fn derive_key_and_nonce()

in src/aes128gcm.rs [521:565]

• 41 lines of code
• 3 McCabe index (conditional complexity)

fn derive_key_and_nonce(
    cryptographer: &dyn Cryptographer,
    ece_mode: EceMode,
    local_prv_key: &dyn LocalKeyPair,
    remote_pub_key: &dyn RemotePublicKey,
    auth_secret: &[u8],
    salt: &[u8],
) -> Result<KeyAndNonce> {
    if auth_secret.len() != ECE_WEBPUSH_AUTH_SECRET_LENGTH {
        return Err(Error::InvalidAuthSecret);
    }
    if salt.len() != ECE_SALT_LENGTH {
        return Err(Error::InvalidSalt);
    }

    let shared_secret = cryptographer.compute_ecdh_secret(remote_pub_key, local_prv_key)?;
    let raw_remote_pub_key = remote_pub_key.as_raw()?;
    let raw_local_pub_key = local_prv_key.pub_as_raw()?;

    // The "aes128gcm" scheme includes the sender and receiver public keys in
    // the info string when deriving the Web Push IKM.
    let ikm_info = match ece_mode {
        EceMode::Encrypt => generate_info(&raw_remote_pub_key, &raw_local_pub_key),
        EceMode::Decrypt => generate_info(&raw_local_pub_key, &raw_remote_pub_key),
    }?;
    let ikm = cryptographer.hkdf_sha256(
        auth_secret,
        &shared_secret,
        &ikm_info,
        ECE_WEBPUSH_IKM_LENGTH,
    )?;
    let key = cryptographer.hkdf_sha256(
        salt,
        &ikm,
        ECE_AES128GCM_KEY_INFO.as_bytes(),
        ECE_AES_KEY_LENGTH,
    )?;
    let nonce = cryptographer.hkdf_sha256(
        salt,
        &ikm,
        ECE_AES128GCM_NONCE_INFO.as_bytes(),
        ECE_NONCE_LENGTH,
    )?;
    Ok((key, nonce))
}