purecrypto 0.1.1

//! TLS 1.3 handshake signatures (RFC 8446 §4.4.3).
//!
//! A `CertificateVerify` proves possession of the certified key by signing a
//! context-bound digest of the handshake transcript. The signature scheme is a
//! 16-bit `SignatureScheme` code (not an X.509 OID); dispatch goes through
//! [`crate::signature_registry`]: the scheme code picks a registry entry,
//! whose `verify(spki, message, signature)` re-parses the SPKI and delegates
//! to the underlying primitive.

use crate::ec::CurveId;
use crate::hash::{Sha256, Sha384, Sha512};
use crate::rng::RngCore;
use crate::signature_registry::{SignaturePolicy, find_by_tls_scheme};
use crate::tls::Error;
use crate::tls::codec::SignatureScheme;
use crate::tls::conn::ServerKey;
use crate::x509::{AnyPublicKey, Error as X509Error};
use alloc::vec::Vec;

/// The 64 `0x20` (space) octets that prefix the signed content (RFC 8446
/// §4.4.3), guarding against cross-protocol signature reuse.
const SIG_PREFIX: [u8; 64] = [0x20; 64];

/// Builds the octet string signed in a `CertificateVerify`:
/// `0x20 * 64 || context_string || 0x00 || Transcript-Hash(Handshake Context)`.
///
/// `server` selects the server context string (the peer that signs during a
/// normal 1-RTT handshake) versus the client one (client authentication).
pub(crate) fn certificate_verify_content(server: bool, transcript_hash: &[u8]) -> Vec<u8> {
    let context: &[u8] = if server {
        b"TLS 1.3, server CertificateVerify"
    } else {
        b"TLS 1.3, client CertificateVerify"
    };
    let mut out = Vec::with_capacity(SIG_PREFIX.len() + context.len() + 1 + transcript_hash.len());
    out.extend_from_slice(&SIG_PREFIX);
    out.extend_from_slice(context);
    out.push(0);
    out.extend_from_slice(transcript_hash);
    out
}

/// The IANA-blessed [`SignatureScheme`] code for the given [`ServerKey`].
pub(crate) fn signature_scheme_for(key: &ServerKey) -> SignatureScheme {
    match key {
        ServerKey::Rsa(_) => SignatureScheme::RSA_PSS_RSAE_SHA256,
        ServerKey::Ecdsa(k) => match k.curve() {
            CurveId::P256 => SignatureScheme::ECDSA_SECP256R1_SHA256,
            CurveId::P384 => SignatureScheme::ECDSA_SECP384R1_SHA384,
            CurveId::P521 => SignatureScheme::ECDSA_SECP521R1_SHA512,
            CurveId::Secp256k1 => SignatureScheme::ECDSA_SECP256R1_SHA256,
        },
        ServerKey::Ed25519(_) => SignatureScheme::ED25519,
        ServerKey::MlDsa44(_) => SignatureScheme::MLDSA44,
        ServerKey::MlDsa65(_) => SignatureScheme::MLDSA65,
        ServerKey::MlDsa87(_) => SignatureScheme::MLDSA87,
    }
}

/// Signs `content` for a TLS 1.3 / DTLS 1.3 `CertificateVerify` using
/// `key`, returning the (scheme, signature_bytes) tuple. Dispatches over
/// every supported key type — RSA-PSS, ECDSA (any curve), Ed25519,
/// ML-DSA-44/65/87.
pub(crate) fn sign_certificate_verify<R: RngCore>(
    key: &ServerKey,
    content: &[u8],
    rng: &mut R,
) -> Result<(SignatureScheme, Vec<u8>), Error> {
    let scheme = signature_scheme_for(key);
    let signature = match key {
        ServerKey::Rsa(k) => k
            .sign_pss::<Sha256, _>(content, rng)
            .map_err(|_| Error::HandshakeFailure)?,
        ServerKey::Ecdsa(k) => {
            let curve = k.curve();
            let sig = match curve {
                CurveId::P384 => k.sign::<Sha384>(content),
                CurveId::P521 => k.sign::<Sha512>(content),
                _ => k.sign::<Sha256>(content),
            }
            .map_err(|_| Error::HandshakeFailure)?;
            sig.to_der(curve)
        }
        ServerKey::Ed25519(k) => k.sign(content).to_bytes().to_vec(),
        // ML-DSA: raw FIPS 204 signature bytes; no DER wrapping. Hedged
        // with the supplied RNG.
        ServerKey::MlDsa44(k) => k
            .sign(rng, content, b"")
            .map_err(|_| Error::HandshakeFailure)?,
        ServerKey::MlDsa65(k) => k
            .sign(rng, content, b"")
            .map_err(|_| Error::HandshakeFailure)?,
        ServerKey::MlDsa87(k) => k
            .sign(rng, content, b"")
            .map_err(|_| Error::HandshakeFailure)?,
    };
    Ok((scheme, signature))
}

