCM Cipher Suites
The cipher suites defined in this document are based on the AES-CCM
Authenticated Encryption with Associated Data (AEAD) algorithms
AEAD_AES_128_CCM and AEAD_AES_256_CCM described in [RFC 5116]. The
following cipher suites are defined:
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM = {0xC0,0xAC}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CCM = {0xC0,0xAD}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 = {0xC0,0xAE}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 = {0xC0,0xAF}
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RFC 7251 AES-CCM ECC TLS June 2014
These cipher suites make use of the AEAD capability in TLS 1.2
[RFC 5246]. Note that each of these AEAD algorithms uses AES-CCM.
Cipher suites ending with "8" use eight-octet authentication tags;
the other cipher suites have 16-octet authentication tags.
The HMAC truncation option described in Section 7 of [RFC 6066] (which
negotiates the "truncated_hmac" TLS extension) does not have an
effect on the cipher suites defined in this note, because they do not
use HMAC to protect TLS records.
The "nonce" input to the AEAD algorithm is as defined in [RFC 6655].
In DTLS, the 64-bit seq_num field is the 16-bit DTLS epoch field
concatenated with the 48-bit sequence_number field. The epoch and
sequence_number appear in the DTLS record layer.
This construction allows the internal counter to be 32 bits long,
which is a convenient size for use with CCM.
These cipher suites make use of the default TLS 1.2 Pseudorandom
Function (PRF), which uses HMAC with the SHA-256 hash function.
The ECDHE_ECDSA key exchange is performed as defined in [RFC 4492],
with the following additional stipulations:
o Curves with a cofactor equal to one SHOULD be used; this
simplifies their use.
o The uncompressed point format MUST be supported. Other point
formats MAY be used.
o The client SHOULD offer the elliptic_curves extension, and the
server SHOULD expect to receive it.
o The client MAY offer the ec_point_formats extension, but the
server need not expect to receive it.
o Fundamental ECC algorithms [RFC 6090] MAY be used as an
implementation method.
o If the server uses a certificate, then the requirements in RFC
4492 apply: "The server's certificate MUST contain an ECDSA-
capable public key and be signed with ECDSA." Guidance on
acceptable choices of hashes and curves that can be used with each
cipher suite is detailed in Section 2.2. The Signature Algorithms
extension (Section 7.4.1.4.1 of [RFC 5246]) SHOULD be used to
indicate support of those signature and hash algorithms. If a
client certificate is used, the same criteria SHOULD apply to it.
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RFC 7251 AES-CCM ECC TLS June 2014
Implementations of these cipher suites will interoperate with
[RFC 4492] but can be more compact than a full implementation of that
RFC.
2.1. AEAD Algorithms
The following AEAD algorithms are used:
AEAD_AES_128_CCM is used in the TLS_ECDHE_ECDSA_WITH_AES_128_CCM
cipher suite,
AEAD_AES_256_CCM is used in the TLS_ECDHE_ECDSA_WITH_AES_256_CCM
cipher suite,
AEAD_AES_128_CCM_8 is used in the
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 cipher suite, and
AEAD_AES_256_CCM_8 is used in the
TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 cipher suite.
2.2. Requirements on Curves and Hashes
Implementations SHOULD select elliptic curves and hash functions so
that AES-128 is used with a curve and a hash function supporting a
128-bit security level, and AES-256 is used with a curve and a hash
function supporting a 192-bit or 256-bit security level. More
detailed guidance on cryptographic parameter selection is given in
[SP800-57] (see especially Tables 2 and 3).
Appendix A describes suitable curves and hash functions that are
widely available.
3. TLS Versions
These cipher suites make use of the authenticated encryption with
additional data defined in TLS 1.2 [RFC 5288]. They MUST NOT be
negotiated in older versions of TLS. Clients MUST NOT offer these
cipher suites if they do not offer TLS 1.2 or later. Servers that
select an earlier version of TLS MUST NOT select one of these cipher
suites. Earlier versions do not have support for AEAD; for instance,
the TLSCiphertext structure does not have the "aead" option in TLS
1.1. Because TLS has no way for the client to indicate that it
supports TLS 1.2 but not earlier versions, a non-compliant server
might potentially negotiate TLS 1.1 or earlier and select one of the
cipher suites in this document. Clients MUST check the TLS version
and generate a fatal "illegal_parameter" alert if they detect an
incorrect version.
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RFC 7251 AES-CCM ECC TLS June 2014
4. IANA Considerations
IANA has assigned the values for the cipher suites defined in
Section 2 from the "TLS Cipher Suite Registry". The DTLS-OK column
has been marked as "Y" for each of these algorithms.
5. Security Considerations
5.1. Perfect Forward Secrecy
The perfect forward secrecy properties of ephemeral Diffie-Hellman
cipher suites are discussed in the security analysis of [RFC 5246].
