AES-CCM for TLS: AES-CCM Cipher Suites for Transport Layer Security (TLS)
Sixteen TLS cipher suites that use AES in CCM mode, built for small devices.
McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for Transport Layer Security (TLS)", RFC 6655, DOI 10.17487/RFC6655, July 2012, <https://www.rfc-editor.org/info/rfc6655>.
@misc{rfc6655,
series = {Request for Comments},
number = 6655,
howpublished = {RFC 6655},
publisher = {RFC Editor},
doi = {10.17487/RFC6655},
url = {https://www.rfc-editor.org/info/rfc6655},
author = {D. McGrew and D. Bailey},
title = {{AES-CCM Cipher Suites for Transport Layer Security (TLS)}},
pagetotal = 8,
year = 2012,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 6655 Cisco Systems
Category: Standards Track D. Bailey
ISSN: 2070-1721 RSA, Security Division of EMC
July 2012
AES-CCM Cipher Suites for Transport Layer Security (TLS)
Abstract
This memo describes the use of the Advanced Encryption Standard (AES)
in the Counter with Cipher Block Chaining - Message Authentication
Code (CBC-MAC) Mode (CCM) of operation within Transport Layer
Security (TLS) and Datagram TLS (DTLS) to provide confidentiality and
data origin authentication. The AES-CCM algorithm is amenable to
compact implementations, making it suitable for constrained
environments.
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 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/rfc6655.
Copyright Notice
Copyright (c) 2012 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
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. RSA-Based AES-CCM Cipher Suites . . . . . . . . . . . . . . . . 3
4. PSK-Based AES-CCM Cipher Suites . . . . . . . . . . . . . . . . 5
5. TLS Versions . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. New AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . 5
6.1. AES-128-CCM with an 8-Octet Integrity Check Value (ICV) . . 6
6.2. AES-256-CCM with a 8-Octet Integrity Check Value (ICV) . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8.1. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . 6
8.2. Counter Reuse . . . . . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document describes the use of Advanced Encryption Standard (AES)
[AES] in Counter with CBC-MAC Mode (CCM) [CCM] in several TLS
ciphersuites. AES-CCM provides both authentication and
confidentiality and uses as its only primitive the AES encrypt
operation (the AES decrypt operation is not needed). This makes it
amenable to compact implementations, which is 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 IPsec
Encapsulating Security Payload (ESP) [RFC4309] 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 [RFC5116], adds the ability to check the integrity and
authenticity of some associated data that is not encrypted. This
document utilizes the AEAD facility within TLS 1.2 [RFC5246] and the
AES-CCM-based AEAD algorithms defined in [RFC5116]. Additional AEAD
algorithms are defined that use AES-CCM but 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 ciphersuites defined in this document use RSA or Pre-Shared Key
(PSK) as their key establishment mechanism; these ciphersuites can be
used with DTLS [RFC6347]. Since the ability to use AEAD ciphers was
introduced in DTLS version 1.2, the ciphersuites defined in this
document cannot be used with earlier versions of that protocol.
2. 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 [RFC2119].
3. RSA-Based AES-CCM Cipher Suites
The ciphersuites 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 [RFC5116]. The
following RSA-based ciphersuites are defined:
CipherSuite TLS_RSA_WITH_AES_128_CCM = {0xC0,0x9C}
CipherSuite TLS_RSA_WITH_AES_256_CCM = {0xC0,0x9D)
CipherSuite TLS_DHE_RSA_WITH_AES_128_CCM = {0xC0,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_CCM = {0xC0,0x9F}
CipherSuite TLS_RSA_WITH_AES_128_CCM_8 = {0xC0,0xA0}
CipherSuite TLS_RSA_WITH_AES_256_CCM_8 = {0xC0,0xA1)
CipherSuite TLS_DHE_RSA_WITH_AES_128_CCM_8 = {0xC0,0xA2}
CipherSuite TLS_DHE_RSA_WITH_AES_256_CCM_8 = {0xC0,0xA3}
These ciphersuites make use of the AEAD capability in TLS 1.2
[RFC5246]. Each uses AES-CCM; those that end in "_8" have an 8-octet
authentication tag, while the other ciphersuites have 16-octet
authentication tags.
