feat: Update SLSA verifier to support a global signing key for GCB V1 which… (#509)

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

* Update SLSA verifier to support a global signing key for GCB V1 which creates the signature in a DSSE-conformant PAE format

- new public key for "global PAE signing key"
- test data and unit tests

Signed-off-by: Kevin Halk <khalk@google.com>

---------

Signed-off-by: Kevin Halk <khalk@google.com>

verifiers/internal/gcb/keys/keys.go

@@ -1,7 +1,9 @@
 package keys
 
 import (
+	"crypto"
 	"crypto/ecdsa"
+	"crypto/sha256"
 	"crypto/x509"
 	"embed"
 	"encoding/pem"
@@ -9,12 +11,16 @@ import (
 	"io/fs"
 	"path"
 
+	dsselib "github.com/secure-systems-lab/go-securesystemslib/dsse"
 	serrors "github.com/slsa-framework/slsa-verifier/v2/errors"
 )
 
 //go:embed materials/*
 var publicKeys embed.FS
 
+const GlobalPAEKeyID = "projects/verified-builder/locations/global/keyRings/attestor/cryptoKeys/provenanceSigner/cryptoKeyVersions/1"
+const GlobalPAEPublicKeyName = "global-pae"
+
 type PublicKey struct {
 	value  []byte
 	pubKey *ecdsa.PublicKey
@@ -22,7 +28,7 @@ type PublicKey struct {
 	// TODO: key type and size
 }
 
-func PublicKeyNew(region string) (*PublicKey, error) {
+func NewPublicKey(region string) (*PublicKey, error) {
 	content, err := fs.ReadFile(publicKeys, path.Join("materials", region+".key"))
 	if err != nil {
 		return nil, fmt.Errorf("%w: cannot read key materials", err)
@@ -61,3 +67,46 @@ func (p *PublicKey) VerifySignature(digest [32]byte, sig []byte) error {
 
 	return nil
 }
+
+type GlobalPAEKey struct {
+	publicKey *PublicKey
+	Verifier  *dsselib.EnvelopeVerifier
+}
+
+func NewGlobalPAEKey() (*GlobalPAEKey, error) {
+	publicKey, err := NewPublicKey(GlobalPAEPublicKeyName)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create public key for Global PAE key: %w", err)
+	}
+
+	globalPaeKey := &GlobalPAEKey{publicKey: publicKey}
+	envVerifier, err := dsselib.NewEnvelopeVerifier(globalPaeKey)
+	if err != nil {
+		return nil, err
+	}
+	globalPaeKey.Verifier = envVerifier
+	return globalPaeKey, nil
+}
+
+func (v *GlobalPAEKey) VerifyPAESignature(envelope *dsselib.Envelope) error {
+	_, err := v.Verifier.Verify(envelope)
+	return err
+}
+
+// Verify implements dsse.Verifier.Verify. It verifies
+// a signature formatted in DSSE-conformant PAE.
+func (v *GlobalPAEKey) Verify(data, sig []byte) error {
+	// Verify the signature.
+	digest := sha256.Sum256(data)
+	return v.publicKey.VerifySignature(digest, sig)
+}
+
+// KeyID implements dsse.Verifier.KeyID.
+func (v *GlobalPAEKey) KeyID() (string, error) {
+	return GlobalPAEKeyID, nil
+}
+
+// Public implements dsse.Verifier.Public.
+func (v *GlobalPAEKey) Public() crypto.PublicKey {
+	return v.publicKey
+}

verifiers/internal/gcb/keys/materials/global-pae.key

@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEdMcJUyKbmarf6dydhfmAjgmK6c42
+oCCNRR1se3Bi3VO65KcGk6qyci6/bsu2s4u+dLKWrsUQomEw4v3FtVctoA==
+-----END PUBLIC KEY-----

verifiers/internal/gcb/provenance.go

@@ -25,6 +25,8 @@ var GCBBuilderIDs = []string{
 	"https://cloudbuild.googleapis.com/GoogleHostedWorker@v0.3",
 }
 
+var regionalKeyRegex = regexp.MustCompile(`^projects\/verified-builder\/locations\/(.*)\/keyRings\/attestor\/cryptoKeys\/builtByGCB\/cryptoKeyVersions\/1$`)
+
 type v01IntotoStatement struct {
 	intoto.StatementHeader
 	Predicate ProvenancePredicate `json:"predicate"`
@@ -416,20 +418,35 @@ func (p *Provenance) verifySignatures(prov *provenance) error {
 	}
 
 	payloadHash := sha256.Sum256(payload)
-
 	// Verify the signatures.
 	if len(prov.Envelope.Signatures) == 0 {
 		return fmt.Errorf("%w: no signatures found in envelope", serrors.ErrorNoValidSignature)
 	}
 
