AES-CCM ECC Cipher Suites for TLS
draft-mcgrew-tls-aes-ccm-ecc-01

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. The ciphersuites defined in this document use Elliptic Curve Cryptography (ECC), and are intended for use in networks with limited bandwidth.

Status of this Memo

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Table of Contents

1.  Introduction
    1.1.  Conventions Used In This Document

2.  ECC based AES-CCM Cipher Suites
    2.1.  Required Algorithms for each CipherSuite

3.  TLS Versions

4.  New AEAD algorithms
    4.1.  AES-128-CCM with an 8-octet ICV
    4.2.  AES-256-CCM with an 8-octet ICV

5.  IANA Considerations

6.  Security Considerations
    6.1.  Perfect Forward Secrecy
    6.2.  Counter Reuse

7.  Acknowledgements

8.  References
    8.1.  Normative References
    8.2.  Informative References

§  Authors' Addresses

1.  Introduction

This document describes the use of Advanced Encryption Standard (AES) [AES] (National Institute of Standards and Technology, “Specification for the Advanced Encryption Standard (AES),” November 2001.) in Counter with CBC-MAC Mode (CCM) [CCM] (National Institute of Standards and Technology, “Recommendation for Block Cipher Modes of Operation: The CCM Mode for Authentication and Confidentiality,” May 2004.) 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. The use of AES-CCM has been specified for IPsec ESP [RFC4309] (Housley, R., “Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP),” December 2005.) and 802.15.4 wireless networks [IEEE802154] (Institute of Electrical and Electronics Engineers, “Wireless Personal Area Networks,” 2006.).

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] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.), adds the ability to check the integrity and authenticity of some associated data that is not encrypted. This note utilizes the AEAD facility within TLS 1.2 [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) and the AES-CCM-based AEAD algorithms defined in [RFC5116] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.). Additional AEAD algorithms are defined in this note; these use AES-CCM but have shorter authentication tags, and therefore are more suitable for use across networks in which bandwidth is constrained and message sizes may be small.

The ciphersuites defined in this document use Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) as their key establishment mechanism; these ciphersuites can be used with DTLS [I‑D.ietf‑tls‑rfc4347‑bis] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security version 1.2,” October 2009.).

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)

2.  ECC 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] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.). The following ciphersuites are defined:

CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM = {TBD1,TBD1} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CCM = {TBD2,TBD2)
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 = {TBD3,TBD3}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 = {TBD4,TBD4)

These ciphersuites make use of the AEAD capability in TLS 1.2 [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.). Note that each of these AEAD algorithms uses AES-CCM. Ciphersuites ending with "8" use eight-octet authentication tags; the other ciphersuites have 16 octet authentication tags.

The HMAC truncation option described in Section 3.5 of [RFC4366] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.) (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 defined as in [RFC5288] (Salowey, J., Choudhury, A., and D. McGrew, “AES Galois Counter Mode (GCM) Cipher Suites for TLS,” August 2008.). The "nonce" SHALL be 12 bytes long and constructed as follows:

struct {
   case client:
      uint32 client_write_IV;  // low order 32-bits
   case server:
      uint32 server_write_IV;  // low order 32-bits
   uint64 seq_num;
} CCMNonce.

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 ciphersuites 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 [RFC4492] (Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, “Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS),” May 2006.), with the following additional stipulations:

The curves secp256r1 and secp384r1 MUST be supported, and the curve secp521r1 MAY be supported; these curves are equivalent to the NIST P-256, P-384, and P-521 curves. Note that all of these curves have cofactor equal to one, which simplifies their use.

The server's certificate MUST contain an ECDSA-capable public key, it MUST be signed with ECDSA, and it MUST use SHA-256, SHA-384, or SHA-512. The Signature Algorithms extension (Section 7.4.1.4.1 of [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.)) MUST be used to indicate support of those signature and hash algorithms. If a client certificate is used, the same conditions apply to it. The acceptable choices of hashes and curves that can be used with each ciphersuite are detailed in Section 2.1 (Required Algorithms for each CipherSuite).

The uncompressed point format MUST be supported. Other point formats MAY be used.

The client MUST offer the elliptic_curves extension and the server MUST expect to receive it.

The client MAY offer the ec_point_formats extension, but the server need not expect to receive it.

[I‑D.mcgrew‑fundamental‑ecc] (McGrew, D., Igoe, K., and M. Salter, “Fundamental Elliptic Curve Cryptography Algorithms,” December 2010.) MAY be used as an implementation method.

Implementations of these ciphersuites will interoperate with [RFC4492] (Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, “Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS),” May 2006.), but can be more compact than a full implementation of that RFC.

2.1.  Required Algorithms for each CipherSuite

The curves and hash algorithms that can be used with each ciphersuite are described in the following table.

3.  TLS Versions

These ciphersuites make use of the authenticated encryption with additional data defined in TLS 1.2 [RFC5288] (Salowey, J., Choudhury, A., and D. McGrew, “AES Galois Counter Mode (GCM) Cipher Suites for TLS,” August 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.

4.  New AEAD algorithms

The following AEAD algorithms are defined:

AEAD_AES_128_CCM_8 = TBD9 AEAD_AES_256_CCM_8 = TBD10
AEAD_AES_128_CCM_12 = TBD11
AEAD_AES_256_CCM_12 = TBD12

4.1.  AES-128-CCM with an 8-octet 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] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.)), except that it uses eight octets for authentication, instead of the full sixteen 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 as for AEAD_AES_128_CCM. An AEAD_AES_128_CCM_8 ciphertext is exactly 8 octets longer than its corresponding plaintext.

4.2.  AES-256-CCM with an 8-octet 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] (McGrew, D., “An Interface and Algorithms for Authenticated Encryption,” January 2008.)), except that it uses eight octets for authentication, instead of the full sixteen 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.

5.  IANA Considerations

IANA has assigned values for the Ciphersuites defined in Section 2 (ECC based AES-CCM Cipher Suites) and the AEAD algorithms defined in Section 4 (New AEAD algorithms) of this note.

6.  Security Considerations

6.1.  Perfect Forward Secrecy

The perfect forward secrecy properties of ephemeral Diffie-Hellman ciphersuites are discussed in the security analysis of [RFC4346] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.1,” April 2006.). This analysis applies to the ECDHE ciphersuites.

6.2.  Counter Reuse

AES-CCM security requires that the counter is never reused. The IV construction in Section 2 (ECC based AES-CCM Cipher Suites) is designed to prevent counter reuse.

7.  Acknowledgements

This draft borrows heavily from [RFC5288] (Salowey, J., Choudhury, A., and D. McGrew, “AES Galois Counter Mode (GCM) Cipher Suites for TLS,” August 2008.).

This draft is motivated by the considerations raised in the Zigbee Smart Energy 2.0 working group.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses