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 $