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1This document describes the chacha20-poly1305@openssh.com authenticated
2encryption cipher supported by OpenSSH.
3
4Background
5----------
6
7ChaCha20 is a stream cipher designed by Daniel Bernstein and described
8in [1]. It operates by permuting 128 fixed bits, 128 or 256 bits of key,
9a 64 bit nonce and a 64 bit counter into 64 bytes of output. This output
10is used as a keystream, with any unused bytes simply discarded.
11
12Poly1305[2], also by Daniel Bernstein, is a one-time Carter-Wegman MAC
13that computes a 128 bit integrity tag given a message and a single-use
14256 bit secret key.
15
16The chacha20-poly1305@openssh.com combines these two primitives into an
17authenticated encryption mode. The construction used is based on that
18proposed for TLS by Adam Langley in [3], but differs in the layout of
19data passed to the MAC and in the addition of encyption of the packet
20lengths.
21
22Negotiation
23-----------
24
25The chacha20-poly1305@openssh.com offers both encryption and
26authentication. As such, no separate MAC is required. If the
27chacha20-poly1305@openssh.com cipher is selected in key exchange,
28the offered MAC algorithms are ignored and no MAC is required to be
29negotiated.
30
31Detailed Construction
32---------------------
33
34The chacha20-poly1305@openssh.com cipher requires 512 bits of key
35material as output from the SSH key exchange. This forms two 256 bit
36keys (K_1 and K_2), used by two separate instances of chacha20.
37The first 256 bits consitute K_2 and the second 256 bits become
38K_1.
39
40The instance keyed by K_1 is a stream cipher that is used only
41to encrypt the 4 byte packet length field. The second instance,
42keyed by K_2, is used in conjunction with poly1305 to build an AEAD
43(Authenticated Encryption with Associated Data) that is used to encrypt
44and authenticate the entire packet.
45
46Two separate cipher instances are used here so as to keep the packet
47lengths confidential but not create an oracle for the packet payload
48cipher by decrypting and using the packet length prior to checking
49the MAC. By using an independently-keyed cipher instance to encrypt the
50length, an active attacker seeking to exploit the packet input handling
51as a decryption oracle can learn nothing about the payload contents or
52its MAC (assuming key derivation, ChaCha20 and Poly1305 are secure).
53
54The AEAD is constructed as follows: for each packet, generate a Poly1305
55key by taking the first 256 bits of ChaCha20 stream output generated
56using K_2, an IV consisting of the packet sequence number encoded as an
57uint64 under the SSH wire encoding rules and a ChaCha20 block counter of
58zero. The K_2 ChaCha20 block counter is then set to the little-endian
59encoding of 1 (i.e. {1, 0, 0, 0, 0, 0, 0, 0}) and this instance is used
60for encryption of the packet payload.
61
62Packet Handling
63---------------
64
65When receiving a packet, the length must be decrypted first. When 4
66bytes of ciphertext length have been received, they may be decrypted
67using the K_1 key, a nonce consisting of the packet sequence number
68encoded as a uint64 under the usual SSH wire encoding and a zero block
69counter to obtain the plaintext length.
70
71Once the entire packet has been received, the MAC MUST be checked
72before decryption. A per-packet Poly1305 key is generated as described
73above and the MAC tag calculated using Poly1305 with this key over the
74ciphertext of the packet length and the payload together. The calculated
75MAC is then compared in constant time with the one appended to the
76packet and the packet decrypted using ChaCha20 as described above (with
77K_2, the packet sequence number as nonce and a starting block counter of
781).
79
80To send a packet, first encode the 4 byte length and encrypt it using
81K_1. Encrypt the packet payload (using K_2) and append it to the
82encrypted length. Finally, calculate a MAC tag and append it.
83
84Rekeying
85--------
86
87ChaCha20 must never reuse a {key, nonce} for encryption nor may it be
88used to encrypt more than 2^70 bytes under the same {key, nonce}. The
89SSH Transport protocol (RFC4253) recommends a far more conservative
90rekeying every 1GB of data sent or received. If this recommendation
91is followed, then chacha20-poly1305@openssh.com requires no special
92handling in this area.
93
94References
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