mirror of
git://anongit.mindrot.org/openssh.git
synced 2024-12-21 09:30:12 +00:00
05855bf2ce
clarify ordering of subkeys; pointed out by ietf-ssh AT stbuehler.de Upstream-ID: 05ebe9f949449a555ebce8e0aad7c8c9acaf8463
108 lines
4.5 KiB
Plaintext
108 lines
4.5 KiB
Plaintext
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 encyption 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 1.3 2016/05/03 13:10:24 djm Exp $
|
|
|