Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track February 4, 2014
Expires: August 8, 2014
ChaCha20 and Poly1305 and their use in IPsec
draft-nir-ipsecme-chacha20-poly1305-01
Abstract
This document describes the use of the ChaCha20 stream cipher in
IPsec, as well as the use of the Poly1305 authenticator, both as
stand-alone algorithms, and as a combined mode AEAD algorithm.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . . 3
2. Algorithms for ESP & AH . . . . . . . . . . . . . . . . . . . . 3
2.1. ENCR_ChaCha20 for ESP . . . . . . . . . . . . . . . . . . . 3
2.2. AUTH_Poly1305 for ESP and AH . . . . . . . . . . . . . . . 4
2.2.1. Example One-Time Key Derivation . . . . . . . . . . . . 5
2.3. ESP_ChaCha20-Poly1305 for ESP . . . . . . . . . . . . . . . 5
2.3.1. AAD Construction . . . . . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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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 implementations 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 with or without the Poly1305 authenticator. 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].
2. Algorithms for ESP & AH
This document defines three algorithms for use with the Encapsulated
Security Protocol (ESP - [RFC4303]) and Authentication Header (AH -
[RFC4302]):
o ChaCha20 for use as an encryption algorithms for ESP.
o Poly1305-MAC for use as a message authentication algorithm for ESP
and AH.
o ChaCha20-Poly1305-ESP as an AEAD algorithm for ESP.
2.1. ENCR_ChaCha20 for ESP
The algorithm for ChaCha20 is described in section 2.4 of
[chacha_poly]. In ESP the following parameters are used:
o The IV is 64-bit. Since this is used for the nonce in the
ChaCha20 function, this IV MUST NOT repeat. The most natural way
to implement this is with a counter, but anything that guarantees
uniqueness can be used, such as a linear feedback shift register
(LFSR). Note that the encrypter can use any IV generation method
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that meets the uniqueness requirement, without coordinating with
the decrypter.
o The Internet Key Exchange protocol (IKE - [RFC5996]) 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 is called with the 256-bit key,
one (1) for the initial counter. The packet IV is prepended by a
32-bit sender ID value to form the 96-bit nonce. For regular
IPsec, the Sender ID is set to zero. For multi-sender SAs, such
as described in [RFC6054], there sender ID can be set to a
different value for each sender. The reason that one (1) is used
for the initial counter rather than zero is that one is used for
encryption in the AEAD algorithm (because zero is reserved for
generating the one-time Poly1305 key), so one was used here for
consistency.
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.
The encryption algorithm transform ID for negotiating this algorithm
in IKE is TBA by IANA.
2.2. AUTH_Poly1305 for ESP and AH
You cannot use a part of the keying material directly, because
Poly1305 requires an unpredictable and non-repeating key for each
authenticated message. So the Poly1305 key for each message is
generated as in section 2.6 of [chacha_poly]:
o The key for AUTH_Poly1305 is 256-bits.
o To determine the per-packet Poly1305 key, the ChaCha20 block
function is called with the following parameters:
* The AUTH_Poly1305 256-bit key is used as the key.
* The nonce is set from the sequence number (in the order as it
appears in the packet). 32 or 64 zero bits are prepended
depending on whether ESN is enabled or not, forming a 96-bit
nonce.
* Zero is used for the block counter.
o The 512-bit result is then truncated. The top 256 bits are used
as the one-time key for Poly1305 to calculate the message
authentication code (MAC).
o The 128-bit output serves as the MAC for the packet. All 16 bytes
are included in the packet.
The integrity algorithm transform ID for negotiating this algorithm
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in IKE is TBA by IANA.
2.2.1. Example One-Time Key Derivation
Suppose our 256-bit MAC key and sequence number are as follows:
AUTH_Poly1305 Key:
000 7b ac 2b 25 2d b4 47 af 09 b6 7a 55 a4 e9 55 84|{.+%-.G...zU..U.
016 0a e1 d6 73 10 75 d9 eb 2a 93 75 78 3e d5 53 ff|...s.u..*.ux>.S.
ESN: off
Sequence Number: 40 (represented on the packet as 00:00:00:28)
So the ChaCha20 block function (section 2.3 of [chacha_poly]) is
called with the following parameters:
o The AUTH_Poly1305 key.
o The 96-bit nonce 00:00:00:00:00:00:00:00:00:00:00:28.
o The block count parameter set to zero (0).
Note in the following ChaCha20 block, that the sequence number
(bottom right) is reversed. This is because the sequence number is
big-endian in the ESP packet, but is treated as a sequence of 32-bit
little-endian numbers in ChaCha20.
Set up ChaCha20 block:
61707865 3320646e 79622d32 6b206574
252bac7b af47b42d 557ab609 8455e9a4
73d6e10a ebd97510 7875932a ff53d53e
00000000 00000000 00000000 28000000
After running the ChaCha20 block operation:
0c15e292 6e2fe7cc dcd9bd92 f60a30b3
4094c018 b0ba041d 88ed8acf 09f9bba5
a29dbf3d e1a6acdc 011e3fe8 d953dff5
32989c29 12be0248 0c267749 e55f2037
The Poly1305 one-time key (256 bits):
000 92 e2 15 0c cc e7 2f 6e 92 bd d9 dc b3 30 0a f6 ....../n.....0..
016 18 c0 94 40 1d 04 ba b0 cf 8a ed 88 a5 bb f9 09 ...@............
2.3. 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]:
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o As in Section 2.1, the IV is 64-bit, and is used as part of the
nonce.
o Also as in Section 2.1, a 32-bit sender ID (zero for regular
IPsec) is prepended to the 64-bit IV to form the nonce.
o Also as in Section 2.1, the encryption key is 256-bit.
o As in Section 2.2, the nonce, along with a block counter of zero
is passed to the ChaCha20 block function, and the top part of the
result used as the Poly1305 key. However, unlike AUTH_Poly1305,
the nonce passed to the block function here does not depend on the
ESP sequence number, but 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 The ChaCha20 encryption function is then called with the nonce,
the key, and an initial counter of zero.
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:
* The Authenticated Additional Data (AAD) - see Section 2.3.1.
* 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.
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.
2.3.1. 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. 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
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is the uniqueness of the nonce used in ChaCha20. This is trivial in
AUTH_Poly1305, because the packet sequence number is used as a nonce,
but is required for ChaCha20 as well. As said in Section 2.1, the
nonce should be selected uniquely for a particular key. 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.
4. IANA Considerations
IANA is requested to assign two values from the IKEv2 "Transform Type
1 - Encryption Algorithm Transform IDs" registry, as follows:
o ENCR_ChaCha20
o ESP_ChaCha20-Poly1305
IANA is also requested to assign one value from the IKEv2 "Transform
Type 3 - Integrity Algorithm Transform IDs" registry with name
"AUTH_Poly1305".
5. 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.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
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[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.
[chacha_poly]
Langley, A. and Y. Nir, "ChaCha20 and Poly1305 for IETF
protocols", draft-nir-cfrg-chacha20-poly1305-01 (work in
progress), January 2014.
6.2. Informative References
[AE] Bellare, M. and C. Namprempre, "Authenticated Encryption:
Relations among notions and analysis of the generic
composition paradigm",
<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>.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[standby-cipher]
McGrew, D., Grieco, A., and Y. Sheffer, "Selection of
Future Cryptographic Standards",
draft-mcgrew-standby-cipher (work in progress).
Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 6789735
Israel
Email: ynir@checkpoint.com
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