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Nir 3 Internet-Draft Check Point 4 Intended status: Standards Track March 30, 2015 5 Expires: October 1, 2015 7 ChaCha20, Poly1305 and their use in IPsec 8 draft-ietf-ipsecme-chacha20-poly1305-00 10 Abstract 12 This document describes the use of the ChaCha20 stream cipher along 13 with the Poly1305 authenticator, combined into an AEAD algorithm for 14 IPsec. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on October 1, 2015. 33 Copyright Notice 35 Copyright (c) 2015 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 1.1. Conventions Used in This Document . . . . . . . . . . . . 2 52 2. ESP_ChaCha20-Poly1305 for ESP . . . . . . . . . . . . . . . . 3 53 2.1. AAD Construction . . . . . . . . . . . . . . . . . . . . 4 54 3. Use in IKEv2 . . . . . . . . . . . . . . . . . . . . . . . . 4 55 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 56 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 57 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 58 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 60 7.2. Informative References . . . . . . . . . . . . . . . . . 5 61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 63 1. Introduction 65 The Advanced Encryption Standard (AES - [FIPS-197]) has become the 66 gold standard in encryption. Its efficient design, wide 67 implementation, and hardware support allow for high performance in 68 many areas, including IPsec VPNs. On most modern platforms, AES is 69 anywhere from 4x to 10x as fast as the previous most-used cipher, 70 3-key Data Encryption Standard (3DES - [FIPS-46]), which makes it not 71 only the best choice, but the only choice. 73 The problem is that if future advances in cryptanalysis reveal a 74 weakness in AES, VPN users will be in an unenviable position. With 75 the only other widely supported cipher being the much slower 3DES, it 76 is not feasible to re-configure IPsec installations to use 3DES. 77 [standby-cipher] describes this issue and the need for a standby 78 cipher in greater detail. 80 This document proposes the ChaCha20 stream cipher as such a standby 81 cipher in an AEAD construction with the Poly1305 authenticator for 82 use with the Encapsulated Security Protocol (ESP - [RFC4303]) and the 83 Internet Key Exchange Protocol (IKEv2 - [RFC7296]). The algorithms 84 are described in a separate document ([chacha_poly]). This document 85 only describes the IPsec-specific things. 87 1.1. Conventions Used in This Document 89 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 90 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 91 document are to be interpreted as described in [RFC2119]. 93 2. ESP_ChaCha20-Poly1305 for ESP 95 ESP_ChaCha20-Poly1305 is a combined mode algorithm, or AEAD. The 96 construction follows the AEAD construction in section 2.7 of 97 [chacha_poly]: 99 o The IV is 64-bit, and is used as part of the nonce. TBD: do we 100 want to skip the IV altogether and just use the packet counter? 101 o A 32-bit sender ID is prepended to the 64-bit IV to form the 102 96-bit nonce. For regular IPsec, this is set to all zeros. IPsec 103 extensions that allow multiple senders, such as GDOI ([RFC6407]) 104 or [RFC6054] may set this to different values. 105 o The encryption key is 256-bit. 106 o The Internet Key Exchange protocol generates a bitstring called 107 KEYMAT that is generated from a PRF. That KEYMAT is divided into 108 keys for encryption, message authentication and whatever else is 109 needed. For the ChaCha20 algorithm, 256 bits are used for the 110 key. TBD: do we want an extra 32 bits as salt for the nonce like 111 in GCM? 112 o The ChaCha20 encryption algorithm requires the following 113 parameters: a 256-bit key, a 96-bit nonce, and a 32-bit initial 114 block counter. For ESP we set these as follows: 116 * The key is set to the key mentioned above. 117 * The 96-bit nonce is formed from a concatenation of the 32-bit 118 sender ID and the 64-bit IV, as described above. 119 * The Initial Block Counter is set to one (1). The reason that 120 one is used for the initial counter rather than zero is that 121 zero is reserved for generating the one-time Poly1305 key (see 122 below) 123 o As ChaCha20 is not a block cipher, no padding should be necessary. 124 However, in keeping with the specification in RFC 4303, the ESP 125 does have padding, so as to align the buffer to an integral 126 multiple of 4 octets. 127 o The same key and nonce, along with a block counter of zero are 128 passed to the ChaCha20 block function, and the top 256 bits of the 129 result are used as the Poly1305 key. The nonce passed to the 130 block function here is the same nonce that is used in ChaCha20, 131 including the 32-bit Sender ID bits, and the key passed is the 132 same as the encryption key. 133 o Finally, the Poly1305 function is run on the data to be 134 authenticated, which is, as specified in section 2.7 of 135 [chacha_poly] a concatenation of the following in the below order: 137 * The Authenticated Additional Data (AAD) - see Section 2.1. 