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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 consitute 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 1.4 2018/04/10 00:10:49 djm Exp $
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