Save consistency proof along with evidence of misbehavior

Although the consistency proof is neither necessary nor sufficient
to prove misbehavior by a log, this will help with debugging if a
log returns a bogus consistency proof erroneously (which seems to
be happening with the Rocketeer log lately...).

cmd/common.go

@@ -84,20 +84,25 @@ func defangLogUri (logUri string) string {
 	return strings.Replace(strings.Replace(logUri, "://", "_", 1), "/", "_", -1)
 }
 
-func saveEvidence (logUri string, firstSTH *ct.SignedTreeHead, secondSTH *ct.SignedTreeHead) (string, string, error) {
+func saveEvidence (logUri string, firstSTH *ct.SignedTreeHead, secondSTH *ct.SignedTreeHead, proof ct.ConsistencyProof) (string, string, string, error) {
 	now := strconv.FormatInt(time.Now().Unix(), 10)
 
 	firstFilename := filepath.Join(stateDir, "evidence", defangLogUri(logUri) + ".inconsistent." + now + ".first")
 	if err := ctwatch.WriteSTHFile(firstFilename, firstSTH); err != nil {
-		return "", "", err
+		return "", "", "", err
 	}
 
 	secondFilename := filepath.Join(stateDir, "evidence", defangLogUri(logUri) + ".inconsistent." + now + ".second")
 	if err := ctwatch.WriteSTHFile(secondFilename, secondSTH); err != nil {
-		return "", "", err
+		return "", "", "", err
 	}
 
-	return firstFilename, secondFilename, nil
+	proofFilename := filepath.Join(stateDir, "evidence", defangLogUri(logUri) + ".inconsistent." + now + ".proof")
+	if err := ctwatch.WriteProofFile(proofFilename, proof); err != nil {
+		return "", "", "", err
+	}
+
+	return firstFilename, secondFilename, proofFilename, nil
 }
 
 func Main (argStateDir string, processCallback ctwatch.ProcessCallback) {
@@ -200,18 +205,19 @@ func Main (argStateDir string, processCallback ctwatch.ProcessCallback) {
 			if prevSTH != nil {
 				var valid bool
 				var err error
-				valid, treeBuilder, err = scanner.CheckConsistency(prevSTH, latestSTH)
+				var proof ct.ConsistencyProof
+				valid, treeBuilder, proof, err = scanner.CheckConsistency(prevSTH, latestSTH)
 				if err != nil {
 					log.Printf("Error fetching consistency proof: %s\n", err)
 					exitCode |= 4
 					continue
 				}
 				if !valid {
-					firstFilename, secondFilename, err := saveEvidence(logUri, prevSTH, latestSTH)
+					firstFilename, secondFilename, proofFilename, err := saveEvidence(logUri, prevSTH, latestSTH, proof)
 					if err != nil {
 						log.Printf("Consistency proof failed - the log has misbehaved!  Saving evidence of misbehavior failed: %s\n", err)
 					} else {
-						log.Printf("Consistency proof failed - the log has misbehaved!  Evidence of misbehavior has been saved to '%s' and '%s'.\n", firstFilename, secondFilename)
+						log.Printf("Consistency proof failed - the log has misbehaved!  Evidence of misbehavior has been saved to '%s' and '%s' (with proof in '%s').\n", firstFilename, secondFilename, proofFilename)
 					}
 					exitCode |= 8
 					continue

helpers.go

@@ -46,6 +46,15 @@ func WriteSTHFile (path string, sth *ct.SignedTreeHead) error {
 	return ioutil.WriteFile(path, sthJson, 0666)
 }
 
+func WriteProofFile (path string, proof ct.ConsistencyProof) error {
+	proofJson, err := json.MarshalIndent(proof, "", "\t")
+	if err != nil {
+		return err
+	}
+	proofJson = append(proofJson, byte('\n'))
+	return ioutil.WriteFile(path, proofJson, 0666)
+}
+
 func IsPrecert (entry *ct.LogEntry) bool {
 	return entry.Leaf.TimestampedEntry.EntryType == ct.PrecertLogEntryType
 }

scanner.go

@@ -202,12 +202,12 @@ func (s *Scanner) GetSTH() (*ct.SignedTreeHead, error) {
 	return latestSth, nil
 }
 
-func (s *Scanner) CheckConsistency(first *ct.SignedTreeHead, second *ct.SignedTreeHead) (bool, *MerkleTreeBuilder, error) {
+func (s *Scanner) CheckConsistency(first *ct.SignedTreeHead, second *ct.SignedTreeHead) (bool, *MerkleTreeBuilder, ct.ConsistencyProof, error) {
 	var proof ct.ConsistencyProof
 
 	if first.TreeSize > second.TreeSize {
 		// No way this can be valid
-		return false, nil, nil
+		return false, nil, nil, nil
 	} else if first.TreeSize == second.TreeSize {
 		// The proof *should* be empty, so don't bother contacting the server.
 		// This is necessary because the digicert server returns a 400 error if first==second.
@@ -216,12 +216,12 @@ func (s *Scanner) CheckConsistency(first *ct.SignedTreeHead, second *ct.SignedTr
 		var err error
 		proof, err = s.logClient.GetConsistencyProof(int64(first.TreeSize), int64(second.TreeSize))
 		if err != nil {
-			return false, nil, err
+			return false, nil, nil, err
 		}
 	}
 
 	valid, treeBuilder := VerifyConsistencyProof(proof, first, second)
-	return valid, treeBuilder, nil
+	return valid, treeBuilder, proof, nil
 }
 
 func (s *Scanner) Scan(startIndex int64, endIndex int64, processCert ProcessCallback, treeBuilder *MerkleTreeBuilder) error {