Add untagged COSE_Sign1 support

RFC8152 specifies that a single-signer token can be either tagged
(COSE_Sign1_Tagged) or untagged (COSE_Sign1).

The existing Sign1Message implementation only handles COSE_Sign1_Tagged
objects. Add UntaggedSign1Message to analogously handle COSE_Sign1
objects.

Signed-off-by: setrofim <setrofim@gmail.com>

README.md

@@ -65,7 +65,7 @@ go get github.com/veraison/go-cose@main
 import "github.com/veraison/go-cose"
 ```
 
-Construct a new COSE_Sign1 message, then sign it using ECDSA w/ SHA-256 and finally marshal it. For example:
+Construct a new COSE_Sign1_Tagged message, then sign it using ECDSA w/ SHA-256 and finally marshal it. For example:
 
 ```go
 package main
@@ -102,7 +102,7 @@ func SignP256(data []byte) ([]byte, error) {
 }
 ```
 
-Verify a raw COSE_Sign1 message. For example:
+Verify a raw COSE_Sign1_Tagged message. For example:
 
 ```go
 package main
@@ -132,6 +132,11 @@ func VerifyP256(publicKey crypto.PublicKey, sig []byte) error {
 
 See [example_test.go](./example_test.go) for more examples.
 
+#### Untagged Signing and Verification
+
+Untagged COSE_Sign1 messages can be signed and verified as above, using
+`cose.UntaggedSign1Message` instead of `cose.Sign1Message`.
+
 ### About hashing
 
 `go-cose` does not import any hash package by its own to avoid linking unnecessary algorithms to the final binary.

example_test.go

@@ -139,7 +139,7 @@ func ExampleSign1Message() {
 	// verification error as expected
 }
 
-// This example demonstrates signing COSE_Sign1 signatures using Sign1().
+// This example demonstrates signing COSE_Sign1_Tagged signatures using Sign1().
 func ExampleSign1() {
 	// create a signer
 	privateKey, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
@@ -170,3 +170,35 @@ func ExampleSign1() {
 	// Output:
 	// message signed
 }
+
+// This example demonstrates signing COSE_Sign1 signatures using Sign1Untagged().
+func ExampleSign1Untagged() {
+	// create a signer
+	privateKey, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
+	if err != nil {
+		panic(err)
+	}
+	signer, err := cose.NewSigner(cose.AlgorithmES512, privateKey)
+	if err != nil {
+		panic(err)
+	}
+
+	// sign message
+	headers := cose.Headers{
+		Protected: cose.ProtectedHeader{
+			cose.HeaderLabelAlgorithm: cose.AlgorithmES512,
+		},
+		Unprotected: cose.UnprotectedHeader{
+			cose.HeaderLabelKeyID: []byte("1"),
+		},
+	}
+	sig, err := cose.Sign1Untagged(rand.Reader, signer, headers, []byte("hello world"), nil)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println("message signed")
+	_ = sig // further process on sig
+	// Output:
+	// message signed
+}

sign1.go

@@ -52,22 +52,11 @@ func NewSign1Message() *Sign1Message {
 
 // MarshalCBOR encodes Sign1Message into a COSE_Sign1_Tagged object.
 func (m *Sign1Message) MarshalCBOR() ([]byte, error) {
-	if m == nil {
-		return nil, errors.New("cbor: MarshalCBOR on nil Sign1Message pointer")
-	}
-	if len(m.Signature) == 0 {
-		return nil, ErrEmptySignature
-	}
-	protected, unprotected, err := m.Headers.marshal()
+	content, err := m.getContent()
 	if err != nil {
 		return nil, err
 	}
-	content := sign1Message{
-		Protected:   protected,
-		Unprotected: unprotected,
-		Payload:     m.Payload,
-		Signature:   m.Signature,
-	}
+
 	return encMode.Marshal(cbor.Tag{
 		Number:  CBORTagSign1Message,
 		Content: content,
@@ -85,28 +74,7 @@ func (m *Sign1Message) UnmarshalCBOR(data []byte) error {
 		return errors.New("cbor: invalid COSE_Sign1_Tagged object")
 	}
 