/// Verifies a TLS 1.3 handshake signature of `message` under `key`, dispatching
/// through [`crate::signature_registry`] on `scheme`.
///
/// `policy` gates the scheme: a scheme not on its whitelist (even one in the
/// registry) is rejected with [`Error::BadCertificate`].
///
/// Returns [`Error::PeerMisbehaved`] if the scheme is unsupported by the
/// registry or does not match the key type, [`Error::Decode`] if the
/// signature wire format is malformed, and [`Error::BadCertificate`] if the
/// signature is otherwise invalid (or policy-rejected).
pub(crate) fn verify_signature(
    scheme: SignatureScheme,
    key: &AnyPublicKey,
    message: &[u8],
    signature: &[u8],
    policy: &SignaturePolicy,
) -> Result<(), Error> {
    let algo = find_by_tls_scheme(scheme.0).ok_or(Error::PeerMisbehaved)?;
    // The registry verifier needs an SPKI; round-trip the parsed key. (A few
    // hundred bytes of allocation per CertificateVerify is negligible next to
    // the asymmetric verify itself.)
    let spki = key.to_spki_der();
    if !policy.permits(algo, &spki) {
        return Err(Error::BadCertificate);
    }
    match algo.verify(&spki, message, signature) {
        Ok(()) => Ok(()),
        // `UnsupportedAlgorithm` here means the SPKI's key type doesn't match
        // the scheme (e.g. an RSA key against `ecdsa_secp256r1_sha256`).
        Err(X509Error::UnsupportedAlgorithm) => Err(Error::PeerMisbehaved),
        Err(X509Error::Malformed) => Err(Error::Decode),
        Err(_) => Err(Error::BadCertificate),
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::hash::{Digest, Sha256};
    use crate::test_util::from_hex_vec;
    use crate::x509::Certificate;

    // RFC 8448 §3: verify the server's CertificateVerify (rsa_pss_rsae_sha256,
    // RSA-1024 certified key) over the reconstructed transcript.
    #[test]
    fn rfc8448_certificate_verify() {
        let ch = from_hex_vec(include_str!("../../../testdata/rfc8448_client_hello.hex"));
        let sh = from_hex_vec(include_str!("../../../testdata/rfc8448_server_hello.hex"));
        let flight = from_hex_vec(include_str!(
            "../../../testdata/rfc8448_server_flight_payload.hex"
        ));

        // Server key from the Certificate message (cert DER at offset 51..483).
        let cert = Certificate::from_der(flight[51..483].to_vec()).unwrap();
        let key = cert.subject_public_key().unwrap();

        // Transcript-Hash(ClientHello .. Certificate): CH || SH || EE || Cert,
        // where EE||Cert is flight[0..485].
        let mut transcript = Vec::new();
        transcript.extend_from_slice(&ch);
        transcript.extend_from_slice(&sh);
        transcript.extend_from_slice(&flight[0..485]);
        let th = Sha256::digest(&transcript);

        let content = certificate_verify_content(true, th.as_ref());

        // CertificateVerify message at flight[485..621]: 0f 00 00 84 | 08 04 |
        // 00 80 | sig(128).
        let scheme = SignatureScheme(u16::from_be_bytes([flight[489], flight[490]]));
        assert_eq!(scheme, SignatureScheme::RSA_PSS_RSAE_SHA256);
        let sig = &flight[493..621];

        // RFC 8448 uses an RSA-1024 server key; the modern default policy
        // floors RSA at 2048. Loosen for this single legacy fixture.
        let policy = SignaturePolicy::modern().with_min_rsa_bits(1024);
        verify_signature(scheme, &key, &content, sig, &policy).unwrap();

        // A tampered transcript must not verify.
        let mut bad = content.clone();
        *bad.last_mut().unwrap() ^= 0x01;
        assert!(matches!(
            verify_signature(scheme, &key, &bad, sig, &policy),
            Err(Error::BadCertificate)
        ));

        // Wrong scheme for an RSA key (ECDSA) is a misbehavior, not a bad sig.
        assert!(matches!(
            verify_signature(
                SignatureScheme::ECDSA_SECP256R1_SHA256,
                &key,
                &content,
                sig,
                &policy,
            ),
            Err(Error::PeerMisbehaved)
        ));
    }
}