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Module dm

huddle_core::crypto

Module dm 

Source
Expand description

huddle 0.7.1: End-to-end DM key derivation via Ed25519→X25519 ECDH.

Both peers in a 1-1 DM derive the same 32-byte room key from their long-term Ed25519 identity keys — no shared passphrase, no central key agreement, no extra round-trip beyond MemberAnnounce for the partner’s pubkey.

Steps:

  1. Ed25519 seed → X25519 secret. We hash the seed with SHA-512 and take the first 32 bytes; StaticSecret::from(bytes) performs the canonical X25519 clamping. This is the same conversion libsodium uses in crypto_sign_ed25519_sk_to_curve25519.
  2. Ed25519 pubkey → X25519 pubkey via the birational Edwards-to-Montgomery map (VerifyingKey::to_montgomery). Matches crypto_sign_ed25519_pk_to_curve25519.
  3. X25519 Diffie-Hellman gives a 32-byte shared secret.
  4. HKDF-SHA256 expands it to the room key, binding the result to the canonical DM room_id via the info parameter so this DM’s key can never collide with any other context.

The output replaces the Argon2id-derived passphrase_key in the existing encrypted-room flow. The wrap / unwrap helpers in crypto::passphrase accept any [u8; 32], so no other changes are needed downstream — DMs and group rooms share the Megolm path.

Functions§

derive_dm_key
Derive the symmetric DM room key from one side’s Ed25519 secret seed and the other side’s Ed25519 public key, plus the canonical DM room_id (which binds the key to this specific 1-1 channel).