The Hashed Message Authentication Code (HMAC) truncation option
described in Section 7 of [RFC6066] (which negotiates the
"truncated_hmac" TLS extension) does not have an effect on cipher
suites that do not use HMAC.
The "nonce" input to the AEAD algorithm is exactly that of [RFC5288]:
the "nonce" SHALL be 12 bytes long and is constructed as follows:
(this is an example of a "partially explicit" nonce; see Section
3.2.1 in [RFC5116]).
struct {
opaque salt[4];
opaque nonce_explicit[8];
} CCMNonce;
The salt is the "implicit" part of the nonce and is not sent in the
packet. Instead, the salt is generated as part of the handshake
process: it is either the client_write_IV (when the client is
sending) or the server_write_IV (when the server is sending). The
salt length (SecurityParameters.fixed_iv_length) is 4 octets. The
nonce_explicit is the "explicit" part of the nonce. It is chosen by
the sender and is carried in each TLS record in the
GenericAEADCipher.nonce_explicit field. The nonce_explicit length
(SecurityParameters.record_iv_length) is 8 octets. Each value of the
nonce_explicit MUST be distinct for each distinct invocation of the
GCM encrypt function for any fixed key. Failure to meet this
uniqueness requirement can significantly degrade security. The
nonce_explicit MAY be the 64-bit sequence number (as long as those
values are assured to meet the distinctness requirement).
In DTLS, the 64-bit seq_num is the 16-bit epoch concatenated with the
48-bit seq_num.
When the nonce_explicit is equal to the sequence number, the CCMNonce
will have the structure of the CCMNonceExample given below.
struct {
uint32 client_write_IV; // low order 32-bits
uint64 seq_num; // TLS sequence number
} CCMClientNonce.
struct {
uint32 server_write_IV; // low order 32-bits
uint64 seq_num; // TLS sequence number
} CCMServerNonce.
struct {
case client:
CCMClientNonce;
case server:
CCMServerNonce:
} CCMNonceExample;
These ciphersuites make use of the default TLS 1.2 Pseudorandom
Function (PRF), which uses HMAC with the SHA-256 hash function. The
RSA and DHE_RSA, key exchange is performed as defined in [RFC5246].
4. PSK-Based AES-CCM Cipher Suites
As in Section 3, these ciphersuites follow [RFC5116]. The PSK and
DHE_PSK key exchange is performed as in [RFC4279]. The following
ciphersuites are defined:
CipherSuite TLS_PSK_WITH_AES_128_CCM = {0xC0,0xA4}
CipherSuite TLS_PSK_WITH_AES_256_CCM = {0xC0,0xA5)
CipherSuite TLS_DHE_PSK_WITH_AES_128_CCM = {0xC0,0xA6}
CipherSuite TLS_DHE_PSK_WITH_AES_256_CCM = {0xC0,0xA7}
CipherSuite TLS_PSK_WITH_AES_128_CCM_8 = {0xC0,0xA8}
CipherSuite TLS_PSK_WITH_AES_256_CCM_8 = {0xC0,0xA9)
CipherSuite TLS_PSK_DHE_WITH_AES_128_CCM_8 = {0xC0,0xAA}
CipherSuite TLS_PSK_DHE_WITH_AES_256_CCM_8 = {0xC0,0xAB}
The "nonce" input to the AEAD algorithm is defined as in Section 3.
These ciphersuites make use of the default TLS 1.2 Pseudorandom
Function (PRF), which uses HMAC with the SHA-256 hash function. The
PSK and DHE_PSK key exchange is performed as defined in [RFC5487].
5. TLS Versions
These ciphersuites make use of the authenticated encryption with
additional data (AEAD) defined in TLS 1.2 [RFC5288]. Earlier
versions of TLS do not have support for AEAD; for instance, the
TLSCiphertext structure does not have the "aead" option in TLS 1.1.
