Add support for RS256, RS384, RS512 constants (#163)

Signed-off-by: hslatman <herman+github@smallstep.com>

algorithm.go

@@ -47,6 +47,25 @@ const (
 	AlgorithmReserved Algorithm = 0
 )
 
+// Algorithms known, but not supported by this library.
+//
+// Signers and Verifiers requiring the algorithms below are not
+// directly supported by this library. They need to be provided
+// as an external [cose.Signer] or [cose.Verifier] implementation.
+//
+// An example use case where RS256 is allowed and used is in
+// WebAuthn: https://www.w3.org/TR/webauthn-2/#sctn-sample-registration.
+const (
+	// RSASSA-PKCS1-v1_5 using SHA-256 by RFC 8812.
+	AlgorithmRS256 Algorithm = -257
+
+	// RSASSA-PKCS1-v1_5 using SHA-384 by RFC 8812.
+	AlgorithmRS384 Algorithm = -258
+
+	// RSASSA-PKCS1-v1_5 using SHA-512 by RFC 8812.
+	AlgorithmRS512 Algorithm = -259
+)
+
 // Algorithm represents an IANA algorithm entry in the COSE Algorithms registry.
 //
 // # See Also
@@ -65,6 +84,12 @@ func (a Algorithm) String() string {
 		return "PS384"
 	case AlgorithmPS512:
 		return "PS512"
+	case AlgorithmRS256:
+		return "RS256"
+	case AlgorithmRS384:
+		return "RS384"
+	case AlgorithmRS512:
+		return "RS512"
 	case AlgorithmES256:
 		return "ES256"
 	case AlgorithmES384:

algorithm_test.go

@@ -18,6 +18,9 @@ func TestAlgorithm_String(t *testing.T) {
 		{AlgorithmPS256, "PS256"},
 		{AlgorithmPS384, "PS384"},
 		{AlgorithmPS512, "PS512"},
+		{AlgorithmRS256, "RS256"},
+		{AlgorithmRS384, "RS384"},
+		{AlgorithmRS512, "RS512"},
 		{AlgorithmES256, "ES256"},
 		{AlgorithmES384, "ES384"},
 		{AlgorithmES512, "ES512"},

conformance_test.go

@@ -308,6 +308,12 @@ func mustNameToAlg(name string) cose.Algorithm {
 		return cose.AlgorithmPS384
 	case "PS512":
 		return cose.AlgorithmPS512
+	case "RS256":
+		return cose.AlgorithmRS256
+	case "RS384":
+		return cose.AlgorithmRS384
+	case "RS512":
+		return cose.AlgorithmRS512
 	case "ES256":
 		return cose.AlgorithmES256
 	case "ES384":

signer.go

@@ -89,6 +89,6 @@ func NewSigner(alg Algorithm, key crypto.Signer) (Signer, error) {
 			key: key,
 		}, nil
 	default:
-		return nil, ErrAlgorithmNotSupported
+		return nil, fmt.Errorf("can't create new Signer for %s: %w", alg, ErrAlgorithmNotSupported)
 	}
 }

signer_test.go

@@ -110,10 +110,20 @@ func TestNewSigner(t *testing.T) {
 			key:     rsaKeyLowEntropy,
 			wantErr: "RSA key must be at least 2048 bits long",
 		},
+		{
+			name:    "unsupported rsa signing algorithm",
+			alg:     AlgorithmRS256,
+			wantErr: "can't create new Signer for RS256: algorithm not supported",
+		},
 		{
 			name:    "unknown algorithm",
 			alg:     0,
-			wantErr: "algorithm not supported",
+			wantErr: "can't create new Signer for Reserved: algorithm not supported",
+		},
+		{
+			name:    "unassigned algorithm",
+			alg:     -1,
+			wantErr: "can't create new Signer for unknown algorithm value -1: algorithm not supported",
 		},
 	}
 	for _, tt := range tests {

verifier.go

@@ -75,6 +75,6 @@ func NewVerifier(alg Algorithm, key crypto.PublicKey) (Verifier, error) {
 			key: vk,
 		}, nil
 	default:
-		return nil, ErrAlgorithmNotSupported
+		return nil, fmt.Errorf("can't create new Verifier for %s: %w", alg, ErrAlgorithmNotSupported)
 	}
 }

verifier_test.go

@@ -106,10 +106,20 @@ func TestNewVerifier(t *testing.T) {
 			key:     rsaKeyLowEntropy,
 			wantErr: "RSA key must be at least 2048 bits long",
 		},
+		{
+			name:    "unsupported rsa signing algorithm",
+			alg:     AlgorithmRS256,
+			wantErr: "can't create new Verifier for RS256: algorithm not supported",
+		},
 		{
 			name:    "unknown algorithm",
 			alg:     0,
-			wantErr: "algorithm not supported",
+			wantErr: "can't create new Verifier for Reserved: algorithm not supported",
+		},
+		{
+			name:    "unassigned algorithm",
+			alg:     -1,
+			wantErr: "can't create new Verifier for unknown algorithm value -1: algorithm not supported",
 		},
 		{
 			name:    "bogus ecdsa public key (point not on curve)",