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//! RSA->Ed25519 cross-certificates
//!
//! These are used in the Tor link handshake to prove that a given ed25519
//! key speaks for a given (deprecated) RSA identity.
use tor_bytes::Reader;
use tor_checkable::{ExternallySigned, timed::TimerangeBound};
use tor_llcrypto as ll;
use digest::Digest;
use crate::{CertType, ExpiryHours};
mod encode;
pub use encode::EncodedRsaCrosscert;
/// A RSA->Ed25519 cross-certificate
///
/// This kind of certificate is used in the channel handshake to prove
/// that the Ed25519 identity key speaks on behalf of the RSA identity key.
/// (There is no converse type for certifying Ed25519 identity keys with
/// RSA identity keys, since the RSA identity keys are too weak to trust.)
#[must_use]
pub struct RsaCrosscert {
/// The key that is being certified
subject_key: ll::pk::ed25519::Ed25519Identity,
/// The expiration time of this certificate, in hours since the
/// unix epoch.
exp_hours: ExpiryHours,
/// The digest of the signed part of the certificate (for checking)
digest: [u8; 32],
/// The (alleged) signature on the certificate.
signature: Vec<u8>,
}
/// Prefix appended when generating a digest for an RsaCrosscert
const PREFIX: &[u8] = b"Tor TLS RSA/Ed25519 cross-certificate";
/// Compute the SHA256 digest of `c`, prefixed with PREFIX.
fn compute_digest(c: &[u8]) -> [u8; 32] {
let mut d = ll::d::Sha256::new();
d.update(PREFIX);
d.update(c);
d.finalize().into()
impl RsaCrosscert {
/// Return the time at which this certificate becomes expired
pub fn expiry(&self) -> std::time::SystemTime {
self.exp_hours.into()
/// Return a reference to the digest.
pub fn digest(&self) -> &[u8; 32] {
&self.digest
/// Return true if the subject key in this certificate matches `other`
pub fn subject_key_matches(&self, other: &ll::pk::ed25519::Ed25519Identity) -> bool {
other == &self.subject_key
/// Return this certificate cert type.
pub fn cert_type(&self) -> CertType {
CertType::RSA_ID_V_IDENTITY
/// Decode a slice of bytes into an RSA crosscert.
pub fn decode(bytes: &[u8]) -> tor_bytes::Result<UncheckedRsaCrosscert> {
let mut r = Reader::from_slice(bytes);
let signed_portion = r.peek(36)?; // TODO(nickm): a bit ugly.
let subject_key = r.extract()?;
let exp_hours = r.extract()?;
let siglen = r.take_u8()?;
let signature = r.take(siglen as usize)?.into();
let digest = compute_digest(signed_portion);
let cc = RsaCrosscert {
subject_key,
exp_hours,
digest,
signature,
};
Ok(UncheckedRsaCrosscert(cc))
/// An RsaCrosscert whose signature has not been checked.
pub struct UncheckedRsaCrosscert(RsaCrosscert);
impl ExternallySigned<TimerangeBound<RsaCrosscert>> for UncheckedRsaCrosscert {
type Key = ll::pk::rsa::PublicKey;
type KeyHint = ();
type Error = tor_bytes::Error;
fn key_is_correct(&self, _k: &Self::Key) -> Result<(), Self::KeyHint> {
// there is no way to check except for trying to verify the signature
Ok(())
fn is_well_signed(&self, k: &Self::Key) -> Result<(), Self::Error> {
k.verify(&self.0.digest[..], &self.0.signature[..])
.map_err(|_| {
tor_bytes::Error::InvalidMessage(
"Invalid signature on RSA->Ed identity crosscert".into(),
)
})?;
fn dangerously_assume_wellsigned(self) -> TimerangeBound<RsaCrosscert> {
let expiration = self.0.expiry();
TimerangeBound::new(self.0, ..expiration)