Use a Factory to create signature verifiers at policy evaluation time (#165)

* Make verifiers composable

* fix: remove unused code and improve signature verification logic

* fix: simplify abstractions and renamed some things

* fix: improve tl interface.

* fix: sort out signer/verifier

signerverifier/aws.go

@@ -20,8 +20,5 @@ func GetAWSSigner(ctx context.Context, keyARN string, region string) (dsse.Signe
 	if err != nil {
 		return nil, fmt.Errorf("error getting aws crypto signer: %w", err)
 	}
-	signer := &ECDSA256SignerVerifier{
-		Signer: cs,
-	}
-	return signer, nil
+	return NewECDSASignerVerifier(cs)
 }

signerverifier/common.go

@@ -9,52 +9,17 @@ import (
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
+	"io"
 
-	"github.com/docker/attest/internal/util"
 	"github.com/secure-systems-lab/go-securesystemslib/dsse"
 )
 
-type ECDSA256SignerVerifier struct {
-	crypto.Signer
-}
-
-// implement keyid function.
-func (s *ECDSA256SignerVerifier) KeyID() (string, error) {
-	keyid, err := KeyID(s.Signer.Public())
-	if err != nil {
-		return "", fmt.Errorf("error getting keyid: %w", err)
-	}
-	return keyid, nil
-}
-
-func (s *ECDSA256SignerVerifier) Public() crypto.PublicKey {
-	return s.Signer.Public()
-}
-
-func (s *ECDSA256SignerVerifier) Sign(_ context.Context, data []byte) ([]byte, error) {
-	return s.Signer.Sign(rand.Reader, data, crypto.SHA256)
-}
-
-func (s *ECDSA256SignerVerifier) Verify(_ context.Context, data []byte, sig []byte) error {
-	pub, ok := s.Signer.Public().(*ecdsa.PublicKey)
-	if !ok {
-		return fmt.Errorf("public key is not ecdsa")
-	}
-	ok = ecdsa.VerifyASN1(pub, util.SHA256(data), sig)
-	if !ok {
-		return fmt.Errorf("payload signature is not valid")
-	}
-	return nil
-}
-
 func LoadKeyPair(priv []byte) (dsse.SignerVerifier, error) {
 	privateKey, err := parsePriv(priv)
 	if err != nil {
 		return nil, err
 	}
-	return &ECDSA256SignerVerifier{
-		Signer: privateKey,
-	}, nil
+	return NewECDSASignerVerifier(privateKey)
 }
 
 func parsePriv(privkeyBytes []byte) (*ecdsa.PrivateKey, error) {
@@ -78,7 +43,26 @@ func GenKeyPair() (dsse.SignerVerifier, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &ECDSA256SignerVerifier{
-		Signer: signer,
-	}, nil
+	return NewECDSASignerVerifier(signer)
+}
+
+// ensure it implements crypto.Signer.
+var _ crypto.Signer = (*cryptoSignerWrapper)(nil)
+
+type cryptoSignerWrapper struct {
+	sv dsse.SignerVerifier
+}
+
+// Public implements crypto.Signer.
+func (c *cryptoSignerWrapper) Public() crypto.PublicKey {
+	return c.sv.Public()
+}
+
+// Sign implements crypto.Signer.
+func (c *cryptoSignerWrapper) Sign(_ io.Reader, digest []byte, _ crypto.SignerOpts) (signature []byte, err error) {
+	return c.sv.Sign(context.Background(), digest)
+}
+
+func AsCryptoSigner(signer dsse.SignerVerifier) (crypto.Signer, error) {
+	return &cryptoSignerWrapper{sv: signer}, nil
 }