138 * The AAD length in bytes as a 32-bit network order quantity. 139 * The ciphertext 140 * The length of the ciphertext as a 32-bit network order 141 quantity. 142 o The 128-bit output of Poly1305 is used as the tag. All 16 bytes 143 are included in the packet. 145 The encryption algorithm transform ID for negotiating this algorithm 146 in IKE is TBA by IANA. 148 2.1. AAD Construction 150 The construction of the Additional Authenticated Data (AAD) is 151 similar to the one in [RFC4106]. For security associations (SAs) 152 with 32-bit sequence numbers the AAD is 8 bytes: 4-byte SPI followed 153 by 4-byte sequence number ordered exactly as it is in the packet. 154 For SAs with ESN the AAD is 12 bytes: 4-byte SPI followed by an 155 8-byte sequence number as a 64-bit network order integer. 157 3. Use in IKEv2 159 AEAD algorithms can be used in IKE, as described in [RFC5282]. More 160 specifically, the Encrypted Payload is as described in section 3 of 161 that document, the IV is 64 bits, as described in Section 2, and the 162 AAD is as described in section 5.1 of RFC 5282, so it's 32 bytes (28 163 for the IKEv2 header + 4 bytes for the encrypted payload header) 164 assuming no unencrypted payloads. 166 4. Security Considerations 168 The ChaCha20 cipher is designed to provide 256-bit security. 170 The Poly1305 authenticator is designed to ensure that forged messages 171 are rejected with a probability of 1-(n/(2^102)) for a 16n-byte 172 message, even after sending 2^64 legitimate messages, so it is SUF- 173 CMA in the terminology of [AE]. 175 The most important security consideration in implementing this draft 176 is the uniqueness of the nonce used in ChaCha20. The nonce should be 177 selected uniquely for a particular key, but unpredictability of the 178 nonce is not required. Counters and LFSRs are both acceptable ways 179 of generating unique nonces, as is encrypting a counter using a 180 64-bit cipher such as DES. Note that it is not acceptable to use a 181 truncation of a counter encrypted with a 128-bit or 256-bit cipher, 182 because such a truncation may repeat after a short time. 184 Another issue with implementing these algorithms is avoiding side 185 channels. This is trivial for ChaCha20, but requires some care for 186 Poly1305. Considerations for implementations of these algorithms are 187 in the [chacha_poly] document. 189 5. IANA Considerations 191 IANA is requested to assign one value from the IKEv2 "Transform Type 192 1 - Encryption Algorithm Transform IDs" registry, with name 193 ESP_ChaCha20-Poly1305, and this document as reference. 195 6. Acknowledgements 197 All of the algorithms in this document were designed by D. J. 198 Bernstein. The AEAD construction was designed by Adam Langley. The 199 author would also like to thank Adam for helpful comments, as well as 200 Yaron Sheffer for telling me to write the algorithms draft. 202 7. References 204 7.1. Normative References 206 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 207 Requirement Levels", BCP 14, RFC 2119, March 1997. 209 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 210 4303, December 2005. 212 [RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption 213 Algorithms with the Encrypted Payload of the Internet Key 214 Exchange version 2 (IKEv2) Protocol", RFC 5282, August 215 2008. 217 [RFC6054] McGrew, D. and B. Weis, "Using Counter Modes with 218 Encapsulating Security Payload (ESP) and Authentication 219 Header (AH) to Protect Group Traffic", RFC 6054, November 220 2010. 222 [RFC7296] Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y., and P. 223 Eronen, "Internet Key Exchange Protocol Version 2 224 (IKEv2)", RFC 7296, October 2014. 226 [chacha_poly] 227 Langley, A. and Y. Nir, "ChaCha20 and Poly1305 for IETF 228 protocols", draft-nir-cfrg-chacha20-poly1305-01 (work in 229 progress), January 2014. 231 7.2. Informative References 233 [AE] Bellare, M. and C. Namprempre, "Authenticated Encryption: 234 Relations among notions and analysis of the generic 235 composition paradigm", 2000, 236 . 238 [FIPS-197] 239 National Institute of Standards and Technology, "Advanced 240 Encryption Standard (AES)", FIPS PUB 197, November 2001, 241 . 244 [FIPS-46] National Institute of Standards and Technology, "Data 245 Encryption Standard", FIPS PUB 46-2, December 1993, 246 . 248 [RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode 249 (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 250 4106, June 2005. 252 [RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain 253 of Interpretation", RFC 6407, October 2011. 255 [standby-cipher] 256 McGrew, D., Grieco, A., and Y. Sheffer, "Selection of 257 Future Cryptographic Standards", draft-mcgrew-standby- 258 cipher (work in progress), January 2013. 260 Author's Address 262 Yoav Nir 263 Check Point Software Technologies Ltd. 264 5 Hasolelim st. 265 Tel Aviv 6789735 266 Israel 268 Email: ynir.ietf@gmail.com