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+This document describes the chacha20-poly1305@openssh.com authenticated
+encryption cipher supported by OpenSSH.
+
+Background
+----------
+
+ChaCha20 is a stream cipher designed by Daniel Bernstein and described
+in [1]. It operates by permuting 128 fixed bits, 128 or 256 bits of key,
+a 64 bit nonce and a 64 bit counter into 64 bytes of output. This output
+is used as a keystream, with any unused bytes simply discarded.
+
+Poly1305[2], also by Daniel Bernstein, is a one-time Carter-Wegman MAC
+that computes a 128 bit integrity tag given a message and a single-use
+256 bit secret key.
+
+The chacha20-poly1305@openssh.com combines these two primitives into an
+authenticated encryption mode. The construction used is based on that
+proposed for TLS by Adam Langley in [3], but differs in the layout of
+data passed to the MAC and in the addition of encryption of the packet
+lengths.
+
+Negotiation
+-----------
+
+The chacha20-poly1305@openssh.com offers both encryption and
+authentication. As such, no separate MAC is required. If the
+chacha20-poly1305@openssh.com cipher is selected in key exchange,
+the offered MAC algorithms are ignored and no MAC is required to be
+negotiated.
+
+Detailed Construction
+---------------------
+
+The chacha20-poly1305@openssh.com cipher requires 512 bits of key
+material as output from the SSH key exchange. This forms two 256 bit
+keys (K_1 and K_2), used by two separate instances of chacha20.
+The first 256 bits constitute K_2 and the second 256 bits become
+K_1.
+
+The instance keyed by K_1 is a stream cipher that is used only
+to encrypt the 4 byte packet length field. The second instance,
+keyed by K_2, is used in conjunction with poly1305 to build an AEAD
+(Authenticated Encryption with Associated Data) that is used to encrypt
+and authenticate the entire packet.
+
+Two separate cipher instances are used here so as to keep the packet
+lengths confidential but not create an oracle for the packet payload
+cipher by decrypting and using the packet length prior to checking
+the MAC. By using an independently-keyed cipher instance to encrypt the
+length, an active attacker seeking to exploit the packet input handling
+as a decryption oracle can learn nothing about the payload contents or
+its MAC (assuming key derivation, ChaCha20 and Poly1305 are secure).
+
+The AEAD is constructed as follows: for each packet, generate a Poly1305
+key by taking the first 256 bits of ChaCha20 stream output generated
+using K_2, an IV consisting of the packet sequence number encoded as an
+uint64 under the SSH wire encoding rules and a ChaCha20 block counter of
+zero. The K_2 ChaCha20 block counter is then set to the little-endian
+encoding of 1 (i.e. {1, 0, 0, 0, 0, 0, 0, 0}) and this instance is used
+for encryption of the packet payload.
+
+Packet Handling
+---------------
+
+When receiving a packet, the length must be decrypted first. When 4
+bytes of ciphertext length have been received, they may be decrypted
+using the K_1 key, a nonce consisting of the packet sequence number
+encoded as a uint64 under the usual SSH wire encoding and a zero block
+counter to obtain the plaintext length.
+
+Once the entire packet has been received, the MAC MUST be checked
+before decryption. A per-packet Poly1305 key is generated as described
+above and the MAC tag calculated using Poly1305 with this key over the
+ciphertext of the packet length and the payload together. The calculated
+MAC is then compared in constant time with the one appended to the
+packet and the packet decrypted using ChaCha20 as described above (with
+K_2, the packet sequence number as nonce and a starting block counter of
+1).
+
+To send a packet, first encode the 4 byte length and encrypt it using
+K_1. Encrypt the packet payload (using K_2) and append it to the
+encrypted length. Finally, calculate a MAC tag and append it.
+
+Rekeying
+--------
+
+ChaCha20 must never reuse a {key, nonce} for encryption nor may it be
+used to encrypt more than 2^70 bytes under the same {key, nonce}. The
+SSH Transport protocol (RFC4253) recommends a far more conservative
+rekeying every 1GB of data sent or received. If this recommendation
+is followed, then chacha20-poly1305@openssh.com requires no special
+handling in this area.
+
+References
+----------
+
+[1] "ChaCha, a variant of Salsa20", Daniel Bernstein
+    http://cr.yp.to/chacha/chacha-20080128.pdf
+
+[2] "The Poly1305-AES message-authentication code", Daniel Bernstein
+    http://cr.yp.to/mac/poly1305-20050329.pdf
+
+[3] "ChaCha20 and Poly1305 based Cipher Suites for TLS", Adam Langley
+    http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-03
+
+$OpenBSD: PROTOCOL.chacha20poly1305,v 1.5 2020/02/21 00:04:43 dtucker Exp $
+