feat: add an option to print provenance (#87)

* add an option to print provenance

Signed-off-by: Asra Ali <asraa@google.com>

fix

Signed-off-by: Asra Ali <asraa@google.com>

* print provenaace

Signed-off-by: Asra Ali <asraa@google.com>

README.md

@@ -61,6 +61,8 @@ $ go run . --help
     	path to an artifact to verify
   -branch string
     	expected branch the binary was compiled from (default "main")
+  -print-provenance
+    	output the verified provenance
   -provenance string
     	path to a provenance file
   -source string
@@ -74,22 +76,20 @@ $ go run . --help
 ### Example
 
 ```bash
-$ go run . --artifact-path ~/Downloads/binary-linux-amd64 --provenance ~/Downloads/binary-linux-amd64.intoto.jsonl --source github.com/origin/repo
-
-Verified against tlog entry 1544571
-verified SLSA provenance produced at 
+$ go run . -artifact-path ~/Downloads/slsa-verifier-linux-amd64 -provenance ~/Downloads/slsa-verifier-linux-amd64.intoto.jsonl -source github.com/slsa-framework/slsa-verifier -tag v1.0.0
+Verified signature against tlog entry index 2592016 at URL: https://rekor.sigstore.dev/api/v1/log/entries/d77621eaf1de74592546f773192f49ed995e8b12f2e5eeed02057ae32b24aa95
+Signing certificate information:
  {
-        "caller": "origin/repo",
-        "commit": "0dfcd24824432c4ce587f79c918eef8fc2c44d7b",
-        "job_workflow_ref": "/slsa-framework/slsa-github-generator/blob/main/.github/workflows/builder_go_slsa3.yml",
-        "trigger": "workflow_dispatch",
-        "issuer": "https://token.actions.githubusercontent.com"
+	"caller": "slsa-framework/slsa-verifier",
+	"commit": "c1b6db643d6134285dc929206fdcfa3712a877eb",
+	"job_workflow_ref": "/slsa-framework/slsa-github-generator/.github/workflows/builder_go_slsa3.yml@refs/tags/v0.0.1",
+	"trigger": "push",
+	"issuer": "https://token.actions.githubusercontent.com"
 }
-{"_type":"https://in-toto.io/Statement/v0.1","predicateType":"https://slsa.dev/provenance/v0.2","subject":[{"name":"binary-linux-amd64","digest":{"sha256":"723ccb85318bc8b1a9dd29340612ce1268cd3418d70f68e775edbdc16d1d9158"}}],"predicate":{...}}
-successfully verified SLSA provenance
+PASSED: Verified SLSA provenance
 ```
 
-The verified in-toto statement is written to stdout and can be used to pipe into policy engines. 
+The verified in-toto statement may be written to stdout with the `--print-provenance` flag to pipe into policy engines. 
 
 ## Technical design
 

main.go

@@ -17,12 +17,13 @@ import (
 )
 
 var (
-	provenancePath string
-	artifactPath   string
-	source         string
-	branch         string
-	tag            string
-	versiontag     string
+	provenancePath  string
+	artifactPath    string
+	source          string
+	branch          string
+	tag             string
+	versiontag      string
+	printProvenance bool
 )
 
 var defaultRekorAddr = "https://rekor.sigstore.dev"
@@ -30,68 +31,63 @@ var defaultRekorAddr = "https://rekor.sigstore.dev"
 func verify(ctx context.Context,
 	provenance []byte, artifactHash, source, branch string,
 	tag, versiontag *string,
-) error {
+) ([]byte, error) {
 	rClient, err := rekor.NewClient(defaultRekorAddr)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	/* Verify signature on the intoto attestation. */
 	env, cert, err := pkg.VerifyProvenanceSignature(ctx, rClient, provenance, artifactHash)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	/* Verify properties of the signing identity. */
 	// Get the workflow info given the certificate information.
 	workflowInfo, err := pkg.GetWorkflowInfoFromCertificate(cert)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	b, err := json.MarshalIndent(workflowInfo, "", "\t")
 	if err != nil {
-		return err
+		return nil, err
 	}
-	fmt.Fprintf(os.Stderr, "verified signature on SLSA provenance produced at \n %s\n", b)
+	fmt.Fprintf(os.Stderr, "Signing certificate information:\n %s\n", b)
 
 	/* Verify properties of the SLSA provenance. */
 	// Verify the workflow identity.
 	if err := pkg.VerifyWorkflowIdentity(workflowInfo, source); err != nil {
-		return err
+		return nil, err
 	}
 
