Internet DRAFT - draft-nir-ipsecme-chacha20-poly1305
draft-nir-ipsecme-chacha20-poly1305
Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track November 24, 2014
Expires: May 28, 2015
ChaCha20, Poly1305 and their use in IPsec
draft-nir-ipsecme-chacha20-poly1305-05
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 . . . . . . . . . . . . . . . . . . . . . . . . 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. UI Suite . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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]). We
call this ESP_ChaCha20-Poly1305. These 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].
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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
[chacha_poly]:
o The IV is 64-bit, and is used as part of the nonce.
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 (IKE - [RFC7296]) 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
below)
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
* The length of the ciphertext as a 32-bit network order
quantity.
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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. UI Suite
This document also defines an RFC 4308-style UI suite for IKE and
IPsec (See [RFC4308]. The suite is called "VPN-C". The name was
chosen for two reasons:
o "VPN-A" and "VPN-B" are already defined in RFC 4308.
o "C" stands for "Civilian", because unlike VPN-A, VPN-B, and the
additional UI suites defined in [RFC6379], most of the algorithm
in this suite come from civilian researchers, not from government
agencies.
The Algorithms:
ESP:
Encryption ESP_ChaCha20-Poly1305
Integrity NULL
IKEv2:
Encryption ESP_ChaCha20-Poly1305
Integrity NULL
Pseudo-random function HMAC-SHA-256 [RFC4868]
Diffie-Hellman group 256-bit random ECP group [RFC5903]
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HMAC-SHA-256 is used here because there is no natural way to use
either ChaCha20 or Poly1305 as an IKEv2 PRF. See discussion in
section 2.7 of [chacha_poly].
TBD: Do we want to define a special PRF function here? Something can
be concocted from using ChaCha20 as the PRF function and Poly1305 for
shortening keys, but somehow this looks unwieldy.
TBD: Should we replace the Diffie-Hellman group with ED25519 ???
5. 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.
6. 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.
IANA is also requested to assign the identifier "VPN-C" with this
document as reference from the "Cryptographic Suites for IKEv1,
IKEv2, and IPsec" registry.
7. Acknowledgements
All of the algorithms in this document were designed by D. J.
Bernstein. The AEAD construction was designed by Adam Langley. The
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author would also like to thank Adam for helpful comments, as well as
Yaron Sheffer for telling me to write the algorithms draft.
8. References
8.1. 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", draft-nir-cfrg-chacha20-poly1305-01 (work in
progress), January 2014.
8.2. Informative References
[AE] Bellare, M. and C. Namprempre, "Authenticated Encryption:
Relations among notions and analysis of the generic
composition paradigm", 2000,
<http://cseweb.ucsd.edu/~mihir/papers/oem.html>.
[FIPS-197]
National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", FIPS PUB 197, November 2001,
<http://csrc.nist.gov/publications/fips/fips197/
fips-197.pdf>.
[FIPS-46] National Institute of Standards and Technology, "Data
Encryption Standard", FIPS PUB 46-2, December 1993,
<http://www.itl.nist.gov/fipspubs/fip46-2.htm>.
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[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)", RFC
4106, June 2005.
[RFC4308] Hoffman, P., "Cryptographic Suites for IPsec", RFC 4308,
December 2005.
[RFC6379] Law, L. and J. Solinas, "Suite B Cryptographic Suites for
IPsec", RFC 6379, October 2011.
[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", 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
Israel
Email: ynir.ietf@gmail.com
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