Add support for verifying dsse-intoto

- Verification should be able to correctly validate a bundle as
  cryptographically valid (VerificationOptions.empty())
- Verifiers may also include signer identity during verification
- Verifiers should extract the embedded attestation to do further
  analysis on the attestation. Sigstore-java does not process
  those in any way
- There is no signing options for DSSE bundles

Signed-off-by: Appu Goundan <[email protected]>

.github/workflows/conformance.yml

@@ -39,4 +39,4 @@ jobs:
         with:
           entrypoint: ${{ github.workspace }}/bin/sigstore-cli
           environment: ${{ matrix.sigstore-env }}
-          xfail: "test_verify_dsse_bundle_with_trust_root test_verify_in_toto_in_dsse_envelope"
+          xfail: "test_verify_dsse_bundle_with_trust_root"

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

@@ -22,7 +22,9 @@
 import dev.sigstore.VerificationOptions.CertificateMatcher;
 import dev.sigstore.VerificationOptions.UncheckedCertificateException;
 import dev.sigstore.bundle.Bundle;
+import dev.sigstore.bundle.Bundle.DsseEnvelope;
 import dev.sigstore.bundle.Bundle.MessageSignature;
+import dev.sigstore.dsse.InTotoPayload;
 import dev.sigstore.encryption.certificates.Certificates;
 import dev.sigstore.encryption.signers.Verifiers;
 import dev.sigstore.fulcio.client.FulcioVerificationException;
@@ -33,6 +35,8 @@
 import dev.sigstore.rekor.client.RekorTypes;
 import dev.sigstore.rekor.client.RekorVerificationException;
 import dev.sigstore.rekor.client.RekorVerifier;
+import dev.sigstore.rekor.dsse.v0_0_1.Dsse;
+import dev.sigstore.rekor.dsse.v0_0_1.PayloadHash;
 import dev.sigstore.tuf.SigstoreTufClient;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
@@ -52,6 +56,7 @@
 import java.util.List;
 import java.util.Objects;
 import java.util.stream.Collectors;
+import org.bouncycastle.util.encoders.DecoderException;
 import org.bouncycastle.util.encoders.Hex;
 
 /** Verify hashrekords from rekor signed using the keyless signing flow with fulcio certificates. */
@@ -125,8 +130,9 @@ public void verify(Path artifact, Bundle bundle, VerificationOptions options)
   public void verify(byte[] artifactDigest, Bundle bundle, VerificationOptions options)
       throws KeylessVerificationException {
 
-    if (bundle.getDsseEnvelope().isPresent()) {
-      throw new KeylessVerificationException("Cannot verify DSSE signature based bundles");
+    if (bundle.getDsseEnvelope().isEmpty() && bundle.getMessageSignature().isEmpty()) {
+      throw new IllegalStateException(
+          "Bundle must contain a message signature or dsse envelope to verify");
     }
     if (bundle.getMessageSignature().isEmpty()) {
       // this should be unreachable
@@ -182,7 +188,12 @@ public void verify(byte[] artifactDigest, Bundle bundle, VerificationOptions opt
       throw new KeylessVerificationException("Signing time was after certificate expiry", e);
     }
 
-    checkMessageSignature(bundle.getMessageSignature().get(), rekorEntry, artifactDigest, leafCert);
+    if (bundle.getMessageSignature().isPresent()) { // hashedrekord
+      checkMessageSignature(
+          bundle.getMessageSignature().get(), rekorEntry, artifactDigest, leafCert);
+    } else { // dsse
+      checkDsseEnvelope(rekorEntry, bundle.getDsseEnvelope().get(), artifactDigest, leafCert);
+    }
   }
 
