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 The first 256 bits consitute K_2 and the second 256 bits become 38 K_1. 39 40 The instance keyed by K_1 is a stream cipher that is used only 41 to encrypt the 4 byte packet length field. The second instance, 42 keyed by K_2, is used in conjunction with poly1305 to build an AEAD 43 (Authenticated Encryption with Associated Data) that is used to encrypt 44 and authenticate the entire packet. 45 46 Two separate cipher instances are used here so as to keep the packet 47 lengths confidential but not create an oracle for the packet payload 48 cipher by decrypting and using the packet length prior to checking 49 the MAC. By using an independently-keyed cipher instance to encrypt the 50 length, an active attacker seeking to exploit the packet input handling 51 as a decryption oracle can learn nothing about the payload contents or 52 its MAC (assuming key derivation, ChaCha20 and Poly1305 are secure). 53 54 The AEAD is constructed as follows: for each packet, generate a Poly1305 55 key by taking the first 256 bits of ChaCha20 stream output generated 56 using K_2, an IV consisting of the packet sequence number encoded as an 57 uint64 under the SSH wire encoding rules and a ChaCha20 block counter of 58 zero. The K_2 ChaCha20 block counter is then set to the little-endian 59 encoding of 1 (i.e. {1, 0, 0, 0, 0, 0, 0, 0}) and this instance is used 60 for encryption of the packet payload. 61 62 Packet Handling 63 --------------- 64 65 When receiving a packet, the length must be decrypted first. When 4 66 bytes of ciphertext length have been received, they may be decrypted 67 using the K_1 key, a nonce consisting of the packet sequence number 68 encoded as a uint64 under the usual SSH wire encoding and a zero block 69 counter to obtain the plaintext length. 70 71 Once the entire packet has been received, the MAC MUST be checked 72 before decryption. A per-packet Poly1305 key is generated as described 73 above and the MAC tag calculated using Poly1305 with this key over the 74 ciphertext of the packet length and the payload together. The calculated 75 MAC is then compared in constant time with the one appended to the 76 packet and the packet decrypted using ChaCha20 as described above (with 77 K_2, the packet sequence number as nonce and a starting block counter of 78 1). 79 80 To send a packet, first encode the 4 byte length and encrypt it using 81 K_1. Encrypt the packet payload (using K_2) and append it to the 82 encrypted length. Finally, calculate a MAC tag and append it. 83 84 Rekeying 85 -------- 86 87 ChaCha20 must never reuse a {key, nonce} for encryption nor may it be 88 used to encrypt more than 2^70 bytes under the same {key, nonce}. The 89 SSH Transport protocol (RFC4253) recommends a far more conservative 90 rekeying every 1GB of data sent or received. If this recommendation 91 is followed, then chacha20-poly1305 (a] openssh.com requires no special 92 handling in this area. 93 94 References 95 ---------- 96 97 [1] "ChaCha, a variant of Salsa20", Daniel Bernstein 98 http://cr.yp.to/chacha/chacha-20080128.pdf 99 100 [2] "The Poly1305-AES message-authentication code", Daniel Bernstein 101 http://cr.yp.to/mac/poly1305-20050329.pdf 102 103 [3] "ChaCha20 and Poly1305 based Cipher Suites for TLS", Adam Langley 104 http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-03 105 106 $OpenBSD: PROTOCOL.chacha20poly1305,v 1.3 2016/05/03 13:10:24 djm Exp $ 107 108