 	var errs []error
-	regex := regexp.MustCompile(`^projects\/verified-builder\/locations\/(.*)\/keyRings\/attestor\/cryptoKeys\/builtByGCB\/cryptoKeyVersions\/1$`)
+
 	for _, sig := range prov.Envelope.Signatures {
-		match := regex.FindStringSubmatch(sig.KeyID)
-		if len(match) == 2 {
-			// Create a public key instance for this region.
-			region := match[1]
-			pubKey, err := keys.PublicKeyNew(region)
+		var region string
+		if sig.KeyID == keys.GlobalPAEKeyID {
+			// If the signature is signed with the global PAE key, use a DSSE verifier
+			// to verify the DSSE/PAE-encoded signature.
+			region = keys.GlobalPAEPublicKeyName
+			globalPaeKey, err := keys.NewGlobalPAEKey()
+			if err != nil {
+				errs = append(errs, err)
+				continue
+			}
+
+			err = globalPaeKey.VerifyPAESignature(&prov.Envelope)
+			if err != nil {
+				errs = append(errs, err)
+				continue
+			}
+		} else if match := regionalKeyRegex.FindStringSubmatch(sig.KeyID); len(match) == 2 {
+			// If the signature is signed with a regional key, verify the legacy
+			// signing which is over the envelope (not PAE-encoded).
+			region = match[1]
+			pubKey, err := keys.NewPublicKey(region)
 			if err != nil {
 				errs = append(errs, err)
 				continue
@@ -448,16 +465,18 @@ func (p *Provenance) verifySignatures(prov *provenance) error {
 				errs = append(errs, err)
 				continue
 			}
-
-			var statement v01IntotoStatement
-			if err := json.Unmarshal(payload, &statement); err != nil {
-				return fmt.Errorf("%w: %s", serrors.ErrorInvalidDssePayload, err.Error())
-			}
-			p.verifiedIntotoStatement = &statement
-			p.verifiedProvenance = prov
-			fmt.Fprintf(os.Stderr, "Verification succeeded with region key '%s'\n", region)
-			return nil
+		} else {
+			continue
 		}
+
+		var statement v01IntotoStatement
+		if err := json.Unmarshal(payload, &statement); err != nil {
+			return fmt.Errorf("%w: %s", serrors.ErrorInvalidDssePayload, err.Error())
+		}
+		p.verifiedIntotoStatement = &statement
+		p.verifiedProvenance = prov
+		fmt.Fprintf(os.Stderr, "Verification succeeded with region key '%s'\n", region)
+		return nil
 	}
 
 	return fmt.Errorf("%w: %v", serrors.ErrorNoValidSignature, errs)

verifiers/internal/gcb/provenance_test.go

@@ -15,7 +15,7 @@ import (
 	"github.com/slsa-framework/slsa-verifier/v2/verifiers/utils"
 )
 
-// This function sets the statement of the proveannce, as if
+// This function sets the statement of the provenance, as if
 // it had been verified. This is necessary because individual functions
 // expect this statement to be populated; and this is done only
 // after the signature is verified.
@@ -506,6 +506,10 @@ func Test_VerifySignature(t *testing.T) {
 			name: "valid gcb provenance",
 			path: "./testdata/gcloud-container-github.json",
 		},
+		{
+			name: "global gcb signing key",
+			path: "./testdata/gcloud-container-global-pae-signing-key-successful.json",
+		},
 		{
 			name:     "invalid signature",
 			path:     "./testdata/gcloud-container-invalid-signature.json",
@@ -516,6 +520,11 @@ func Test_VerifySignature(t *testing.T) {
 			path:     "./testdata/gcloud-container-invalid-signature-payloadtype.json",
 			expected: serrors.ErrorNoValidSignature,
 		},
+		{
+			name:     "invalid signature - global PAE key",
+			path:     "./testdata/gcloud-container-invalid-signature-global-pae-key.json",
+			expected: serrors.ErrorNoValidSignature,
+		},
 		{
 			name:     "invalid signature empty",
 			path:     "./testdata/gcloud-container-empty-signature.json",
@@ -559,6 +568,10 @@ func Test_VerifySignature(t *testing.T) {
 			name: "signature multiple 3rd valid",
 			path: "./testdata/gcloud-container-multiple-signatures-3rdvalid.json",
 		},
+		{
+			name: "signature multiple global pae valid",
+			path: "./testdata/gcloud-container-multiple-signatures-global-pae-valid.json",
+		},
 	}
 	for _, tt := range tests {
 		tt := tt // Re-initializing variable so it is not changed while executing the closure below
@@ -578,7 +591,6 @@ func Test_VerifySignature(t *testing.T) {
 			if err := setStatement(prov); err != nil {
 				panic(fmt.Errorf("setStatement: %w", err))
 			}
-
 			err = prov.VerifySignature()
 			if !cmp.Equal(err, tt.expected, cmpopts.EquateErrors()) {
 				t.Errorf(cmp.Diff(err, tt.expected, cmpopts.EquateErrors()))
@@ -799,6 +811,10 @@ func Test_VerifyTextProvenance(t *testing.T) {
 			name: "valid gcb provenance",
 			path: "./testdata/gcloud-container-github.json",
 		},
+		{
+			name: "valid gcb provenance with global signing key",
+			path: "./testdata/gcloud-container-global-pae-signing-key-successful.json",
+		},
 		{
 			name:     "mismatch everything",
 			path:     "./testdata/gcloud-container-github.json",