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potto216 / double-ratchet-rs

A pure Rust implementation of the Double Ratchet algorithm as described by Signal.
MIT License
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modify kdf_ck so it matches the Signal protocol spec #1

Closed potto216 closed 2 weeks ago

potto216 commented 2 weeks ago

The Signal protocol page says for the kdf_ck function

KDF_CK(ck): HMAC [2] with SHA-256 or SHA-512 [8] is recommended, using ck as the HMAC key and using separate constants as input (e.g. a single byte 0x01 as input to produce the message key, and a single byte 0x02 as input to produce the next chain key). KDF_CK(ck): Returns a pair (32-byte chain key, 32-byte message key) as the output of applying a KDF keyed by a 32-byte chain key ck to some constant.

So modify the function so a single byte 0x01 as input to produce the message key, and a single byte 0x02 as input to produce the next chain key.

potto216 commented 2 weeks ago

kdf_ck_v2 has the modification. Eventually replace kdf_ck with kdf_ck_v2.