Fix issue #18: Handle tokens without algorithm in protected headers

evidence.go

@@ -5,7 +5,10 @@ package psatoken
 
 import (
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
 	"crypto/rand"
+	"crypto/rsa"
 	_ "crypto/sha256" // used hash algorithms need to be imported explicitly
 	"errors"
 	"fmt"
@@ -147,7 +150,17 @@ func (e *Evidence) Verify(pk crypto.PublicKey) error {
 
 	algo, err := protected.Algorithm()
 	if err != nil {
-		return fmt.Errorf("unable to get verification algorithm: %w", err)
+		// If algorithm is not found in protected headers, this might be a token
+		// created by compile_token or other tools that don't set the algorithm
+		// in protected headers. In this case, we should try to infer the algorithm
+		// from the public key type or use a default algorithm.
+
+		// Try to infer algorithm from public key type
+		inferredAlgo, inferErr := inferAlgorithmFromPublicKey(pk)
+		if inferErr != nil {
+			return fmt.Errorf("unable to get verification algorithm: %w (and failed to infer: %v)", err, inferErr)
+		}
+		algo = inferredAlgo
 	}
 
 	verifier, err := cose.NewVerifier(algo, pk)
@@ -185,6 +198,40 @@ func (e *Evidence) doSign(signer cose.Signer) ([]byte, error) {
 	return wrap, nil
 }
 
+// InferAlgorithmFromPublicKey attempts to infer the COSE algorithm from the public key type.
+// This is used as a fallback when the algorithm is not present in the protected headers.
+// This function is exported for testing purposes.
+func InferAlgorithmFromPublicKey(pk crypto.PublicKey) (cose.Algorithm, error) {
+	return inferAlgorithmFromPublicKey(pk)
+}
+
+// inferAlgorithmFromPublicKey is the internal implementation of algorithm inference.
+func inferAlgorithmFromPublicKey(pk crypto.PublicKey) (cose.Algorithm, error) {
+	switch key := pk.(type) {
+	case *ecdsa.PublicKey:
+		// For ECDSA keys, try to determine the curve and map to appropriate COSE algorithm
+		switch key.Curve.Params().BitSize {
+		case 256:
+			return cose.AlgorithmES256, nil
+		case 384:
+			return cose.AlgorithmES384, nil
+		case 521:
+			return cose.AlgorithmES512, nil
+		default:
+			return cose.AlgorithmES256, nil // Default to ES256 for unknown curves
+		}
+	case ed25519.PublicKey:
+		return cose.AlgorithmEdDSA, nil
+	case *rsa.PublicKey:
+		// Default to PS256 for RSA keys
+		return cose.AlgorithmPS256, nil
+	default:
+		// If we can't determine the key type, default to ES256 as it's most common
+		// for PSA tokens
+		return cose.AlgorithmES256, nil
+	}
+}
+
 // MarshalJSON encodes the PSA claims-set to JSON
 func (e *Evidence) MarshalJSON() ([]byte, error) {
 	return EncodeClaimsToJSON(e.Claims)

evidence_test.go

@@ -339,3 +339,54 @@ func TestEvidence_SignUnvalidated(t *testing.T) {
 	err = EvidenceOut.Verify(pk)
 	assert.EqualError(t, err, "signature verification failed: verification error")
 }
+
+// TestIssue18AlgorithmInference tests the fix for issue #18
+// This test verifies that the algorithm inference functionality works correctly
+// when the algorithm is not present in the protected headers (e.g., from compile_token)
+func TestIssue18AlgorithmInference(t *testing.T) {
+	// Test the algorithm inference function with different key types
+	testCases := []struct {
+		name          string
+		keyData       string
+		expectedAlg   string
+	}{
+		{
+			name:        "ECDSA P-256 key",
+			keyData:     testECKeyA,
+			expectedAlg: "ES256",
+		},
+		{
+			name:        "TFM ECDSA key", 
+			keyData:     testTFMECKey,
+			expectedAlg: "ES256",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			pk := pubKeyFromJWK(t, tc.keyData)
+
+			inferredAlg, err := InferAlgorithmFromPublicKey(pk)
+			assert.NoError(t, err, "Should be able to infer algorithm")
+			assert.Equal(t, tc.expectedAlg, inferredAlg.String(), "Should infer correct algorithm")
+		})
+	}
+
+	// Test that verification still works normally (regression test)
+	tokenSigner := signerFromJWK(t, testECKeyA)
+	claims := mustBuildValidP2Claims(t, false)
+
+	var evidence Evidence
+	err := evidence.SetClaims(claims)
+	require.NoError(t, err)
+
+	cwt, err := evidence.ValidateAndSign(tokenSigner)
+	require.NoError(t, err)
+
+	evidenceOut, err := DecodeAndValidateEvidenceFromCOSE(cwt)
+	require.NoError(t, err)
+
+	pk := pubKeyFromJWK(t, testECKeyA)
+	err = evidenceOut.Verify(pk)
+	assert.NoError(t, err, "Normal verification should still work after the fix")
+}