quicproquo/crates/quicnprotochat-core/src/identity.rs

//! Ed25519 identity keypair for MLS credentials and AS registration.
//!
//! The [`IdentityKeypair`] is the long-term identity key embedded in MLS
//! `BasicCredential`s. It is used for signing MLS messages and as the
//! indexing key for the Authentication Service.
//!
//! # Zeroize
//!
//! The 32-byte private seed is stored as `Zeroizing<[u8; 32]>`, which zeroes
//! the bytes on drop. `[u8; 32]` is `Copy + Default` and satisfies zeroize's
//! `DefaultIsZeroes` constraint, avoiding a conflict with ed25519-dalek's
//! `SigningKey` zeroize impl.
//!
//! # Fingerprint
//!
//! A 32-byte SHA-256 digest of the raw public key bytes is used as a compact,
//! collision-resistant identifier for logging.

use ed25519_dalek::{Signer as DalekSigner, SigningKey, VerifyingKey};
use openmls_traits::signatures::Signer;
use openmls_traits::types::{Error as MlsError, SignatureScheme};
use serde::{Deserialize, Serialize};
use sha2::{Digest, Sha256};
use zeroize::Zeroizing;

/// An Ed25519 identity keypair.
///
/// Created with [`IdentityKeypair::generate`]. The private signing key seed
/// is zeroed when this struct is dropped.
pub struct IdentityKeypair {
    /// Raw 32-byte private seed — zeroized on drop.
    ///
    /// Stored as bytes rather than `SigningKey` to satisfy zeroize's
    /// `DefaultIsZeroes` bound on `Zeroizing<T>`.
    seed: Zeroizing<[u8; 32]>,
    /// Corresponding 32-byte public verifying key.
    verifying: VerifyingKey,
}

impl IdentityKeypair {
    /// Recreate an identity keypair from a 32-byte seed.
    pub fn from_seed(seed: [u8; 32]) -> Self {
        let signing = SigningKey::from_bytes(&seed);
        let verifying = signing.verifying_key();
        Self {
            seed: Zeroizing::new(seed),
            verifying,
        }
    }

    /// Return the raw 32-byte private seed (for persistence).
    pub fn seed_bytes(&self) -> [u8; 32] {
        *self.seed
    }
}

impl IdentityKeypair {
    /// Generate a fresh random Ed25519 identity keypair.
    pub fn generate() -> Self {
        use rand::rngs::OsRng;
        let signing = SigningKey::generate(&mut OsRng);
        let verifying = signing.verifying_key();
        let seed = Zeroizing::new(signing.to_bytes());
        Self { seed, verifying }
    }

    /// Return the raw 32-byte Ed25519 public key.
    ///
    /// This is the byte array used as `identityKey` in `auth.capnp` calls.
    pub fn public_key_bytes(&self) -> [u8; 32] {
        self.verifying.to_bytes()
    }

    /// Return the SHA-256 fingerprint of the public key (32 bytes).
    pub fn fingerprint(&self) -> [u8; 32] {
        let mut hasher = Sha256::new();
        hasher.update(self.verifying.to_bytes());
        hasher.finalize().into()
    }

    /// Reconstruct the `SigningKey` from the stored seed bytes.
    fn signing_key(&self) -> SigningKey {
        SigningKey::from_bytes(&self.seed)
    }
}

/// Implement the openmls `Signer` trait so `IdentityKeypair` can be passed
/// directly to `KeyPackage::builder().build(...)` without needing the external
/// `openmls_basic_credential` crate.
impl Signer for IdentityKeypair {
    fn sign(&self, payload: &[u8]) -> Result<Vec<u8>, MlsError> {
        let sk = self.signing_key();
        let sig: ed25519_dalek::Signature = sk.sign(payload);
        Ok(sig.to_bytes().to_vec())
    }

    fn signature_scheme(&self) -> SignatureScheme {
        SignatureScheme::ED25519
    }
}

impl Serialize for IdentityKeypair {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        serializer.serialize_bytes(&self.seed[..])
    }
}

impl<'de> Deserialize<'de> for IdentityKeypair {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        let bytes: Vec<u8> = serde::Deserialize::deserialize(deserializer)?;
        let seed: [u8; 32] = bytes
            .as_slice()
            .try_into()
            .map_err(|_| serde::de::Error::custom("identity seed must be 32 bytes"))?;
        Ok(IdentityKeypair::from_seed(seed))
    }
}

impl std::fmt::Debug for IdentityKeypair {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let fp = self.fingerprint();
        f.debug_struct("IdentityKeypair")
            .field(
                "fingerprint",
                &format!("{:02x}{:02x}{:02x}{:02x}…", fp[0], fp[1], fp[2], fp[3]),
            )
            .finish_non_exhaustive()
    }
}