AES-GCM Cipher Suites for TLS
draft-ietf-tls-rsa-aes-gcm-03

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Abstract

This memo describes the use of the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS) authenticated encryption operation. GCM provides both confidentiality and data origin authentication, can be efficiently implemented in hardware for speeds of 10 gigabits per second and above, and is also well-suited to software implementations. This memo defines TLS cipher suites that use AES-GCM with RSA, DSS and Diffie-Hellman based key exchange mechanisms.

Table of Contents

1.  Introduction

2.  Conventions Used In This Document

3.  AES-GCM Cipher Suites

4.  TLS Versions

5.  IANA Considerations

6.  Security Considerations
    6.1.  Counter Reuse
    6.2.  Recommendations for Multiple Encryption Processors

7.  Acknowledgements

8.  References
    8.1.  Normative References
    8.2.  Informative References

§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

This document describes the use of AES [AES] (National Institute of Standards and Technology, “Advanced Encryption Standard (AES),” November 2001.) in Galois Counter Mode (GCM) [GCM] (Dworkin, M., “Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC,” November 2007.) (AES-GCM) with various 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 [I‑D.ietf‑tls‑rfc4346‑bis] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.)) providing both confidentiality and data origin authentication. The following sections define cipher suites based on RSA, DSS and Diffie-Hellman key exchanges; ECC based cipher suites are defined in a separate document [I‑D.ietf‑tls‑ecc‑new‑mac] (Rescorla, E., “TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode,” May 2008.).

AES-GCM is not only efficient and secure, but hardware implementations can achieve high speeds with low cost and low latency, because the mode can be pipelined. Applications that require high data throughput can benefit from these high-speed implementations. AES-GCM has been specified as a mode that can be used with IPsec ESP [RFC4106] (Viega, J. and D. McGrew, “The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP),” June 2005.) and 802.1AE MAC Security [IEEE8021AE] (Institute of Electrical and Electronics Engineers, “Media Access Control Security,” August 2006.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  AES-GCM Cipher Suites

The following cipher suites use the new authenticated encryption modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM] (Dworkin, M., “Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC,” November 2007.):

CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {TBD,TBD} CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {TBD,TBD}

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] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.). Note that each of these AEAD algorithms uses a 128-bit authentication tag with GCM. The "nonce" SHALL be 12 bytes long consisting of two parts as follows: (this is an example of a "partially explicit" nonce; see section 3.2.1 in [RFC5116] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.)).

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

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 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.

The RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS, and DH_anon key exchanges are performed as defined in [I‑D.ietf‑tls‑rfc4346‑bis] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.).

The PRF algorithms SHALL be as follows:

For cipher suites ending with _SHA256, the PRF is the TLS PRF [I‑D.ietf‑tls‑rfc4346‑bis] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.) with SHA-256 as the hash function.

For cipher suites ending with _SHA384, the PRF is the TLS PRF [I‑D.ietf‑tls‑rfc4346‑bis] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.) with SHA-384 as the hash function.

Implementations MUST send TLS Alert bad_record_mac for all types of failures encountered in processing the AES-GCM algorithm.

4.  TLS Versions

These cipher suites make use of the authenticated encryption with additional data defined in TLS 1.2 [I‑D.ietf‑tls‑rfc4346‑bis] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.). 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 which 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.

5.  IANA Considerations

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

CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {TBD,TBD} CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {TBD,TBD}
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {TBD,TBD}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {TBD,TBD}

6.  Security Considerations

The security considerations in [I‑D.ietf‑tls‑rfc4346‑bis] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” March 2008.) apply to this document as well. The remainder of this section describes 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 (AES-GCM Cipher Suites) is designed to prevent counter reuse.

6.2.  Recommendations for Multiple Encryption Processors

If multiple cryptographic processors are in use by the sender, then 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

where the FixedDistinct field is distinct for each encryption processor, but is fixed for a given processor, and 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] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.), the Salt is the Fixed-Common part of the nonce (it is fixed, and it is common across all encryption processors), the FixedDistinct field exactly corresponds to the Fixed-Distinct field, and the Variable field corresponds to the Counter field, and the explicit part exactly corresponds to the nonce_explicit.

For clarity, we provide an example for TLS in which there are two distinct encryption processors, each of which uses 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 draft borrows heavily from [I‑D.ietf‑tls‑ecc‑new‑mac] (Rescorla, E., “TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode,” May 2008.). The authors would like to thank Alex Lam, Simon Josefsson and Pasi Eronen for providing useful comments during the review of this draft.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses

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