   @VisibleForTesting
@@ -255,4 +266,114 @@ void checkMessageSignature(
       throw new KeylessVerificationException("Unexpected rekor type", re);
     }
   }
+
+  // do all dsse specific checks
+  void checkDsseEnvelope(
+      RekorEntry rekorEntry,
+      DsseEnvelope dsseEnvelope,
+      byte[] artifactDigest,
+      X509Certificate leafCert)
+      throws KeylessVerificationException {
+
+    // verify the artifact is in the subject list of the envelope
+    if (!Objects.equals(InTotoPayload.PAYLOAD_TYPE, dsseEnvelope.getPayloadType())) {
+      throw new KeylessVerificationException(
+          "DSSE envelope must have payload type "
+              + InTotoPayload.PAYLOAD_TYPE
+              + ", but found '"
+              + dsseEnvelope.getPayloadType()
+              + "'");
+    }
+    InTotoPayload payload = InTotoPayload.from(dsseEnvelope);
+
+    // find one sha256 hash in the subject list that matches the artifact hash
+    if (payload.getSubject().stream()
+        .noneMatch(
+            subject -> {
+              if (subject.getDigest().containsKey("sha256")) {
+                try {
+                  var digestBytes = Hex.decode(subject.getDigest().get("sha256"));
+                  return Arrays.equals(artifactDigest, digestBytes);
+                } catch (DecoderException de) {
+                  // ignore (assume false)
+                }
+              }
+              return false;
+            })) {
+      var providedHashes =
+          payload.getSubject().stream()
+              .map(s -> s.getDigest().getOrDefault("sha256", "no-sha256-hash"))
+              .collect(Collectors.joining(",", "[", "]"));
+
+      throw new KeylessVerificationException(
+          "Provided artifact digest does not match any subject sha256 digests in DSSE payload"
+              + "\nprovided(hex) : "
+              + Hex.toHexString(artifactDigest)
+              + "\nverification  : "
+              + providedHashes);
+    }
+
+    // verify the dsse signature
+    if (dsseEnvelope.getSignatures().size() != 1) {
+      throw new KeylessVerificationException(
+          "DSSE envelope must have exactly 1 signature, but found: "
+              + dsseEnvelope.getSignatures().size());
+    }
+    try {
+      if (!Verifiers.newVerifier(leafCert.getPublicKey())
+          .verify(dsseEnvelope.getPAE(), dsseEnvelope.getSignature())) {
+        throw new KeylessVerificationException("DSSE signature was not valid");
+      }
+    } catch (NoSuchAlgorithmException | InvalidKeyException ex) {
+      throw new RuntimeException(ex);
+    } catch (SignatureException se) {
+      throw new KeylessVerificationException("Signature could not be processed", se);
+    }
+
+    // check if the digest over the dsse payload matches the digest in the rekorEntry
+    Dsse rekorDsse;
+    try {
+      rekorDsse = RekorTypes.getDsse(rekorEntry);
+    } catch (RekorTypeException re) {
+      throw new KeylessVerificationException("Unexpected rekor type", re);
+    }
+
+    var algorithm = rekorDsse.getPayloadHash().getAlgorithm();
+    if (algorithm != PayloadHash.Algorithm.SHA_256) {
+      throw new KeylessVerificationException(
+          "Cannot process DSSE entry with hashing algorithm " + algorithm.toString());
+    }
+
+    byte[] payloadDigest;
+    try {
+      payloadDigest = Hex.decode(rekorDsse.getPayloadHash().getValue());
+    } catch (DecoderException de) {
+      throw new KeylessVerificationException(
+          "Could not decode hex sha256 artifact hash in hashrekord", de);
+    }
+
+    byte[] calculatedDigest = Hashing.sha256().hashBytes(dsseEnvelope.getPayload()).asBytes();
+    if (!Arrays.equals(calculatedDigest, payloadDigest)) {
+      throw new KeylessVerificationException(
+          "Digest of DSSE payload in bundle does not match DSSE payload digest in log entry"
+              + "\nbundle(hex) : "
+              + Hex.toHexString(calculatedDigest)
+              + "\nlog(hex) : "
+              + Hex.toHexString(payloadDigest));
+    }
+
+    // check if the signature over the dsse payload matches the signature in the rekorEntry
+    if (rekorDsse.getSignatures().size() != 1) {
+      throw new KeylessVerificationException(
+          "DSSE log entry must have exactly 1 signature, but found: "
+              + rekorDsse.getSignatures().size());
+    }
+
+    if (!Base64.getEncoder()
+        .encodeToString(dsseEnvelope.getSignature())
+        .equals(rekorDsse.getSignatures().get(0).getSignature())) {
+      throw new KeylessVerificationException(
+          "Provided DSSE signature materials are inconsistent with DSSE log entry");
+    }
+  }
 }

sigstore-java/src/test/java/dev/sigstore/KeylessVerifierTest.java

@@ -16,6 +16,7 @@
 package dev.sigstore;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.hash.Hashing;
 import com.google.common.io.Resources;
 import dev.sigstore.VerificationOptions.CertificateMatcher;
 import dev.sigstore.bundle.Bundle;
@@ -27,6 +28,8 @@
 import java.nio.file.Path;
 import java.security.cert.X509Certificate;
 import java.util.List;
+import org.hamcrest.CoreMatchers;
+import org.hamcrest.MatcherAssert;
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
@@ -105,26 +108,6 @@ public void testVerify_badCheckpointSignature() throws Exception {
                 VerificationOptions.empty()));
   }
 
