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Nir 3 Internet-Draft Check Point 4 Intended status: Standards Track April 5, 2015 5 Expires: October 7, 2015 7 ChaCha20, Poly1305 and their use in IKE & IPsec 8 draft-ietf-ipsecme-chacha20-poly1305-02 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 7, 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. ChaCha20 & Poly1305 for ESP . . . . . . . . . . . . . . . . . 3 53 2.1. AAD Construction . . . . . . . . . . . . . . . . . . . . 4 54 3. Use in IKEv2 . . . . . . . . . . . . . . . . . . . . . . . . 4 55 4. Negotiating in IKE . . . . . . . . . . . . . . . . . . . . . 4 56 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 57 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 58 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 59 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 60 8.1. Normative References . . . . . . . . . . . . . . . . . . 5 61 8.2. Informative References . . . . . . . . . . . . . . . . . 6 62 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 64 1. Introduction 66 The Advanced Encryption Standard (AES - [FIPS-197]) has become the 67 gold standard in encryption. Its efficient design, wide 68 implementation, and hardware support allow for high performance in 69 many areas, including IPsec VPNs. On most modern platforms, AES is 70 anywhere from 4x to 10x as fast as the previous most-used cipher, 71 3-key Data Encryption Standard (3DES - [FIPS-46]), which makes it not 72 only the best choice, but the only choice. 74 The problem is that if future advances in cryptanalysis reveal a 75 weakness in AES, VPN users will be in an unenviable position. With 76 the only other widely supported cipher being the much slower 3DES, it 77 is not feasible to re-configure IPsec installations to use 3DES. 78 [standby-cipher] describes this issue and the need for a standby 79 cipher in greater detail. 81 This document proposes the ChaCha20 stream cipher as such a standby 82 cipher in an Authenticated Encryption with Associated Data (AEAD) 83 construction with the Poly1305 authenticator for use with the 84 Encapsulated Security Protocol (ESP - [RFC4303]) and the Internet Key 85 Exchange Protocol (IKEv2 - [RFC7296]). The algorithms are described 86 in a separate document ([chacha_poly]). This document only describes 87 the IPsec-specific things. 89 1.1. Conventions Used in This Document 91 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 92 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 93 document are to be interpreted as described in [RFC2119]. 95 2. ChaCha20 & Poly1305 for ESP 97 AEAD_CHACHA20_POLY1305 is a combined mode algorithm, or AEAD. The 98 construction follows the AEAD construction in section 2.8 of 99 [chacha_poly]: 101 o The Initialization Vector (IV) is 64-bit, and is used as part of 102 the nonce. The IV MUST be unique for each SA but does not need to 103 be unpredictable. The use of a counter or an LFSR is RECOMMENDED. 104 o A 32-bit sender ID is prepended to the 64-bit IV to form the 105 96-bit nonce. For regular IPsec, this is set to all zeros. IPsec 106 extensions that allow multiple senders, such as GDOI ([RFC6407]) 107 or [RFC6054] may set this to different values. 108 o The encryption key is 256-bit. 109 o The Internet Key Exchange protocol generates a bitstring called 110 KEYMAT that is generated from a PRF. That KEYMAT is divided into 111 keys for encryption, message authentication and whatever else is 112 needed. For the ChaCha20 algorithm, 256 bits are used for the 113 key. TBD: do we want an extra 32 bits as salt for the nonce like 114 in GCM, or keep the salt (=SenderID) at zero? 115 o The ChaCha20 encryption algorithm requires the following 116 parameters: a 256-bit key, a 96-bit nonce, and a 32-bit initial 117 block counter. For ESP we set these as follows: 119 * The key is set to the key mentioned above. 120 * The 96-bit nonce is formed from a concatenation of the 32-bit 121 sender ID and the 64-bit IV, as described above. 122 * The Initial Block Counter is set to one (1). The reason that 123 one is used for the initial counter rather than zero is that 124 zero is reserved for generating the one-time Poly1305 key (see 125 below) 126 o As ChaCha20 is not a block cipher, no padding should be necessary. 127 However, in keeping with the specification in RFC 4303, the ESP 128 does have padding, so as to align the buffer to an integral 129 multiple of 4 octets. 130 o The same key and nonce, along with a block counter of zero are 131 passed to the ChaCha20 block function, and the top 256 bits of the 132 result are used as the Poly1305 key. The nonce passed to the 133 block function here is the same nonce that is used in ChaCha20, 134 including the 32-bit Sender ID bits, and the key passed is the 135 same as the encryption key. 136 o Finally, the Poly1305 function is run on the data to be 137 authenticated, which is, as specified in section 2.8 of 138 [chacha_poly] a concatenation of the following in the below order: 140 * The Authenticated Additional Data (AAD) - see Section 2.1. 142 * Padding that rounds the length up to 16 bytes. This is 4 or 8 143 bytes depending on whether extended sequence numbers (ESN) is 144 set for the SA. The padding is all zeros. 145 * The ciphertext 146 * Padding that rounds the total length up to an integral multiple 147 of 16 bytes. This padding is also all zeros. 