sign: Signed Certificate Timestamp validation

Signed-off-by: Andrew Pan <andrew.pan@trailofbits.com>
Co-authored-by: Alex Cameron <alex.cameron@trailofbits.com>
Signed-off-by: Andrew Pan <andrew.pan@trailofbits.com>

src/crypto/keyring.rs

@@ -0,0 +1,165 @@
+// Copyright 2023 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::collections::HashMap;
+
+use const_oid::db::rfc5912::{ID_EC_PUBLIC_KEY, RSA_ENCRYPTION, SECP_256_R_1};
+use digest::Digest;
+use ring::{signature as ring_signature, signature::UnparsedPublicKey};
+use thiserror::Error;
+use x509_cert::{
+    der,
+    der::{Decode, Encode},
+    spki::SubjectPublicKeyInfoOwned,
+};
+
+#[derive(Error, Debug)]
+pub enum KeyringError {
+    #[error("malformed key")]
+    KeyMalformed(#[from] x509_cert::der::Error),
+    #[error("unsupported algorithm")]
+    AlgoUnsupported,
+
+    #[error("requested key not in keyring")]
+    KeyNotFound,
+    #[error("verification failed")]
+    VerificationFailed,
+}
+type Result<T> = std::result::Result<T, KeyringError>;
+
+/// A CT signing key.
+struct Key {
+    inner: UnparsedPublicKey<Vec<u8>>,
+    /// The key's RFC 6962-style "key ID".
+    /// <https://datatracker.ietf.org/doc/html/rfc6962#section-3.2>
+    fingerprint: [u8; 32],
+}
+
+impl Key {
+    /// Creates a `Key` from a DER blob containing a SubjectPublicKeyInfo object.
+    pub fn new(spki_bytes: &[u8]) -> Result<Self> {
+        let spki = SubjectPublicKeyInfoOwned::from_der(spki_bytes)?;
+        let (algo, params) = if let Some(params) = &spki.algorithm.parameters {
+            // Special-case RSA keys, which don't have SPKI parameters.
+            if spki.algorithm.oid == RSA_ENCRYPTION && params == &der::Any::null() {
+                // TODO(tnytown): Do we need to support RSA keys?
+                return Err(KeyringError::AlgoUnsupported);
+            };
+
+            (spki.algorithm.oid, params.decode_as()?)
+        } else {
+            return Err(KeyringError::AlgoUnsupported);
+        };
+
+        match (algo, params) {
+            // TODO(tnytown): should we also accept ed25519, p384, ... ?
+            (ID_EC_PUBLIC_KEY, SECP_256_R_1) => Ok(Key {
+                inner: UnparsedPublicKey::new(
+                    &ring_signature::ECDSA_P256_SHA256_ASN1,
+                    spki.subject_public_key.raw_bytes().to_owned(),
+                ),
+                fingerprint: {
+                    let mut hasher = sha2::Sha256::new();
+                    spki.encode(&mut hasher).expect("failed to hash key!");
+                    hasher.finalize().into()
+                },
+            }),
+            _ => Err(KeyringError::AlgoUnsupported),
+        }
+    }
+}
+
+/// Represents a set of CT signing keys, each of which is potentially a valid signer for
+/// Signed Certificate Timestamps (SCTs) or Signed Tree Heads (STHs).
+pub struct Keyring(HashMap<[u8; 32], Key>);
+
+impl Keyring {
+    /// Creates a `Keyring` from DER encoded SPKI-format public keys.
+    pub fn new<'a>(keys: impl IntoIterator<Item = &'a [u8]>) -> Result<Self> {
+        Ok(Self(
+            keys.into_iter()
+                .flat_map(Key::new)
+                .map(|k| Ok((k.fingerprint, k)))
+                .collect::<Result<_>>()?,
+        ))
+    }
+
+    /// Verifies `data` against a `signature` with a public key identified by `key_id`.
+    pub fn verify(&self, key_id: &[u8; 32], signature: &[u8], data: &[u8]) -> Result<()> {
+        let key = self.0.get(key_id).ok_or(KeyringError::KeyNotFound)?;
+
+        key.inner
+            .verify(data, signature)
+            .or(Err(KeyringError::VerificationFailed))?;
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Keyring;
+    use crate::crypto::signing_key::ecdsa::{ECDSAKeys, EllipticCurve};
+    use digest::Digest;
+    use std::io::Write;
+
+    #[test]
+    fn verify_keyring() {
+        let message = b"some message";
+
+        // Create a key pair and a keyring containing the public key.
+        let key_pair = ECDSAKeys::new(EllipticCurve::P256).unwrap();
+        let signer = key_pair.to_sigstore_signer().unwrap();
+        let pub_key = key_pair.as_inner().public_key_to_der().unwrap();
+        let keyring = Keyring::new([pub_key.as_slice()]).unwrap();
+
+        // Generate the signature.
+        let signature = signer.sign(message).unwrap();
+
+        // Generate the key id.
+        let mut hasher = sha2::Sha256::new();
+        hasher.write(pub_key.as_slice()).unwrap();
+        let key_id: [u8; 32] = hasher.finalize().into();
+
+        // Check for success.
+        assert!(keyring
+            .verify(&key_id, signature.as_slice(), message)
+            .is_ok());
+
+        // Check for failure with incorrect key id.
+        assert!(keyring
+            .verify(&[0; 32], signature.as_slice(), message)
+            .is_err());
+
+        // Check for failure with incorrect payload.
+        let incorrect_message = b"another message";
+
+        assert!(keyring
+            .verify(&key_id, signature.as_slice(), incorrect_message)
+            .is_err());
+
+        // Check for failure with incorrect keyring.
+        let incorrect_key_pair = ECDSAKeys::new(EllipticCurve::P256).unwrap();
+        let incorrect_keyring = Keyring::new([incorrect_key_pair
+            .as_inner()
+            .public_key_to_der()
+            .unwrap()
+            .as_slice()])
+        .unwrap();
+
+        assert!(incorrect_keyring
+            .verify(&key_id, signature.as_slice(), message)
+            .is_err());
+    }
+}

src/crypto/mod.rs

@@ -177,6 +177,8 @@ pub(crate) mod certificate;
 pub(crate) mod certificate_pool;
 #[cfg(feature = "cert")]
 pub(crate) use certificate_pool::CertificatePool;
+#[cfg(feature = "cert")]
+pub(crate) mod keyring;
 
 pub mod verification_key;
 
@@ -188,6 +190,9 @@ use self::signing_key::{
 
 pub mod signing_key;
 
+#[cfg(feature = "sign")]
+pub(crate) mod transparency;
+
 #[cfg(test)]
 pub(crate) mod tests {
     use chrono::{DateTime, TimeDelta, Utc};