Consequently, these ciphersuites 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. Because
TLS has no way for the client to indicate that it supports TLS 1.2
but not earlier, 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.
6. New AEAD Algorithms
The following AEAD algorithms are defined:
AEAD_AES_128_CCM_8 = 18
AEAD_AES_256_CCM_8 = 19
6.1. AES-128-CCM with an 8-Octet Integrity Check Value (ICV)
The AEAD_AES_128_CCM_8 authenticated encryption algorithm is
identical to the AEAD_AES_128_CCM algorithm (see Section 5.3 of
[RFC5116]), except that it uses 8 octets for authentication, instead
of the full 16 octets used by AEAD_AES_128_CCM. The
AEAD_AES_128_CCM_8 ciphertext consists of the ciphertext output of
the CCM encryption operation concatenated with the 8-octet
authentication tag output of the CCM encryption operation. Test
cases are provided in [CCM]. The input and output lengths are the
same as those for AEAD_AES_128_CCM. An AEAD_AES_128_CCM_8 ciphertext
is exactly 8 octets longer than its corresponding plaintext.
6.2. AES-256-CCM with a 8-Octet Integrity Check Value (ICV)
The AEAD_AES_256_CCM_8 authenticated encryption algorithm is
identical to the AEAD_AES_256_CCM algorithm (see Section 5.4 of
[RFC5116]), except that it uses 8 octets for authentication, instead
of the full 16 octets used by AEAD_AES_256_CCM. The
AEAD_AES_256_CCM_8 ciphertext consists of the ciphertext output of
the CCM encryption operation concatenated with the 8-octet
authentication tag output of the CCM encryption operation. Test
cases are provided in [CCM]. The input and output lengths are as for
AEAD_AES_128_CCM. An AEAD_AES_128_CCM_8 ciphertext is exactly 8
octets longer than its corresponding plaintext.
7. IANA Considerations
IANA has assigned the values for the ciphersuites defined in Sections
3 and 4 from the "TLS Cipher Suite" registry and the values of the
AEAD algorithms defined in Section 6 from the "AEAD Algorithms"
registry.
8. Security Considerations
8.1. Perfect Forward Secrecy
The perfect forward secrecy properties of RSA-based TLS ciphersuites
are discussed in the security analysis of [RFC5246]. It should be
noted that only the ephemeral Diffie-Hellman-based ciphersuites are
capable of providing perfect forward secrecy.
8.2. Counter Reuse
AES-CCM security requires that the counter is never reused. The IV
construction in Section 3 is designed to prevent counter reuse.
9. Acknowledgements
This document borrows heavily from [RFC5288]. Thanks are due to
Stephen Farrell and Robert Cragie for their input.
10. References
10.1. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, December 2005.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption", RFC 5116, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC5487] Badra, M., "Pre-Shared Key Cipher Suites for TLS with
SHA-256/384 and AES Galois Counter Mode", RFC 5487,
March 2009.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
January 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
10.2. Informative References
[IEEE802154] Institute of Electrical and Electronics Engineers,
"Wireless Personal Area Networks", IEEE
Standard 802.15.4-2006, 2006.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES)
CCM Mode with IPsec Encapsulating Security Payload
(ESP)", RFC 4309, December 2005.
Authors' Addresses
David McGrew
Cisco Systems
13600 Dulles Technology Drive
Herndon, VA 20171
USA
EMail: mcgrew@cisco.com
Daniel V. Bailey
RSA, Security Division of EMC
174 Middlesex Tpke.
Bedford, MA 01463
USA
EMail: dbailey@rsa.com
ISO Plain Language
On this page
§1Introduction
This document describes the use of the Advanced Encryption Standard in Counter with CBC-MAC Mode, or CCM, in several TLS ciphersuites. AES-CCM provides both authentication and confidentiality and uses as its only primitive the AES encrypt operation, because the AES decrypt operation is not needed. This makes it amenable to compact implementations, which is advantageous in constrained environments. 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 IPsec Encapsulating Security Payload and for 802.15.4 wireless networks.