-  @Test
-  public void testVerify_errorsOnDSSEBundle() throws Exception {
-    var bundleFile =
-        Resources.toString(
-            Resources.getResource("dev/sigstore/samples/bundles/bundle.dsse.sigstore"),
-            StandardCharsets.UTF_8);
-    var artifact = Resources.getResource("dev/sigstore/samples/bundles/artifact.txt").getPath();
-
-    var verifier = KeylessVerifier.builder().sigstorePublicDefaults().build();
-    var ex =
-        Assertions.assertThrows(
-            KeylessVerificationException.class,
-            () ->
-                verifier.verify(
-                    Path.of(artifact),
-                    Bundle.from(new StringReader(bundleFile)),
-                    VerificationOptions.empty()));
-    Assertions.assertEquals("Cannot verify DSSE signature based bundles", ex.getMessage());
-  }
-
   @Test
   public void testVerify_canVerifyV01Bundle() throws Exception {
     // note that this v1 bundle contains an inclusion proof
@@ -231,4 +214,61 @@ public void verifyCertificateMatches_noneMatch() throws Exception {
         "No provided certificate identities matched values in certificate: [{issuer:'String: not-match',san:'String: not-match'},{issuer:'String: not-match-again',san:'String: not-match-again'}]",
         ex.getMessage());
   }
+
+  @Test
+  public void testVerify_dsseBundle() throws Exception {
+    var bundleFile =
+        Resources.toString(
+            Resources.getResource("dev/sigstore/samples/bundles/bundle.dsse.sigstore"),
+            StandardCharsets.UTF_8);
+    var artifact = Resources.getResource("dev/sigstore/samples/bundles/artifact.txt").getPath();
+
+    var verifier = KeylessVerifier.builder().sigstorePublicDefaults().build();
+    verifier.verify(
+        Path.of(artifact), Bundle.from(new StringReader(bundleFile)), VerificationOptions.empty());
+  }
+
+  @Test
+  public void testVerify_dsseBundleBadSignature() throws Exception {
+    var bundleFile =
+        Resources.toString(
+            Resources.getResource("dev/sigstore/samples/bundles/bundle.dsse.bad.sig.sigstore"),
+            StandardCharsets.UTF_8);
+    var artifact = Resources.getResource("dev/sigstore/samples/bundles/artifact.txt").getPath();
+    var verifier = KeylessVerifier.builder().sigstorePublicDefaults().build();
+
+    var ex =
+        Assertions.assertThrows(
+            KeylessVerificationException.class,
+            () ->
+                verifier.verify(
+                    Path.of(artifact),
+                    Bundle.from(new StringReader(bundleFile)),
+                    VerificationOptions.empty()));
+    Assertions.assertEquals("DSSE signature was not valid", ex.getMessage());
+  }
+
+  @Test
+  public void testVerify_dsseBundleArtifactNotInSubjects() throws Exception {
+    var bundleFile =
+        Resources.toString(
+            Resources.getResource("dev/sigstore/samples/bundles/bundle.dsse.sigstore"),
+            StandardCharsets.UTF_8);
+    var badArtifactDigest =
+        Hashing.sha256().hashString("nonsense", StandardCharsets.UTF_8).asBytes();
+    var verifier = KeylessVerifier.builder().sigstorePublicDefaults().build();
+
+    var ex =
+        Assertions.assertThrows(
+            KeylessVerificationException.class,
+            () ->
+                verifier.verify(
+                    badArtifactDigest,
+                    Bundle.from(new StringReader(bundleFile)),
+                    VerificationOptions.empty()));
+    MatcherAssert.assertThat(
+        ex.getMessage(),
+        CoreMatchers.startsWith(
+            "Provided artifact digest does not match any subject sha256 digests in DSSE payload"));
+  }
 }