The regular encryption format provides privacy and authenticity using long-term asymmetric keys, but sometimes that's not possible or practical:
For these cases, we need to combine encryption for privacy with signing for authenticity. This is what the saltpack signcryption mode provides. Note that this is not as simple as layering the encryption mode the signing mode. Note also that this gives up the repudiability property of the encryption mode.
At a high-level, the message is encrypted once using a symmetric key, which is shared with all recipients. The recipients may be either public Curve25519 encryption keys, or long-term 32-byte symmetric secrets. It is then signed with a long-term signing key belonging to the sender.
The message is chunked into 1MB chunks. A sequential nonce used for the encryption and MAC's ensures that the 1MB chunks cannot be reordered. The end of the message is marked with an authenticated flag to prevent truncation attacks.
As in the encryption mode, each encrypted copy of the message encryption key is given a recipient identifier. Unlike the encryption mode, signcryption doesn't use the literal public key bytes as the identifier (in the case of an asymmetric recipient), but instead an opaque identifier derived from the recipient's key and the ephemeral sender key. This preserves recipient anonymity by default. Applications lose the ability to make suggestions like "your phone can't read this message, but your laptop can" based on the recipients list, but the intention is that distributing long-term shared symmetric keys to all of a user's devices can avoid that situation entirely. The recipient identifiers for symmetric recipients are entirely up to the application.
The signcryption scheme allows an anonymous sender by specifying a signing key that is all zeroes. In this case the signature is filled with zeroes also, and the verification step is skipped. This would be similar to using the encryption mode with an anonymous sender and a shared asymmetric recipient key, though the encryption mode wasn't designed with shared recipient keys in mind. (In particular it would be very wrong to share recipient keys in the encryption mode, if the sender was authenticated).
An signcrypted message is a series of concatenated MessagePack objects. The first is a header packet, followed by one or more payload packets, the last of which is indicated with a final packet flag.
The header packet is a MessagePack list with these contents:
[ format name, version, mode, ephemeral public key, sender secretbox, recipients list, ]
[2, 0]. Note that saltpack version 1 did not include a signcryption mode.
crypto_secretboxcontaining the sender's long-term public signing key, encrypted with the payload key from below.
A recipient pair is a two-element list:
[ recipient identifier, payload key box, ]
crypto_boxcontaining a copy of the payload key, encrypted with the recipient's public key, the ephemeral private key, and a counter nonce.
When composing a message, the sender follows these steps to generate the header:
crypto_secretboxwith the payload key and the nonce
saltpack_sender_key_sbox, to create the sender secretbox.
Encrypt a copy of the payload key for each recipient, and create an identifier for each resulting secretbox. The procedure here is different for the two different types of recipients:
For Curve25519 recipient public keys, first derive a shared symmetric key by
boxing 32 zero bytes with the recipient public key, the ephemeral private
key, and the nonce
saltpack_derived_sboxkey, and taking the last 32 bytes
of the resulting box. Secretbox the payload key using this derived
symmetric key, with the nonce
is the 8-byte big-endian unsigned recipient index. To compute the recipient
identifier, concatenate the derived symmetric key and the
saltpack_recipsbXXXXXXXX nonce together, and HMAC-SHA512 them under the
saltpack signcryption box key identifier. The identifier is the first
32 bytes of that HMAC.
For recipient symmetric keys, first derive a shared symmetric key.
Concatenate the ephemeral public Curve25519 key and the recipient symmetric
key, and HMAC-SHA512 them under the key
saltpack signcryption derived
symmetric key. The derived key is the first 32 bytes of that HMAC.
Secretbox the payload key using this derived symmetric key, with the
XXXXXXXX is the 8-byte big-endian
unsigned recipient index. The recipient identifier in this case is up to the
Collect the format name, version, and mode into a list, followed by the ephemeral public key, the sender secretbox, and the nested recipients list.
crypto_hash(SHA512) of the bytes from #6. This is the header hash.
binobject. These twice-encoded bytes are the header packet.
Encrypting the sender's long-term public key in step #3 allows Alice to stay anonymous to eavesdroppers. If Alice wants to be anonymous to recipients as well, she can supply an all-zero signing public key in step #3. In this case, recipients should skip the signature verification step and indicate that the message is from an anonymous sender.
Recipients parse the header of a message using the following steps:
crypto_hash(SHA512) of the bytes from #1 to give the header hash.
crypto_secretbox_openwith the payload key from #6 and the nonce
saltpack_sender_key_sbox. This gives the sender signing key.
When parsing lists in general, if a list is longer than expected, clients should allow the extra fields and ignore them. That allows us to make future additions to the format without breaking backward compatibility.
A payload packet is a MessagePack list with these contents:
[ signcrypted chunk, final flag, ]
The sender creates the signcrypted chunk with the following steps. For each 1 MB chunk of plaintext:
nonce |= 0x01), otherwise set it to zero (
nonce &= 0xfe). Finally, append the 8-byte unsigned big-endian packet number, where the first payload packet is zero.
saltpack encrypted signature
0x00for false and
The recipient performs those steps in reverse:
If a message ends with without the last packet setting the final flag to true, the receiving client must report an error that the message has been truncated.
Unlike the twice-encoded header above, payload packets are once-encoded directly to the output stream.
# header packet (on the wire, this is twice-encoded) [ # format name "saltpack", # major and minor version [1, 0], # mode (3 = signcryption) 3, # ephemeral public key 895e690ba0fd8d15f51adf59e161af3f67518fa6e2eaadd8a666b8a1629c2349, # sender secretbox b49c4c8791cd97f2c244c637df90e343eda4aaa56e37d975d2b7c81d36f44850d77706a51e2ccd57e7f7606565db4b1e, # recipient pairs [ # the first recipient pair [ # a recipient identifier (the symmetric-vs-asymmetric type is indistinguishable) 6dfcf73ef7ad77b0f20ea28f022647c02a3f3aaf57952a8c5cc9ecc33ca87223 # payload key box c16b6126d155d7a39db20825d6c43f856689d0f8665a8da803270e0106ed91a90ef599961492bd6e49c69b43adc22724, ], # subsequent recipient pairs... ], ] # payload packet [ # signcrypted chunk 197b102766befc2d52d09728c0b9d749392f6a8c38229a682891c6b1ee28ce06402e53196c408dd716c2a97185270076e94a6e7bd6e549fb2935641981be1809604316e0e868260687dac537a7f4d027c9d278, # final flag True, ]