148 * The length of the additional data in octets (as a 64-bit 149 little-endian integer). 150 * The length of the ciphertext in octets (as a 64-bit little- 151 endian integer). 152 o The 128-bit output of Poly1305 is used as the tag. All 16 bytes 153 are included in the packet. 155 The encryption algorithm transform ID for negotiating this algorithm 156 in IKE is TBA by IANA. 158 2.1. AAD Construction 160 The construction of the Additional Authenticated Data (AAD) is 161 similar to the one in [RFC4106]. For security associations (SAs) 162 with 32-bit sequence numbers the AAD is 8 bytes: 4-byte SPI followed 163 by 4-byte sequence number ordered exactly as it is in the packet. 164 For SAs with ESN the AAD is 12 bytes: 4-byte SPI followed by an 165 8-byte sequence number as a 64-bit network order integer. 167 3. Use in IKEv2 169 AEAD algorithms can be used in IKE, as described in [RFC5282]. More 170 specifically, the Encrypted Payload is as described in section 3 of 171 that document, the IV is 64 bits, as described in Section 2, and the 172 AAD is as described in section 5.1 of RFC 5282, so it's 32 bytes (28 173 for the IKEv2 header + 4 bytes for the encrypted payload header) 174 assuming no unencrypted payloads. 176 4. Negotiating in IKE 178 When negotiating the ChaCha20-Poly1305 algorithm for use in IKE or 179 IPsec, the value xxx (TBA by IANA) should be used in the transform 180 substructure of the SA payload as the ENCR (type 1) transform ID. As 181 with other AEAD algorithms, INTEG (type 3) transform substructures 182 SHOULD NOT be specified unless at least one of the ENCR transforms is 183 non-AEAD. 185 5. Security Considerations 187 The ChaCha20 cipher is designed to provide 256-bit security. 189 The Poly1305 authenticator is designed to ensure that forged messages 190 are rejected with a probability of 1-(n/(2^102)) for a 16n-byte 191 message, even after sending 2^64 legitimate messages, so it is SUF- 192 CMA in the terminology of [AE]. 194 The most important security consideration in implementing this draft 195 is the uniqueness of the nonce used in ChaCha20. The nonce should be 196 selected uniquely for a particular key, but unpredictability of the 197 nonce is not required. Counters and LFSRs are both acceptable ways 198 of generating unique nonces, as is encrypting a counter using a 199 64-bit cipher such as DES. Note that it is not acceptable to use a 200 truncation of a counter encrypted with a 128-bit or 256-bit cipher, 201 because such a truncation may repeat after a short time. 203 Another issue with implementing these algorithms is avoiding side 204 channels. This is trivial for ChaCha20, but requires some care for 205 Poly1305. Considerations for implementations of these algorithms are 206 in the [chacha_poly] document. 208 6. IANA Considerations 210 IANA is requested to assign one value from the IKEv2 "Transform Type 211 1 - Encryption Algorithm Transform IDs" registry, with name 212 ENCR_CHACHA20_POLY1305, and this document as reference. 214 7. Acknowledgements 216 All of the algorithms in this document were designed by D. J. 217 Bernstein. The AEAD construction was designed by Adam Langley. The 218 author would also like to thank Adam for helpful comments, as well as 219 Yaron Sheffer for telling me to write the algorithms draft. Thanks 220 also to Martin Willi for pointing out the discrepancy with the final 221 version of the algorithm document. 223 8. References 225 8.1. Normative References 227 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 228 Requirement Levels", BCP 14, RFC 2119, March 1997. 230 [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 231 4303, December 2005. 233 [RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption 234 Algorithms with the Encrypted Payload of the Internet Key 235 Exchange version 2 (IKEv2) Protocol", RFC 5282, August 236 2008. 238 [RFC6054] McGrew, D. and B. Weis, "Using Counter Modes with 239 Encapsulating Security Payload (ESP) and Authentication 240 Header (AH) to Protect Group Traffic", RFC 6054, November 241 2010. 243 [RFC7296] Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y., and P. 244 Eronen, "Internet Key Exchange Protocol Version 2 245 (IKEv2)", RFC 7296, October 2014. 247 [chacha_poly] 248 Langley, A. and Y. Nir, "ChaCha20 and Poly1305 for IETF 249 protocols", draft-nir-cfrg-chacha20-poly1305-01 (work in 250 progress), January 2014. 252 8.2. Informative References 254 [AE] Bellare, M. and C. Namprempre, "Authenticated Encryption: 255 Relations among notions and analysis of the generic 256 composition paradigm", 2000, 257 . 259 [FIPS-197] 260 National Institute of Standards and Technology, "Advanced 261 Encryption Standard (AES)", FIPS PUB 197, November 2001, 262 . 265 [FIPS-46] National Institute of Standards and Technology, "Data 266 Encryption Standard", FIPS PUB 46-2, December 1993, 267 . 269 [RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode 270 (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 271 4106, June 2005. 273 [RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain 274 of Interpretation", RFC 6407, October 2011. 276 [standby-cipher] 277 McGrew, D., Grieco, A., and Y. Sheffer, "Selection of 278 Future Cryptographic Standards", draft-mcgrew-standby- 279 cipher (work in progress), January 2013. 281 Author's Address 282 Yoav Nir 283 Check Point Software Technologies Ltd. 284 5 Hasolelim st. 285 Tel Aviv 6789735 286 Israel 288 Email: ynir.ietf@gmail.com