Add checkpoint verification

sigstore-java/src/main/java/dev/sigstore/KeylessVerifier.java

@@ -177,7 +177,7 @@ public void verify(byte[] artifactDigest, KeylessVerificationRequest request)
     try {
       rekorVerifier.verifyEntry(rekorEntry);
     } catch (RekorVerificationException ex) {
-      throw new KeylessVerificationException("Rekor entry signature was not valid");
+      throw new KeylessVerificationException("Rekor entry signature was not valid", ex);
     }
 
     // check if the time of entry inclusion in the log (a stand-in for signing time) is within the

sigstore-java/src/main/java/dev/sigstore/rekor/client/RekorVerifier.java

@@ -17,12 +17,15 @@
 
 import com.google.common.hash.Hashing;
 import dev.sigstore.encryption.signers.Verifiers;
+import dev.sigstore.rekor.client.RekorEntry.Checkpoint;
 import dev.sigstore.trustroot.SigstoreTrustedRoot;
+import dev.sigstore.trustroot.TransparencyLog;
 import dev.sigstore.trustroot.TransparencyLogs;
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
 import java.security.SignatureException;
 import java.security.spec.InvalidKeySpecException;
+import java.util.Arrays;
 import java.util.Base64;
 import org.bouncycastle.util.encoders.Hex;
 
@@ -84,6 +87,7 @@ public void verifyEntry(RekorEntry entry) throws RekorVerificationException {
 
     // verify inclusion proof
     verifyInclusionProof(entry);
+    verifyCheckpoint(entry, tlog);
   }
 
   /** Verify that a Rekor Entry is in the log by checking inclusion proof. */
@@ -134,6 +138,43 @@ private void verifyInclusionProof(RekorEntry entry) throws RekorVerificationExce
     }
   }
 
+  private void verifyCheckpoint(RekorEntry entry, TransparencyLog tlog)
+      throws RekorVerificationException {
+    Checkpoint checkpoint;
+    try {
+      checkpoint = entry.getVerification().getInclusionProof().parsedCheckpoint();
+    } catch (RekorParseException ex) {
+      throw new RekorVerificationException("Could not parse checkpoint", ex);
+    }
+
+    byte[] inclusionRootHash =
+        Hex.decode(entry.getVerification().getInclusionProof().getRootHash());
+    byte[] checkpointRootHash = Base64.getDecoder().decode(checkpoint.getBase64Hash());
+
+    if (!Arrays.equals(inclusionRootHash, checkpointRootHash)) {
+      throw new RekorVerificationException(
+          "Checkpoint root hash does not match root hash provided in inclusion proof");
+    }
+    var keyHash = Hashing.sha256().hashBytes(tlog.getPublicKey().getRawBytes()).asBytes();
+    // checkpoint 0 is always the log, not any of the cross signing verifiers/monitors
+    var sig = checkpoint.getSignatures().get(0);
+    for (int i = 0; i < 4; i++) {
+      if (sig.getKeyHint()[i] != keyHash[i]) {
+        throw new RekorVerificationException(
+            "Checkpoint key hint did not match provided log public key");
+      }
+    }
+    try {
+      Verifiers.newVerifier(tlog.getPublicKey().toJavaPublicKey())
+          .verifyDigest(inclusionRootHash, sig.getSignature());
+    } catch (NoSuchAlgorithmException
+        | InvalidKeySpecException
+        | SignatureException
+        | InvalidKeyException ex) {
+      throw new RekorVerificationException("Could not verify checkpoint signature", ex);
+    }
+  }
+
   private static byte[] combineBytes(byte[] first, byte[] second) {
     byte[] result = new byte[first.length + second.length];
     System.arraycopy(first, 0, result, 0, first.length);

sigstore-java/src/test/resources/dev/sigstore/samples/rekor-response/valid/entry.json

@@ -11,17 +11,23 @@
           "67e9d9f66f0ad388f7e1a20991e9a2ae3efad5cbf281e8b3d2aaf1ef99a4618c",
           "16a106400c53465f6e18c2475df6ba889ca30f5667bacf32b1a5661f14a5080c",
           "b4439e8d71edbc96271723cb7a969dd725e23e73d139361864a62ed76ce8dc11",
-          "f4926d3efb0abd42b18e18372886854825a2b378e249bd333f81d8d4534485c4",
+          "49b3e90806c7b63b5a86f5748e3ecb7d264ea0828eb74a45bc1a2cd7962408e8",
+          "5059ad9b48fa50bd9adcbff0dd81c5a0dcb60f37e0716e723a33805a464f72f8",
           "6c2ce64219799e61d72996884eee9e19fb906e4d7fa04b71625fde4108f21762",
           "784f79c817abb78db3ae99b6c1ede640470bf4bb678673a05bf3a6b50aaaddd6",
-          "0da021f68571b65e49e926e4c69024de3ac248a1319d254bc51a85a657b93c33"
+          "c6d92ebf4e10cdba500ca410166cd0a8d8b312154d2f45bc4292d63dea6112f6",
+          "1768732027401f6718b0df7769e2803127cfc099eb130a8ed7d913218f6a65f6",
+          "0da021f68571b65e49e926e4c69024de3ac248a1319d254bc51a85a657b93c33",
+          "bc8cf0c8497d5c24841de0c9bef598ec99bbd59d9538d58568340646fe289e9a",
+          "be328fa737b8fa9461850b8034250f237ff5b0b590b9468e6223968df294872b",
+          "6f06f4025d0346f04830352b23f65c8cd9e3ce4b8cb899877c35282521ddaf85"
         ],
         "logIndex": 1227,
-        "rootHash": "c2aaeaf36d5899ee2ab6d931bdc39d0c50e6a9dccee0322b6a9483538f4ef079",
-        "checkpoint": "The checkpoint (signed tree head) that the inclusion proof is based on",
-        "treeSize": 1237
+        "rootHash": "effa4fa4575f72829016a64e584441203de533212f9470d63a56d1992e73465d",
+        "treeSize": 14358,
+        "checkpoint": "rekor.sigstage.dev - 108574341321668964\n14358\n7/pPpFdfcoKQFqZOWERBID3lMyEvlHDWOlbRmS5zRl0=\n\n— rekor.sigstage.dev 0y8wozBFAiB8OkuzdwlL6/rDEu2CsIfqmesaH/KLfmIMvlH3YTdIYgIhAPFZeXK6+b0vbWy4GSU/YZxiTpFrrzjsVOShN4LlPdZb\n"
       },
-      "signedEntryTimestamp": "MEUCIAM2WgNNS1xwUBuoX/rYUBxbyKa6PVwNakss5KJzxw4jAiEA1//71yKdbHUi+rZRX7UyWBf4yBRp1vrkvOcWx6bbqfY="
+      "signedEntryTimestamp": "MEUCIQCO8dFvolJwFZDHkhkSdsW3Ny+07fG8CF7G32feG8NJMgIgd2qfJ5shezuXX8I1S6DsudvIZ8xN/+y95at/V5xHfEQ="
     }
   }
 }