1 This document describes the chacha20-poly1305 (a] openssh.com authenticated 2 encryption cipher supported by OpenSSH. 3 4 Background 5 ---------- 6 7 ChaCha20 is a stream cipher designed by Daniel Bernstein and described 8 in [1]. It operates by permuting 128 fixed bits, 128 or 256 bits of key, 9 a 64 bit nonce and a 64 bit counter into 64 bytes of output. This output 10 is used as a keystream, with any unused bytes simply discarded. 11 12 Poly1305[2], also by Daniel Bernstein, is a one-time Carter-Wegman MAC 13 that computes a 128 bit integrity tag given a message and a single-use 14 256 bit secret key. 15 16 The chacha20-poly1305 (a] openssh.com combines these two primitives into an 17 authenticated encryption mode. The construction used is based on that 18 proposed for TLS by Adam Langley in [3], but differs in the layout of 19 data passed to the MAC and in the addition of encyption of the packet 20 lengths. 21 22 Negotiation 23 ----------- 24 25 The chacha20-poly1305 (a] openssh.com offers both encryption and 26 authentication. As such, no separate MAC is required. If the 27 chacha20-poly1305 (a] openssh.com cipher is selected in key exchange, 28 the offered MAC algorithms are ignored and no MAC is required to be 29 negotiated. 30 31 Detailed Construction 32 --------------------- 33 34 The chacha20-poly1305 (a] openssh.com cipher requires 512 bits of key 35 material as output from the SSH key exchange. This forms two 256 bit 36 keys (K_1 and K_2), used by two separate instances of chacha20. 37 38 The instance keyed by K_1 is a stream cipher that is used only 39 to encrypt the 4 byte packet length field. The second instance, 40 keyed by K_2, is used in conjunction with poly1305 to build an AEAD 41 (Authenticated Encryption with Associated Data) that is used to encrypt 42 and authenticate the entire packet. 43 44 Two separate cipher instances are used here so as to keep the packet 45 lengths confidential but not create an oracle for the packet payload 46 cipher by decrypting and using the packet length prior to checking 47 the MAC. By using an independently-keyed cipher instance to encrypt the 48 length, an active attacker seeking to exploit the packet input handling 49 as a decryption oracle can learn nothing about the payload contents or 50 its MAC (assuming key derivation, ChaCha20 and Poly1305 are secure). 51 52 The AEAD is constructed as follows: for each packet, generate a Poly1305 53 key by taking the first 256 bits of ChaCha20 stream output generated 54 using K_2, an IV consisting of the packet sequence number encoded as an 55 uint64 under the SSH wire encoding rules and a ChaCha20 block counter of 56 zero. The K_2 ChaCha20 block counter is then set to the little-endian 57 encoding of 1 (i.e. {1, 0, 0, 0, 0, 0, 0, 0}) and this instance is used 58 for encryption of the packet payload. 59 60 Packet Handling 61 --------------- 62 63 When receiving a packet, the length must be decrypted first. When 4 64 bytes of ciphertext length have been received, they may be decrypted 65 using the K_1 key, a nonce consisting of the packet sequence number 66 encoded as a uint64 under the usual SSH wire encoding and a zero block 67 counter to obtain the plaintext length. 68 69 Once the entire packet has been received, the MAC MUST be checked 70 before decryption. A per-packet Poly1305 key is generated as described 71 above and the MAC tag calculated using Poly1305 with this key over the 72 ciphertext of the packet length and the payload together. The calculated 73 MAC is then compared in constant time with the one appended to the 74 packet and the packet decrypted using ChaCha20 as described above (with 75 K_2, the packet sequence number as nonce and a starting block counter of 76 1). 77 78 To send a packet, first encode the 4 byte length and encrypt it using 79 K_1. Encrypt the packet payload (using K_2) and append it to the 80 encrypted length. Finally, calculate a MAC tag and append it. 81 82 Rekeying 83 -------- 84 85 ChaCha20 must never reuse a {key, nonce} for encryption nor may it be 86 used to encrypt more than 2^70 bytes under the same {key, nonce}. The 87 SSH Transport protocol (RFC4253) recommends a far more conservative 88 rekeying every 1GB of data sent or received. If this recommendation 89 is followed, then chacha20-poly1305 (a] openssh.com requires no special 90 handling in this area. 91 92 References 93 ---------- 94 95 [1] "ChaCha, a variant of Salsa20", Daniel Bernstein 96 http://cr.yp.to/chacha/chacha-20080128.pdf 97 98 [2] "The Poly1305-AES message-authentication code", Daniel Bernstein 99 http://cr.yp.to/mac/poly1305-20050329.pdf 100 101 [3] "ChaCha20 and Poly1305 based Cipher Suites for TLS", Adam Langley 102 http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-03 103 104 $OpenBSD: PROTOCOL.chacha20poly1305,v 1.2 2013/12/02 02:50:27 djm Exp $ 105 106
