Secure Shell (SSH) authenticated encryption cipher: chacha20-poly1305

ChaCha20 is a stream cipher designed by Daniel Bernstein and described in . 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, 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" cipher combines these two primitives into an authenticated encryption mode. The construction used is based on that proposed for TLS by Adam Langley in , but differs in the layout of data passed to the MAC and in the addition of encyption of the packet lengths. This document specify and register "chacha20-poly1305" to be identical to the already widely deployed "chacha20-poly1305@openssh.com".

The "chacha20-poly1305" offers both encryption and authentication. As such, no separate MAC is required. If the "chacha20-poly1305" cipher is selected in key exchange, the offered MAC algorithms are ignored and no MAC is required to be negotiated.

The "chacha20-poly1305" 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 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.

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.

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 recommends a far more conservative rekeying every 1GB of data sent or received. If this recommendation is followed, then "chacha20-poly1305" requires no special handling in this area.

Markus Friedl helped on the design. Theo de Raadt supported to register the OpenSSH private-use cipher for generic use.

IANA Considerations IANA is requested to add a new "Encryption Algorithm Name" of "chacha20-poly1305" the "Encryption Algorithm Names" registry for Secure Shell (SSH) Protocol Parameters with a "Reference" field referring this RFC.