This analysis applies to the ECDHE cipher suites.
5.2. Counter Reuse
AES-CCM security requires that the counter never be reused. The IV
construction in Section 2 is designed to prevent counter reuse.
5.3. Hardware Security Modules
A cipher suite can be implemented in such a way that the secret keys
and private keys are stored inside a Hardware Security Module (HSM),
and the cryptographic operations involving those keys are performed
by the HSM on data provided by an application interacting with the
HSM through an interface such as that defined by the Cryptographic
Token Interface Standard [PKCS11]. When an AEAD cipher suite, such
as those in this note, are implemented in this way, special handling
of the nonce is required. This is because the "salt" part of the
nonce is set to the client_write_IV or server_write_IV, which is a
function of the TLS master secret.
Another potential issue with the Cryptographic Token Interface
Standard is that the use of the DecryptUpdate function is not
possible with the CCM decrypt operation or the decrypt operation of
any other authenticated encryption method. This is because the
DecryptUpdate requires that post-decryption plaintext be returned
before the authentication check is completed.
6. Acknowledgements
This document borrows heavily from [RFC 5288]. Thanks are due to
Robert Cragie for his great help in making this work complete,
correct, and useful, and to Peter Dettman for his review. Thanks
also to Mike StJohns for pointing out the HSM issues.
This document is motivated by the considerations raised in the Zigbee
Smart Energy 2.0 working group.
McGrew, et al. Informational PAGE 6
RFC 7251 AES-CCM ECC TLS June 2014
7. References
7.1. Normative References
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", FIPS 197, November 2001.
[CCM] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation: The
CCM Mode for Authentication and Confidentiality", SP
800-38C, May 2004.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC 5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC 5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC 5639] Lochter, M. and J. Merkle, "Elliptic Curve Cryptography
(ECC) Brainpool Standard Curves and Curve Generation", RFC
5639, March 2010.
[RFC 6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC 6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC 6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC 6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, July 2012.
McGrew, et al. Informational PAGE 7
RFC 7251 AES-CCM ECC TLS June 2014
[SP800-57] National Institute of Standards and Technology,
"Recommendation for Key Management - Part 1: General
(Revision 3)", SP 800-57 Part 1, July 2012.
7.2. Informative References
[IEEE802154]
IEEE, "Wireless Personal Area Networks", IEEE Standard
802.15.4-2006, 2006.
[PKCS11] RSA Laboratories, "PKCS #11: Cryptographic Token Interface
Standard version 2.20", Public Key Cryptography Standards
PKCS#11-v2.20, 2004.
[RFC 4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)", RFC
4309, December 2005.
McGrew, et al. Informational PAGE 8
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Appendix A. Recommended Curves and Algorithms
This memo does not mandate any particular elliptic curves or
cryptographic algorithms, for the sake of flexibility. However,
since the main motivation for the AES-CCM-ECC cipher suites is their
suitability for constrained environments, it is valuable to identify
a particular suitable set of curves and algorithms.
This appendix identifies a set of elliptic curves and cryptographic
algorithms that meet the requirements of this note and that are
widely supported and believed to be secure.
The curves and hash algorithms recommended for each cipher suite are:
An implementation that includes either
TLS_ECDHE_ECDSA_WITH_AES_128_CCM or
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 SHOULD support the secp256r1
curve and the SHA-256 hash function.
An implementation that includes either
TLS_ECDHE_ECDSA_WITH_AES_256_CCM or
TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 SHOULD support the secp384r1
curve and the SHA-384 hash function, and MAY support the secp521r1
curve and the SHA-512 hash function.
More information about the secp256r1, secp384r1, and secp521r1 curves
is available in Appendix A of [RFC 4492].
It is not necessary to implement the above curves and hash functions
in order to conform to this specification. Other elliptic curves,
such as the Brainpool curves [RFC 5639], for example, meet the
criteria laid out in this memo.
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RFC 7251 AES-CCM ECC TLS June 2014
Authors' Addresses
David McGrew
Cisco Systems
13600 Dulles Technology Drive
Herndon, VA 20171
USA
EMail: mcgrew@cisco.com
Daniel V. Bailey
Ruhr-University Bochum
Universitatsstr. 150
44801 Bochum
Germany
EMail: danbailey@sth.rub.de
Matthew Campagna
Certicom Corp.
5520 Explorer Drive #400
Mississauga, Ontario L4W 5L1
Canada
EMail: mcampagna@gmail.com
Robert Dugal
Certicom Corp.
4701 Tahoe Blvd., Building A
Mississauga, Ontario L4W 0B5
Canada
EMail: rdugal@certicom.com
McGrew, et al. Informational PAGE 10
RFC TOTAL SIZE: 18851 bytes
PUBLICATION DATE: Friday, June 27th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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