signerverifier/ecdsa.go

@@ -0,0 +1,80 @@
+package signerverifier
+
+import (
+	"context"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/rand"
+	"fmt"
+
+	"github.com/docker/attest/internal/util"
+	"github.com/secure-systems-lab/go-securesystemslib/dsse"
+)
+
+type ecdsaVerifier struct {
+	publicKey *ecdsa.PublicKey
+	keyID     string
+}
+
+// ensure ECDSAVerifier implements dsse.Verifier.
+var _ dsse.Verifier = (*ecdsaVerifier)(nil)
+
+func NewECDSAVerifier(publicKey crypto.PublicKey) (dsse.Verifier, error) {
+	ecdsaPublicKey, ok := (publicKey).(*ecdsa.PublicKey)
+	if !ok {
+		return nil, fmt.Errorf("public key is not an ECDSA public key")
+	}
+	return &ecdsaVerifier{
+		publicKey: ecdsaPublicKey,
+	}, nil
+}
+
+func (v *ecdsaVerifier) Verify(_ context.Context, data, signature []byte) error {
+	// verify payload ecdsa signature
+	ok := ecdsa.VerifyASN1(v.publicKey, util.SHA256(data), signature)
+	if !ok {
+		return fmt.Errorf("payload signature is not valid")
+	}
+
+	return nil
+}
+
+func (v *ecdsaVerifier) Public() crypto.PublicKey {
+	return v.publicKey
+}
+
+func (v *ecdsaVerifier) KeyID() (string, error) {
+	if v.keyID != "" {
+		return v.keyID, nil
+	}
+	keyID, err := KeyID(v.publicKey)
+	if err != nil {
+		return "", fmt.Errorf("failed to get key ID: %w", err)
+	}
+	v.keyID = keyID
+	return v.keyID, nil
+}
+
+// must implement dsse.SignerVerifier interface.
+var _ dsse.SignerVerifier = (*ecdsa256SignerVerifier)(nil)
+
+type ecdsa256SignerVerifier struct {
+	signer crypto.Signer
+	dsse.Verifier
+}
+
+func NewECDSASignerVerifier(signer crypto.Signer) (dsse.SignerVerifier, error) {
+	verifier, err := NewECDSAVerifier(signer.Public())
+	if err != nil {
+		return nil, fmt.Errorf("failed to create verifier: %w", err)
+	}
+	sv := &ecdsa256SignerVerifier{
+		signer:   signer,
+		Verifier: verifier,
+	}
+	return sv, nil
+}
+
+func (s *ecdsa256SignerVerifier) Sign(_ context.Context, data []byte) ([]byte, error) {
+	return s.signer.Sign(rand.Reader, data, crypto.SHA256)
+}

signerverifier/gcp.go

@@ -21,8 +21,5 @@ func GetGCPSigner(ctx context.Context, reference string, opts ...option.ClientOp
 	if err != nil {
 		return nil, fmt.Errorf("error getting gcp crypto signer: %w", err)
 	}
-	signer := &ECDSA256SignerVerifier{
-		Signer: cs,
-	}
-	return signer, nil
+	return NewECDSASignerVerifier(cs)
 }

signerverifier/gcp_test.go

@@ -40,6 +40,14 @@ func TestGCPKMS_Signer(t *testing.T) {
 	publicKey, err := ParsePublicKey([]byte(publicKeyPEM))
 	require.NoError(t, err)
 	// verify payload ecdsa signature
-	ok := ecdsa.VerifyASN1(publicKey, hash, sig)
+
+	ecdsaPublicKey, ok := publicKey.(*ecdsa.PublicKey)
+	if !ok {
+		t.Fatal("Failed to convert publicKey to *ecdsa.PublicKey")
+	}
+	ok = ecdsa.VerifyASN1(ecdsaPublicKey, hash, sig)
 	assert.True(t, ok)
+
+	err = signer.Verify(ctx, msg, sig)
+	require.NoError(t, err)
 }

signerverifier/parse.go

@@ -1,6 +1,7 @@
 package signerverifier
 
 import (
+	"crypto"
 	"crypto/ecdsa"
 	"crypto/x509"
 	"encoding/pem"
@@ -9,7 +10,7 @@ import (
 
 const pemType = "PUBLIC KEY"
 
-func ParsePublicKey(pubkeyBytes []byte) (*ecdsa.PublicKey, error) {
+func ParsePublicKey(pubkeyBytes []byte) (crypto.PublicKey, error) {
 	p, _ := pem.Decode(pubkeyBytes)
 	if p == nil {
 		return nil, fmt.Errorf("pubkey file does not contain any PEM data")
@@ -17,12 +18,15 @@ func ParsePublicKey(pubkeyBytes []byte) (*ecdsa.PublicKey, error) {
 	if p.Type != pemType {
 		return nil, fmt.Errorf("pubkey file does not contain a public key")
 	}
-	pubKey, err := x509.ParsePKIXPublicKey(p.Bytes)
-	if err != nil {
-		return nil, fmt.Errorf("error failed to parse public key: %w", err)
-	}
+	return x509.ParsePKIXPublicKey(p.Bytes)
+}
 
-	ecdsaPubKey, ok := pubKey.(*ecdsa.PublicKey)
+func ParseECDSAPublicKey(pubkeyBytes []byte) (*ecdsa.PublicKey, error) {
+	pk, err := ParsePublicKey(pubkeyBytes)
+	if err != nil {
+		return nil, err
+	}
+	ecdsaPubKey, ok := pk.(*ecdsa.PublicKey)
 	if !ok {
 		return nil, fmt.Errorf("error public key is not an ecdsa key: %w", err)
 	}
@@ -34,6 +38,5 @@ func ConvertToPEM(ecdsaPubKey *ecdsa.PublicKey) ([]byte, error) {
 	if err != nil {
 		return nil, fmt.Errorf("error failed to marshal public key: %w", err)
 	}
-
 	return pem.EncodeToMemory(&pem.Block{Type: pemType, Bytes: pubKeyBytes}), nil
 }