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, adds the ability to check the integrity and authenticity of some associated data that is not encrypted. This document uses the AEAD facility within TLS 1.2 and the AES-CCM-based AEAD algorithms defined in RFC 5116. Additional AEAD algorithms are defined that use AES-CCM but 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 ciphersuites defined in this document use RSA or Pre-Shared Key as their key establishment mechanism, and can be used with DTLS. Since the ability to use AEAD ciphers was introduced in DTLS version 1.2, the ciphersuites defined in this document cannot be used with earlier versions of that protocol.
Some small devices have to run TLS but do not have room for a large crypto library. AES-CCM helps them: it encrypts and checks the message in one operation, and it only needs the encrypt direction of the AES cipher, never the decrypt direction. This document defines the TLS cipher suites that use it, including versions with a shorter check tag for networks where every byte counts.
§3RSA-Based AES-CCM Cipher Suites
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SALT | salt | 4 bytes | The implicit part, 4 octets, not sent in the packet. It is the client_write_IV or the server_write_IV. | |
NONCE-EXPLICIT | nonce_explicit | 8 bytes | The explicit part, 8 octets, carried in each TLS record. Each value must be distinct for each distinct invocation for any fixed key. |
The ciphersuites are based on the AES-CCM AEAD algorithms AEAD_AES_128_CCM and AEAD_AES_256_CCM described in RFC 5116. Eight RSA-based ciphersuites are defined: TLS_RSA_WITH_AES_128_CCM at 0xC0,0x9C; TLS_RSA_WITH_AES_256_CCM at 0xC0,0x9D; TLS_DHE_RSA_WITH_AES_128_CCM at 0xC0,0x9E; TLS_DHE_RSA_WITH_AES_256_CCM at 0xC0,0x9F; TLS_RSA_WITH_AES_128_CCM_8 at 0xC0,0xA0; TLS_RSA_WITH_AES_256_CCM_8 at 0xC0,0xA1; TLS_DHE_RSA_WITH_AES_128_CCM_8 at 0xC0,0xA2; and TLS_DHE_RSA_WITH_AES_256_CCM_8 at 0xC0,0xA3.
Each uses AES-CCM; those that end in _8 have an 8-octet authentication tag, while the other ciphersuites have 16-octet authentication tags. The HMAC truncation option described in RFC 6066, which negotiates the truncated_hmac TLS extension, does not have an effect on cipher suites that do not use HMAC.
The nonce input to the AEAD algorithm is exactly that of the AES-GCM cipher suites: it SHALL be 12 bytes long and is constructed from a 4-octet salt and an 8-octet nonce_explicit. The salt is the implicit part of the nonce and is not sent in the packet. Instead, the salt is generated as part of the handshake process: it is either the client_write_IV when the client is sending, or the server_write_IV when the server is sending. The nonce_explicit is the explicit part of the nonce, chosen by the sender and carried in each TLS record in the GenericAEADCipher.nonce_explicit field. Each value of the nonce_explicit MUST be distinct for each distinct invocation of the encrypt function for any fixed key, and failure to meet this uniqueness requirement can significantly degrade security. The nonce_explicit MAY be the 64-bit sequence number, as long as those values are assured to meet the distinctness requirement. In DTLS the 64-bit seq_num is the 16-bit epoch concatenated with the 48-bit seq_num.
These ciphersuites make use of the default TLS 1.2 pseudorandom function, which uses HMAC with the SHA-256 hash function, and the RSA and DHE_RSA key exchange is performed as defined in the TLS 1.2 specification.
Eight RSA-based suites are defined, in four pairs: one for each combination of key exchange and key size, with a full-length or shortened tag. The nonce is 12 bytes long. Four of those bytes are the salt, which comes from the handshake and never travels in the packet, and eight are the explicit part, which the sender puts in each record and which must never repeat for a given key.