 	// Unpack and verify info in the provenance, including the Subject Digest.
 	if err := pkg.VerifyProvenance(env, artifactHash); err != nil {
-		return err
+		return nil, err
 	}
 
 	// Verify the branch.
 	if err := pkg.VerifyBranch(env, branch); err != nil {
-		return err
+		return nil, err
 	}
 
 	// Verify the tag.
 	if tag != nil {
 		if err := pkg.VerifyTag(env, *tag); err != nil {
-			return err
+			return nil, err
 		}
 	}
 
 	// Verify the versioned tag.
 	if versiontag != nil {
 		if err := pkg.VerifyVersionedTag(env, *versiontag); err != nil {
-			return err
+			return nil, err
 		}
 	}
 
-	// Print verified provenance to stdout.
-	pyld, err := base64.StdEncoding.DecodeString(env.Payload)
-	if err != nil {
-		return fmt.Errorf("%w: %s", pkg.ErrorInvalidDssePayload, "decoding payload")
-	}
-	fmt.Fprintf(os.Stdout, "%s\n", string(pyld))
-	return nil
+	// Return verified provenance.
+	return base64.StdEncoding.DecodeString(env.Payload)
 }
 
 func main() {
@@ -101,6 +97,7 @@ func main() {
 	flag.StringVar(&branch, "branch", "main", "expected branch the binary was compiled from")
 	flag.StringVar(&tag, "tag", "", "[optional] expected tag the binary was compiled from")
 	flag.StringVar(&versiontag, "versioned-tag", "", "[optional] expected version the binary was compiled from. Uses semantic version to match the tag")
+	flag.BoolVar(&printProvenance, "print-provenance", false, "print the verified provenance to std out")
 	flag.Parse()
 
 	if provenancePath == "" || artifactPath == "" || source == "" {
@@ -122,13 +119,18 @@ func main() {
 		os.Exit(1)
 	}
 
-	if err := runVerify(artifactPath, provenancePath, source, branch,
-		ptag, pversiontag); err != nil {
-		fmt.Fprintf(os.Stderr, "verification failed: %v\n", err)
+	verifiedProvenance, err := runVerify(artifactPath, provenancePath, source, branch,
+		ptag, pversiontag)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "FAILED: SLSA verification failed: %v\n", err)
 		os.Exit(2)
 	}
 
-	fmt.Fprintf(os.Stderr, "successfully verified SLSA provenance\n")
+	fmt.Fprintf(os.Stderr, "PASSED: Verified SLSA provenance\n")
+
+	if printProvenance {
+		fmt.Fprintf(os.Stdout, "%s\n", string(verifiedProvenance))
+	}
 }
 
 func isFlagPassed(name string) bool {
@@ -143,7 +145,7 @@ func isFlagPassed(name string) bool {
 
 func runVerify(artifactPath, provenancePath, source, branch string,
 	ptag, pversiontag *string,
-) error {
+) ([]byte, error) {
 	f, err := os.Open(artifactPath)
 	if err != nil {
 		log.Fatal(err)
@@ -152,7 +154,7 @@ func runVerify(artifactPath, provenancePath, source, branch string,
 
 	provenance, err := os.ReadFile(provenancePath)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	h := sha256.New()

main_test.go

@@ -345,7 +345,7 @@ func Test_runVerify(t *testing.T) {
 				artifactPath = filepath.Clean(fmt.Sprintf("./testdata/%v/%s", v, tt.artifact))
 				provenancePath = fmt.Sprintf("%s.intoto.jsonl", artifactPath)
 
-				err := runVerify(artifactPath,
+				_, err := runVerify(artifactPath,
 					provenancePath,
 					tt.source, branch,
 					tt.ptag, tt.pversiontag)

pkg/provenance.go

@@ -359,8 +359,15 @@ func FindSigningCertificate(ctx context.Context, uuids []string, dssePayload dss
 		if err := cosign.CheckExpiry(cert, it); err != nil {
 			continue
 		}
+		uuid, err := cosign.ComputeLeafHash(entry)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Error computing leaf hash for tlog entry at index: %d\n", *entry.LogIndex)
+			continue
+		}
+
 		// success!
-		fmt.Fprintf(os.Stderr, "Verified against tlog entry %d\n", *entry.LogIndex)
+		url := fmt.Sprintf("%v/%v/%v", defaultRekorAddr, "api/v1/log/entries", hex.EncodeToString(uuid))
+		fmt.Fprintf(os.Stderr, "Verified signature against tlog entry index %d at URL: %s\n", *entry.LogIndex, url)
 		return cert, nil
 	}