RFC 5288

AES Galois Counter Mode (GCM) Cipher Suites for TLS

📅 published August 2008, from ietf

RFC 5288 Part 1 ✨

Okay bestie, here's the title block 🎀. This is RFC 5288, called AES Galois Counter Mode (GCM) Cipher Suites for TLS. It came out of the Network Working Group and was written by J. Salowey, A. Choudhury, and D. McGrew at Cisco Systems, Inc. It's a Standards Track doc, dated August 2008. The title block doesn't list anything it obsoletes or updates, so I'm not making anything up 🙅‍♀️

Status of This Memo 💅

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Abstract 🌸

This memo is about using the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS) authenticated encryption operation. GCM gives you both confidentiality and data origin authentication 🔐 It can be efficiently implemented in hardware for speeds of 10 gigabits per second and above, and it's also super well-suited to software. The memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and Diffie-Hellman-based key exchange mechanisms. Iconic.

Table of Contents 🧚

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.  Conventions Used in This Document . . . . . . . . . . . . . . . 2
3.  AES-GCM Cipher Suites . . . . . . . . . . . . . . . . . . . . . 2
4.  TLS Versions  . . . . . . . . . . . . . . . . . . . . . . . . . 3
5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
  6.1.  Counter Reuse . . . . . . . . . . . . . . . . . . . . . . . 4
  6.2.  Recommendations for Multiple Encryption Processors  . . . . 4
7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
  8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
  8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

1. Introduction 🦋

Okay so this doc is all about using AES [AES] in Galois Counter Mode (GCM) [GCM] (AES-GCM) with a bunch of different key exchange mechanisms as a cipher suite for TLS. AES-GCM is an authenticated encryption with associated data (AEAD) cipher (as defined in TLS 1.2 [RFC5246]) that gives you both confidentiality and data origin authentication. The next sections define cipher suites based on RSA, DSA, and Diffie-Hellman key exchanges. ECC-based (Elliptic Curve Cryptography) cipher suites get their own separate document [RFC5289], so go check that out too 💜

AES-GCM is not only efficient and secure, it's also a total power move for hardware. Hardware can hit high speeds with low cost and low latency, because the mode can be pipelined 🚀 Apps that need high data throughput can totally benefit from these high-speed implementations. AES-GCM has also been specified as a mode that can be used with IPsec ESP [RFC4106] and 802.1AE Media Access Control (MAC) Security [IEEE8021AE].

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. AES-GCM Cipher Suites 🍓

These cipher suites use the new authenticated encryption modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM]:

   CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
   CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
   CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
   CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
   CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
   CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
   CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
   CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
   CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
   CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
   CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
   CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}

These cipher suites use the AES-GCM authenticated encryption with associated data (AEAD) algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM described in [RFC5116]. Heads up: each of these AEAD algorithms uses a 128-bit authentication tag with GCM. Also, as described in Section 3.5 of [RFC4366], the "truncated_hmac" extension does not have an effect on cipher suites that do not use HMAC 🙈

The "nonce" SHALL be 12 bytes long, and it's made of two parts as follows. This is an example of a "partially explicit" nonce, so see Section 3.2.1 in [RFC5116] for the full story ✨

   struct {
      opaque salt[4];
      opaque nonce_explicit[8];
   } GCMNonce;

The salt is the "implicit" part of the nonce, and it's never sent in the packet. Instead, it gets generated as part of the handshake process. It's 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. The sender picks it, and it's 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. Failing this uniqueness requirement can significantly degrade security, so please don't be that person 💀 The nonce_explicit MAY be the 64-bit sequence number.

The RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS, and DH_anon key exchanges are performed as defined in [RFC5246].

The Pseudo Random Function (PRF) algorithms SHALL be as follows:

For cipher suites ending with _SHA256, the PRF is the TLS PRF [RFC5246] with SHA-256 as the hash function. For cipher suites ending with _SHA384, the PRF is the TLS PRF [RFC5246] with SHA-384 as the hash function.

Implementations MUST send TLS Alert bad_record_mac for all types of failures encountered while processing the AES-GCM algorithm. No exceptions, periodt 💅

4. TLS Versions 👑

These cipher suites make use of the authenticated encryption with additional data defined in TLS 1.2 [RFC5246]. They MUST NOT be negotiated in older versions of TLS. Clients MUST NOT offer these cipher suites if they don't offer TLS 1.2 or later. Servers that select an earlier version of TLS MUST NOT select one of these cipher suites.

