#58 Add shard id verification against state id

Author: martti007

src/main/java/org/unicitylabs/sdk/transaction/verification/InclusionProofVerificationRule.java

@@ -65,8 +65,20 @@ public static VerificationResult<InclusionProofVerificationStatus> verify(RootTr
               InclusionProofVerificationStatus.PATH_INVALID);
     }
 
+    VerificationResult<?> result = ShardIdMatchesStateIdRule.verify(
+            stateId,
+            inclusionProof.getUnicityCertificate().getShardTreeCertificate()
+    );
+    if (result.getStatus() != VerificationStatus.OK) {
+      return new VerificationResult<>(
+              "InclusionProofVerificationRule",
+              InclusionProofVerificationStatus.SHARD_ID_MISMATCH,
+              "",
+              result
+      );
+    }
 
-    VerificationResult<?> result = UnicityCertificateVerification.verify(trustBase, inclusionProof);
+    result = UnicityCertificateVerification.verify(trustBase, inclusionProof);
     if (result.getStatus() != VerificationStatus.OK) {
       return new VerificationResult<>(
               "InclusionProofVerificationRule",

src/main/java/org/unicitylabs/sdk/transaction/verification/InclusionProofVerificationStatus.java

@@ -18,6 +18,8 @@ public enum InclusionProofVerificationStatus {
   INCLUSION_CERTIFICATE_MISSING,
   /** Proof path structure or hashes are invalid. */
   PATH_INVALID,
+  /** Shard id of the unicity certificate does not match the transaction state id. */
+  SHARD_ID_MISMATCH,
   /** Inclusion proof verification succeeded. */
   OK
 }

src/main/java/org/unicitylabs/sdk/transaction/verification/ShardIdMatchesStateIdRule.java

@@ -0,0 +1,42 @@
+package org.unicitylabs.sdk.transaction.verification;
+
+import org.unicitylabs.sdk.api.StateId;
+import org.unicitylabs.sdk.api.bft.ShardId;
+import org.unicitylabs.sdk.api.bft.ShardTreeCertificate;
+import org.unicitylabs.sdk.util.verification.VerificationResult;
+import org.unicitylabs.sdk.util.verification.VerificationStatus;
+
+/**
+ * Rule to verify that the shard id of the shard tree certificate is a prefix of the transaction
+ * state id. An empty shard id matches any state id.
+ */
+public class ShardIdMatchesStateIdRule {
+
+  private ShardIdMatchesStateIdRule() {
+  }
+
+  /**
+   * Verify that the shard id is a prefix of the state id.
+   *
+   * @param stateId state id of the transaction being verified
+   * @param shardTreeCertificate shard tree certificate carrying the shard id
+   *
+   * @return verification result with {@link VerificationStatus#OK} on match (or empty shard id),
+   *     otherwise {@link VerificationStatus#FAIL}
+   */
+  public static VerificationResult<VerificationStatus> verify(
+          StateId stateId,
+          ShardTreeCertificate shardTreeCertificate
+  ) {
+    ShardId shardId = shardTreeCertificate.getShard();
+    if (shardId.getLength() == 0) {
+      return new VerificationResult<>("ShardIdMatchesStateIdRule", VerificationStatus.OK);
+    }
+
+    if (!shardId.isPrefixOf(stateId.getData())) {
+      return new VerificationResult<>("ShardIdMatchesStateIdRule", VerificationStatus.FAIL);
+    }
+
+    return new VerificationResult<>("ShardIdMatchesStateIdRule", VerificationStatus.OK);
+  }
+}