- 0xC0,0x9CTLS_RSA_WITH_AES_128_CCM — RSA key exchange, AES-128 in CCM mode, 16-octet tag.
- 0xC0,0x9DTLS_RSA_WITH_AES_256_CCM — RSA key exchange, AES-256 in CCM mode, 16-octet tag.
- 0xC0,0x9ETLS_DHE_RSA_WITH_AES_128_CCM — Ephemeral Diffie-Hellman with RSA, AES-128 in CCM mode, 16-octet tag.
- 0xC0,0x9FTLS_DHE_RSA_WITH_AES_256_CCM — Ephemeral Diffie-Hellman with RSA, AES-256 in CCM mode, 16-octet tag.
- 0xC0,0xA0TLS_RSA_WITH_AES_128_CCM_8 — RSA key exchange, AES-128 in CCM mode, 8-octet tag.
- 0xC0,0xA1TLS_RSA_WITH_AES_256_CCM_8 — RSA key exchange, AES-256 in CCM mode, 8-octet tag.
- 0xC0,0xA2TLS_DHE_RSA_WITH_AES_128_CCM_8 — Ephemeral Diffie-Hellman with RSA, AES-128 in CCM mode, 8-octet tag.
- 0xC0,0xA3TLS_DHE_RSA_WITH_AES_256_CCM_8 — Ephemeral Diffie-Hellman with RSA, AES-256 in CCM mode, 8-octet tag.
§4PSK-Based AES-CCM Cipher Suites
As in the previous section, these ciphersuites follow RFC 5116, and the PSK and DHE_PSK key exchange is performed as in RFC 4279. Eight PSK-based ciphersuites are defined: TLS_PSK_WITH_AES_128_CCM at 0xC0,0xA4; TLS_PSK_WITH_AES_256_CCM at 0xC0,0xA5; TLS_DHE_PSK_WITH_AES_128_CCM at 0xC0,0xA6; TLS_DHE_PSK_WITH_AES_256_CCM at 0xC0,0xA7; TLS_PSK_WITH_AES_128_CCM_8 at 0xC0,0xA8; TLS_PSK_WITH_AES_256_CCM_8 at 0xC0,0xA9; TLS_PSK_DHE_WITH_AES_128_CCM_8 at 0xC0,0xAA; and TLS_PSK_DHE_WITH_AES_256_CCM_8 at 0xC0,0xAB.
The nonce input to the AEAD algorithm is defined as in the previous section. These ciphersuites make use of the default TLS 1.2 pseudorandom function, which uses HMAC with the SHA-256 hash function, and the PSK and DHE_PSK key exchange is performed as defined in RFC 5487.
Eight more suites are defined for the pre-shared key key exchange, with the same AEAD algorithms, the same nonce and the same tag lengths as the RSA-based ones. They exist so that a device that already shares a secret with the server can use the same small cipher without any public-key arithmetic.
- 0xC0,0xA4TLS_PSK_WITH_AES_128_CCM — Pre-shared key, AES-128 in CCM mode, 16-octet tag.
- 0xC0,0xA5TLS_PSK_WITH_AES_256_CCM — Pre-shared key, AES-256 in CCM mode, 16-octet tag.
- 0xC0,0xA6TLS_DHE_PSK_WITH_AES_128_CCM — Ephemeral Diffie-Hellman with a pre-shared key, AES-128 in CCM mode, 16-octet tag.
- 0xC0,0xA7TLS_DHE_PSK_WITH_AES_256_CCM — Ephemeral Diffie-Hellman with a pre-shared key, AES-256 in CCM mode, 16-octet tag.
- 0xC0,0xA8TLS_PSK_WITH_AES_128_CCM_8 — Pre-shared key, AES-128 in CCM mode, 8-octet tag.
- 0xC0,0xA9TLS_PSK_WITH_AES_256_CCM_8 — Pre-shared key, AES-256 in CCM mode, 8-octet tag.
