Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track March 31, 2015
Expires: October 2, 2015

ChaCha20, Poly1305 and their use in IKE & IPsec
draft-ietf-ipsecme-chacha20-poly1305-01

Abstract

This document describes the use of the ChaCha20 stream cipher along with the Poly1305 authenticator, combined into an AEAD algorithm for IPsec.

Status of This Memo

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Table of Contents

1. Introduction

The Advanced Encryption Standard (AES - [FIPS-197]) has become the gold standard in encryption. Its efficient design, wide implementation, and hardware support allow for high performance in many areas, including IPsec VPNs. On most modern platforms, AES is anywhere from 4x to 10x as fast as the previous most-used cipher, 3-key Data Encryption Standard (3DES - [FIPS-46]), which makes it not only the best choice, but the only choice.

The problem is that if future advances in cryptanalysis reveal a weakness in AES, VPN users will be in an unenviable position. With the only other widely supported cipher being the much slower 3DES, it is not feasible to re-configure IPsec installations to use 3DES. [standby-cipher] describes this issue and the need for a standby cipher in greater detail.

This document proposes the ChaCha20 stream cipher as such a standby cipher in an Authenticated Encryption with Associated Data (AEAD) construction with the Poly1305 authenticator for use with the Encapsulated Security Protocol (ESP - [RFC4303]) and the Internet Key Exchange Protocol (IKEv2 - [RFC7296]). The algorithms are described in a separate document ([chacha_poly]). This document only describes the IPsec-specific things.

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].

2. ChaCha20 & Poly1305 for ESP

AEAD_CHACHA20_POLY1305 is a combined mode algorithm, or AEAD. The construction follows the AEAD construction in section 2.7 of [chacha_poly]:

The encryption algorithm transform ID for negotiating this algorithm in IKE is TBA by IANA.

2.1. AAD Construction

The construction of the Additional Authenticated Data (AAD) is similar to the one in [RFC4106]. For security associations (SAs) with 32-bit sequence numbers the AAD is 8 bytes: 4-byte SPI followed by 4-byte sequence number ordered exactly as it is in the packet. For SAs with ESN the AAD is 12 bytes: 4-byte SPI followed by an 8-byte sequence number as a 64-bit network order integer.

3. Use in IKEv2

AEAD algorithms can be used in IKE, as described in [RFC5282]. More specifically, the Encrypted Payload is as described in section 3 of that document, the IV is 64 bits, as described in Section 2, and the AAD is as described in section 5.1 of RFC 5282, so it's 32 bytes (28 for the IKEv2 header + 4 bytes for the encrypted payload header) assuming no unencrypted payloads.

4. Security Considerations

The ChaCha20 cipher is designed to provide 256-bit security.

The Poly1305 authenticator is designed to ensure that forged messages are rejected with a probability of 1-(n/(2^102)) for a 16n-byte message, even after sending 2^64 legitimate messages, so it is SUF-CMA in the terminology of [AE].

The most important security consideration in implementing this draft is the uniqueness of the nonce used in ChaCha20. The nonce should be selected uniquely for a particular key, but unpredictability of the nonce is not required. Counters and LFSRs are both acceptable ways of generating unique nonces, as is encrypting a counter using a 64-bit cipher such as DES. Note that it is not acceptable to use a truncation of a counter encrypted with a 128-bit or 256-bit cipher, because such a truncation may repeat after a short time.

Another issue with implementing these algorithms is avoiding side channels. This is trivial for ChaCha20, but requires some care for Poly1305. Considerations for implementations of these algorithms are in the [chacha_poly] document.

5. IANA Considerations

IANA is requested to assign one value from the IKEv2 "Transform Type 1 - Encryption Algorithm Transform IDs" registry, with name ENCR_ChaCha20_Poly1305, and this document as reference.

6. Acknowledgements

All of the algorithms in this document were designed by D. J. Bernstein. The AEAD construction was designed by Adam Langley. The author would also like to thank Adam for helpful comments, as well as Yaron Sheffer for telling me to write the algorithms draft.

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.
[RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption Algorithms with the Encrypted Payload of the Internet Key Exchange version 2 (IKEv2) Protocol", RFC 5282, August 2008.
[RFC6054] McGrew, D. and B. Weis, "Using Counter Modes with Encapsulating Security Payload (ESP) and Authentication Header (AH) to Protect Group Traffic", RFC 6054, November 2010.
[RFC7296] Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y. and P. Eronen, "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 7296, October 2014.
[chacha_poly] Langley, A. and Y. Nir, "ChaCha20 and Poly1305 for IETF protocols", Internet-Draft draft-nir-cfrg-chacha20-poly1305-01, January 2014.

7.2. Informative References

[AE] Bellare, M. and C. Namprempre, "Authenticated Encryption: Relations among notions and analysis of the generic composition paradigm", 2000.
[FIPS-197] National Institute of Standards and Technology, "Advanced Encryption Standard (AES)", FIPS PUB 197, November 2001.
[FIPS-46] National Institute of Standards and Technology, "Data Encryption Standard", FIPS PUB 46-2, December 1993.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 4106, June 2005.
[RFC6407] Weis, B., Rowles, S. and T. Hardjono, "The Group Domain of Interpretation", RFC 6407, October 2011.
[standby-cipher] McGrew, D., Grieco, A. and Y. Sheffer, "Selection of Future Cryptographic Standards", Internet-Draft draft-mcgrew-standby-cipher, January 2013.

Author's Address

Yoav Nir Check Point Software Technologies Ltd. 5 Hasolelim st. Tel Aviv, 6789735 Israel EMail: ynir.ietf@gmail.com