Network Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Intended status: Standards Track                          March 30, 2015
Expires: October 1, 2015

               ChaCha20, Poly1305 and their use in IPsec


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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   2
   2.  ESP_ChaCha20-Poly1305 for ESP . . . . . . . . . . . . . . . .   3
     2.1.  AAD Construction  . . . . . . . . . . . . . . . . . . . .   4
   3.  Use in IKEv2  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

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 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",
   document are to be interpreted as described in [RFC2119].

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2.  ESP_ChaCha20-Poly1305 for ESP

   ESP_ChaCha20-Poly1305 is a combined mode algorithm, or AEAD.  The
   construction follows the AEAD construction in section 2.7 of

   o  The IV is 64-bit, and is used as part of the nonce.  TBD: do we
      want to skip the IV altogether and just use the packet counter?
   o  A 32-bit sender ID is prepended to the 64-bit IV to form the
      96-bit nonce.  For regular IPsec, this is set to all zeros.  IPsec
      extensions that allow multiple senders, such as GDOI ([RFC6407])
      or [RFC6054] may set this to different values.
   o  The encryption key is 256-bit.
   o  The Internet Key Exchange protocol generates a bitstring called
      KEYMAT that is generated from a PRF.  That KEYMAT is divided into
      keys for encryption, message authentication and whatever else is
      needed.  For the ChaCha20 algorithm, 256 bits are used for the
      key.  TBD: do we want an extra 32 bits as salt for the nonce like
      in GCM?
   o  The ChaCha20 encryption algorithm requires the following
      parameters: a 256-bit key, a 96-bit nonce, and a 32-bit initial
      block counter.  For ESP we set these as follows:

      *  The key is set to the key mentioned above.
      *  The 96-bit nonce is formed from a concatenation of the 32-bit
         sender ID and the 64-bit IV, as described above.
      *  The Initial Block Counter is set to one (1).  The reason that
         one is used for the initial counter rather than zero is that
         zero is reserved for generating the one-time Poly1305 key (see
   o  As ChaCha20 is not a block cipher, no padding should be necessary.
      However, in keeping with the specification in RFC 4303, the ESP
      does have padding, so as to align the buffer to an integral
      multiple of 4 octets.
   o  The same key and nonce, along with a block counter of zero are
      passed to the ChaCha20 block function, and the top 256 bits of the
      result are used as the Poly1305 key.  The nonce passed to the
      block function here is the same nonce that is used in ChaCha20,
      including the 32-bit Sender ID bits, and the key passed is the
      same as the encryption key.
   o  Finally, the Poly1305 function is run on the data to be
      authenticated, which is, as specified in section 2.7 of
      [chacha_poly] a concatenation of the following in the below order:

      *  The Authenticated Additional Data (AAD) - see Section 2.1.
      *  The AAD length in bytes as a 32-bit network order quantity.
      *  The ciphertext

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      *  The length of the ciphertext as a 32-bit network order
   o  The 128-bit output of Poly1305 is used as the tag.  All 16 bytes
      are included in the packet.

   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.

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5.  IANA Considerations

   IANA is requested to assign one value from the IKEv2 "Transform Type
   1 - Encryption Algorithm Transform IDs" registry, with name
   ESP_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

   [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

   [RFC7296]  Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y., and P.
              Eronen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 7296, October 2014.

              Langley, A. and Y. Nir, "ChaCha20 and Poly1305 for IETF
              protocols", draft-nir-cfrg-chacha20-poly1305-01 (work in
              progress), 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,

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              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.

              McGrew, D., Grieco, A., and Y. Sheffer, "Selection of
              Future Cryptographic Standards", draft-mcgrew-standby-
              cipher (work in progress), January 2013.

Author's Address

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  6789735


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