- 0xC0,0xAATLS_PSK_DHE_WITH_AES_128_CCM_8 — Pre-shared key with ephemeral Diffie-Hellman, AES-128 in CCM mode, 8-octet tag.
- 0xC0,0xABTLS_PSK_DHE_WITH_AES_256_CCM_8 — Pre-shared key with ephemeral Diffie-Hellman, AES-256 in CCM mode, 8-octet tag.
§5TLS Versions
These ciphersuites make use of the authenticated encryption with additional data defined in TLS 1.2. Earlier versions of TLS do not have support for AEAD; for instance, the TLSCiphertext structure does not have the aead option in TLS 1.1. Consequently, these ciphersuites 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, and servers that select an earlier version of TLS MUST NOT select one of these cipher suites. Because TLS has no way for the client to indicate that it supports TLS 1.2 but not earlier, 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.
These suites need the AEAD support that arrived in TLS 1.2, so they cannot be used with older versions. A client must not offer them unless it also offers TLS 1.2 or later, and a server that picks an older version must not choose one of them. Because a client cannot say which versions it supports, it must check the version that was actually negotiated and abort with an illegal_parameter alert if it is too old.
§6New AEAD Algorithms
Two AEAD algorithms are defined: AEAD_AES_128_CCM_8, value 18, and AEAD_AES_256_CCM_8, value 19.
The AEAD_AES_128_CCM_8 authenticated encryption algorithm is identical to the AEAD_AES_128_CCM algorithm, except that it uses 8 octets for authentication instead of the full 16 octets used by AEAD_AES_128_CCM. Its ciphertext consists of the ciphertext output of the CCM encryption operation concatenated with the 8-octet authentication tag output of that operation. Test cases are provided in the CCM specification. The input and output lengths are the same as those for AEAD_AES_128_CCM, and an AEAD_AES_128_CCM_8 ciphertext is exactly 8 octets longer than its corresponding plaintext.
The AEAD_AES_256_CCM_8 authenticated encryption algorithm is identical to the AEAD_AES_256_CCM algorithm, except that it uses 8 octets for authentication instead of the full 16 octets used by AEAD_AES_256_CCM. Its ciphertext consists of the ciphertext output of the CCM encryption operation concatenated with the 8-octet authentication tag output of that operation. The input and output lengths are as for AEAD_AES_128_CCM, and an AEAD_AES_128_CCM_8 ciphertext is exactly 8 octets longer than its corresponding plaintext.
Two new AEAD algorithm identifiers are added, values 18 and 19. They are the existing AES-128 and AES-256 CCM algorithms with the authentication tag cut from 16 octets to 8. Everything else, including the input and output lengths, stays the same, so the ciphertext ends up 8 octets longer than the plaintext.
- AEAD_AES_128_CCM_8, value 18AES-128 in CCM mode with an 8-octet authentication tag instead of 16.
- AEAD_AES_256_CCM_8, value 19AES-256 in CCM mode with an 8-octet authentication tag instead of 16.
- OverheadA CCM_8 ciphertext is exactly 8 octets longer than its plaintext.
§7IANA Considerations
IANA has assigned the values for the ciphersuites defined in Sections 3 and 4 from the TLS Cipher Suite registry, and the values of the AEAD algorithms defined in Section 6 from the AEAD Algorithms registry.
IANA has registered the sixteen cipher suite numbers and the two new AEAD algorithm numbers.
§8Security Considerations
The perfect forward secrecy properties of RSA-based TLS ciphersuites are discussed in the security analysis of the TLS 1.2 specification, and it should be noted that only the ephemeral Diffie-Hellman-based ciphersuites are capable of providing perfect forward secrecy.
AES-CCM security requires that the counter is never reused. The IV construction in Section 3 is designed to prevent counter reuse.
Only the suites that use ephemeral Diffie-Hellman give forward secrecy, so the plain RSA suites do not. And AES-CCM is only safe if the counter never repeats, which is why the nonce construction is fixed the way it is.