refactor: Provenance tests (#628)

Refactors GHA provenance tests to use `testProvenance` which makes it clearer what is actually being tested. This will also make it easier to support `buildType` as a way to have different verification logic as the tests no longer rely on testdata with the `"https://github.com/Attestations/GitHubActionsWorkflow@v1"` build type, which isn't used by any supported builders.

A couple of updates to utilities:
- `VerifyTag` will now validate the ref returned by the `Provenance` instance.
- `VerifyBranch` will now validate the ref returned by the `Provenance` instance.
- `VerifyDigest` now supports the 160 bit `"sha1"` algo (FWIW) and will now search all subject entries even if one subject entry's algorithm does not match the expected algorithm.

---------

Signed-off-by: Ian Lewis <ianlewis@google.com>

cli/slsa-verifier/main_regression_test.go

@@ -156,14 +156,14 @@ func Test_runVerifyGHAArtifactPath(t *testing.T) {
 			artifacts: []string{"binary-linux-amd64-workflow_dispatch"},
 			source:    "github.com/slsa-framework/example-package",
 			ptag:      pString("v1.2.3"),
-			err:       serrors.ErrorMismatchTag,
+			err:       serrors.ErrorInvalidRef,
 		},
 		{
 			name:        "versioned tag no match empty tag workflow_dispatch",
 			artifacts:   []string{"binary-linux-amd64-workflow_dispatch"},
 			source:      "github.com/slsa-framework/example-package",
 			pversiontag: pString("v1"),
-			err:         serrors.ErrorInvalidSemver,
+			err:         serrors.ErrorInvalidRef,
 		},
 		// Provenance contains tag = v13.0.30.
 		{
@@ -477,7 +477,7 @@ func Test_runVerifyGHAArtifactPath(t *testing.T) {
 			source:      "github.com/asraa/slsa-on-github-test",
 			pversiontag: pString("v1.5.0"),
 			pbranch:     pString("main"),
-			err:         serrors.ErrorMismatchBranch,
+			err:         serrors.ErrorInvalidRef,
 			noversion:   true,
 		},
 		// Workflow inputs.
@@ -746,14 +746,14 @@ func Test_runVerifyGHAArtifactImage(t *testing.T) {
 			artifact: "container_workflow_dispatch",
 			source:   "github.com/slsa-framework/example-package",
 			ptag:     pString("v1.2.3"),
-			err:      serrors.ErrorMismatchTag,
+			err:      serrors.ErrorInvalidRef,
 		},
 		{
 			name:        "versioned tag no match empty tag workflow_dispatch",
 			artifact:    "container_workflow_dispatch",
 			source:      "github.com/slsa-framework/example-package",
 			pversiontag: pString("v1"),
-			err:         serrors.ErrorInvalidSemver,
+			err:         serrors.ErrorInvalidRef,
 		},
 	}
 	for _, tt := range tests {
@@ -1327,14 +1327,14 @@ func Test_runVerifyGHAContainerBased(t *testing.T) {
 			artifacts:   []string{"workflow_dispatch.main.default"},
 			source:      "github.com/slsa-framework/example-package",
 			pversiontag: pString("v1"),
-			err:         serrors.ErrorInvalidSemver,
+			err:         serrors.ErrorInvalidRef,
 		},
 		{
 			name:      "tag no match empty tag workflow_dispatch",
 			artifacts: []string{"workflow_dispatch.main.default"},
 			source:    "github.com/slsa-framework/example-package",
 			ptag:      pString("v1.2.3"),
-			err:       serrors.ErrorMismatchTag,
+			err:       serrors.ErrorInvalidRef,
 		},
 		{
 			name:      "wrong branch master",

verifiers/internal/gha/provenance.go

@@ -179,12 +179,19 @@ func verifyDigest(prov slsaprovenance.Provenance, expectedHash string) error {
 	}
 
 	// 8 bit represented in hex, so 8/2=4.
-	l := len(expectedHash) * 4
+	bitLength := len(expectedHash) * 4
+	expectedAlgo := fmt.Sprintf("sha%v", bitLength)
+	// TODO(#630): Add subject digest minimum bit length check.
+	// sha1 is 160 bit (FWIW).
+	if bitLength == 160 {
+		expectedAlgo = "sha1"
+	}
+
 	for _, subject := range subjects {
 		digestSet := subject.Digest
-		hash, exists := digestSet[fmt.Sprintf("sha%v", l)]
+		hash, exists := digestSet[expectedAlgo]
 		if !exists {
-			return fmt.Errorf("%w: %s", serrors.ErrorInvalidDssePayload, fmt.Sprintf("no sha%v subject digest", l))
+			continue
 		}
 		if hash == expectedHash {
 			return nil
@@ -383,44 +390,64 @@ func VerifyWorkflowInputs(prov slsaprovenance.Provenance, inputs map[string]stri
 	return nil
 }
 
+// VerifyBranch verifies that the source branch in the provenance matches the
+// expected value.
 func VerifyBranch(prov slsaprovenance.Provenance, expectedBranch string) error {
-	branch, err := prov.GetBranch()
+	ref, err := prov.GetBranch()
 	if err != nil {
 		return err
 	}
 
-	expectedBranch = "refs/heads/" + expectedBranch
+	branch, err := utils.BranchFromGitRef(ref)
+	if err != nil {
+		return fmt.Errorf("verifying branch: %w", err)
+	}
+
 	if branch != expectedBranch {
 		return fmt.Errorf("expected branch '%s', got '%s': %w", expectedBranch, branch, serrors.ErrorMismatchBranch)
 	}
 
 	return nil
 }
 
+// VerifyTag verifies that the source tag in the provenance matches the
+// expected value.
 func VerifyTag(prov slsaprovenance.Provenance, expectedTag string) error {
-	tag, err := prov.GetTag()
+	ref, err := prov.GetTag()
 	if err != nil {
 		return err
 	}
 
-	expectedTag = "refs/tags/" + expectedTag
+	tag, err := utils.TagFromGitRef(ref)
+	if err != nil {
+		return fmt.Errorf("verifying tag: %w", err)
+	}
+
 	if tag != expectedTag {
 		return fmt.Errorf("expected tag '%s', got '%s': %w", expectedTag, tag, serrors.ErrorMismatchTag)
 	}
 
 	return nil
 }
 
+// VerifyVersionedTag verifies that the source tag in the provenance matches the
+// expected semver value.
 func VerifyVersionedTag(prov slsaprovenance.Provenance, expectedTag string) error {
 	// Retrieve, validate and canonicalize the provenance tag.
 	// Note: prerelease is validated as part of patch validation
 	// and must be equal. Build is discarded as per https://semver.org/:
 	// "Build metadata MUST be ignored when determining version precedence",
-	tag, err := prov.GetTag()
+	ref, err := prov.GetTag()
 	if err != nil {
 		return err
 	}
-	return utils.VerifyVersionedTag(strings.TrimPrefix(tag, "refs/tags/"), expectedTag)
+
+	tag, err := utils.TagFromGitRef(ref)
+	if err != nil {
+		return fmt.Errorf("verifying tag: %w", err)
+	}
+
+	return utils.VerifyVersionedTag(tag, expectedTag)
 }
 
 // hasCertInEnvelope checks if a valid x509 certificate is present in the