Here's the tricky bit 🙃 TLS has no way for the client to say "I support TLS 1.2 but not earlier," so a non-compliant server might potentially negotiate TLS 1.1 or earlier and pick one of the cipher suites in this document. Clients MUST check the TLS version, and if they detect an incorrect version, they MUST generate a fatal "illegal_parameter" alert.

5. IANA Considerations 🌈

IANA has assigned the following values for the cipher suites defined in this document:

   CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
   CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
   CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
   CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
   CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
   CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
   CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
   CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
   CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
   CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
   CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
   CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}

6. Security Considerations 🔥

Pay attention to this one, bestie, it's serious 😭 The security considerations in [RFC5246] apply to this document as well. The rest of this section covers security considerations specific to the cipher suites described in this document.

6.1. Counter Reuse 💀

AES-GCM security requires that the counter is never reused. The IV construction in Section 3 is designed to prevent counter reuse. Implementers should also understand the practical considerations of IV handling outlined in Section 9 of [GCM].

6.2. Recommendations for Multiple Encryption Processors 🧚‍♀️

If multiple cryptographic processors are in use by the sender, the sender MUST ensure that, for a particular key, each value of the nonce_explicit used with that key is distinct. In this case, each encryption processor SHOULD include, in the nonce_explicit, a fixed value that is distinct for each processor. The recommended format is:

   nonce_explicit = FixedDistinct || Variable

The FixedDistinct field is distinct for each encryption processor, but it's fixed for a given processor. The Variable field is distinct for each distinct nonce used by a particular encryption processor. When this method is used, the FixedDistinct fields used by the different processors MUST have the same length.

In the terms of Figure 2 in [RFC5116], the Salt is the Fixed-Common part of the nonce (it's fixed, and it's common across all encryption processors). The FixedDistinct field exactly corresponds to the Fixed-Distinct field, the Variable field corresponds to the Counter field, and the explicit part exactly corresponds to the nonce_explicit.

For clarity, here's an example for TLS with two distinct encryption processors, each using a one-byte FixedDistinct field:

   Salt          = eedc68dc
   FixedDistinct = 01       (for the first encryption processor)
   FixedDistinct = 02       (for the second encryption processor)

The GCMnonces generated by the first encryption processor, and their corresponding nonce_explicit, are:

   GCMNonce                    nonce_explicit
   ------------------------    ----------------------------
   eedc68dc0100000000000000    0100000000000000
   eedc68dc0100000000000001    0100000000000001
   eedc68dc0100000000000002    0100000000000002
   ...

The GCMnonces generated by the second encryption processor, and their corresponding nonce_explicit, are:

   GCMNonce                    nonce_explicit
   ------------------------    ----------------------------
   eedc68dc0200000000000000    0200000000000000
   eedc68dc0200000000000001    0200000000000001
   eedc68dc0200000000000002    0200000000000002
   ...

7. Acknowledgements 🫶

This document borrows heavily from [RFC5289]. The authors would like to thank Alex Lam, Simon Josefsson, and Pasi Eronen for providing useful comments during the review of this document. Y'all are the real MVPs ✨

8. References 📚

The reference list continues in part 2 💖

8.1. Normative References ✨

Okay bestie, these are the normative references, meaning the docs this spec leans on so hard that you can't really read it without them 📚💖

[AES] National Institute of Standards and Technology, "Advanced Encryption Standard (AES)", FIPS 197, November 2001. 🔐
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", National Institute of Standards and Technology SP 800-38D, November 2007. 🌸
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 🎀
[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. 🔥

8.2. Informative References

These ones are the supporting cast. Helpful, good to know, not strictly required to make the spec work 🧚✨

[IEEE8021AE] Institute of Electrical and Electronics Engineers, "Media Access Control Security", IEEE Standard 802.1AE, August 2006. 🌈
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 4106, June 2005. 💜
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366, April 2006. 🍓
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode", RFC 5289, August 2008. 👑

Authors' Addresses ✨

Meet the squad who built this 🫶 Here's where you can find them:

Joseph Salowey
Cisco Systems, Inc.
2901 3rd. Ave
Seattle, WA  98121
USA

EMail: jsalowey@cisco.com


Abhijit Choudhury
Cisco Systems, Inc.
3625 Cisco Way
San Jose, CA  95134
USA

EMail: abhijitc@cisco.com


David McGrew
Cisco Systems, Inc.
170 W Tasman Drive
San Jose, CA  95134
USA

EMail: mcgrew@cisco.com

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