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X3DH Key Agreement

Extended Triple Diffie-Hellman establishes the initial shared secret between two parties using their identity keys and pre-keys. No interactive handshake required.

pub fn initiate(
    alice_identity: &IdentityKeyPair,
    bob_bundle: &PreKeyBundle,
) -> Result<X3DHResult> {
    let ephemeral = StaticSecret::random_from_rng(OsRng);

    // DH1 = DH(IK_a, SPK_b)
    let dh1 = alice_identity.dh(&bob_bundle.signed_pre_key);

    // DH2 = DH(EK_a, IK_b)
    let dh2 = ephemeral.diffie_hellman(&bob_bundle.identity_key);

    // DH3 = DH(EK_a, SPK_b)
    let dh3 = ephemeral.diffie_hellman(&bob_bundle.signed_pre_key);

    // Optional: DH4 = DH(EK_a, OPK_b)
    let dh4 = bob_bundle.one_time_pre_key
        .as_ref()
        .map(|opk| ephemeral.diffie_hellman(opk));

    // Derive shared secret via HKDF
    let shared_secret = kdf::derive(
        &[dh1, dh2, dh3, dh4].concat(),
        X3DH_SALT,
        X3DH_INFO,
        32,
    )?;

    Ok(X3DHResult { shared_secret, ephemeral_key: ephemeral.into() })
}