-	// decode to sign1Message and parse
-	var raw sign1Message
-	if err := decModeWithTagsForbidden.Unmarshal(data[1:], &raw); err != nil {
-		return err
-	}
-	if len(raw.Signature) == 0 {
-		return ErrEmptySignature
-	}
-	msg := Sign1Message{
-		Headers: Headers{
-			RawProtected:   raw.Protected,
-			RawUnprotected: raw.Unprotected,
-		},
-		Payload:   raw.Payload,
-		Signature: raw.Signature,
-	}
-	if err := msg.Headers.UnmarshalFromRaw(); err != nil {
-		return err
-	}
-
-	*m = msg
-	return nil
+	return m.doUnmarshal(data[1:])
 }
 
 // Sign signs a Sign1Message using the provided Signer.
@@ -218,6 +186,53 @@ func (m *Sign1Message) toBeSigned(external []byte) ([]byte, error) {
 	return encMode.Marshal(sigStructure)
 }
 
+func (m *Sign1Message) getContent() (sign1Message, error) {
+	if m == nil {
+		return sign1Message{}, errors.New("cbor: MarshalCBOR on nil Sign1Message pointer")
+	}
+	if len(m.Signature) == 0 {
+		return sign1Message{}, ErrEmptySignature
+	}
+	protected, unprotected, err := m.Headers.marshal()
+	if err != nil {
+		return sign1Message{}, err
+	}
+
+	content := sign1Message{
+		Protected:   protected,
+		Unprotected: unprotected,
+		Payload:     m.Payload,
+		Signature:   m.Signature,
+	}
+
+	return content, nil
+}
+
+func (m *Sign1Message) doUnmarshal(data []byte) error {
+	// decode to sign1Message and parse
+	var raw sign1Message
+	if err := decModeWithTagsForbidden.Unmarshal(data, &raw); err != nil {
+		return err
+	}
+	if len(raw.Signature) == 0 {
+		return ErrEmptySignature
+	}
+	msg := Sign1Message{
+		Headers: Headers{
+			RawProtected:   raw.Protected,
+			RawUnprotected: raw.Unprotected,
+		},
+		Payload:   raw.Payload,
+		Signature: raw.Signature,
+	}
+	if err := msg.Headers.UnmarshalFromRaw(); err != nil {
+		return err
+	}
+
+	*m = msg
+	return nil
+}
+
 // Sign1 signs a Sign1Message using the provided Signer.
 //
 // This method is a wrapper of `Sign1Message.Sign()`.
@@ -234,3 +249,71 @@ func Sign1(rand io.Reader, signer Signer, headers Headers, payload []byte, exter
 	}
 	return msg.MarshalCBOR()
 }
+
+type UntaggedSign1Message Sign1Message
+
+// MarshalCBOR encodes UntaggedSign1Message into a COSE_Sign1 object.
+func (m *UntaggedSign1Message) MarshalCBOR() ([]byte, error) {
+	content, err := (*Sign1Message)(m).getContent()
+	if err != nil {
+		return nil, err
+	}
+
+	return encMode.Marshal(content)
+}
+
+// UnmarshalCBOR decodes a COSE_Sign1 object into an UnataggedSign1Message.
+func (m *UntaggedSign1Message) UnmarshalCBOR(data []byte) error {
+	if m == nil {
+		return errors.New("cbor: UnmarshalCBOR on nil UntaggedSign1Message pointer")
+	}
+
+	if len(data) == 0 {
+		return errors.New("cbor: zero length data")
+	}
+
+	// fast message check - ensure the frist byte indicates a four-element array
+	if data[0] != sign1MessagePrefix[1] {
+		return errors.New("cbor: invalid COSE_Sign1 object")
+	}
+
+	return (*Sign1Message)(m).doUnmarshal(data)
+}
+
+// Sign signs an UnttaggedSign1Message using the provided Signer.
+// The signature is stored in m.Signature.
+//
+// Note that m.Signature is only valid as long as m.Headers.Protected and
+// m.Payload remain unchanged after calling this method.
+// It is possible to modify m.Headers.Unprotected after signing,
+// i.e., add counter signatures or timestamps.
+//
+// Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-4.4
+func (m *UntaggedSign1Message) Sign(rand io.Reader, external []byte, signer Signer) error {
+	return (*Sign1Message)(m).Sign(rand, external, signer)
+}
+
+// Verify verifies the signature on the UntaggedSign1Message returning nil on success or
+// a suitable error if verification fails.
+//
+// Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-4.4
+func (m *UntaggedSign1Message) Verify(external []byte, verifier Verifier) error {
+	return (*Sign1Message)(m).Verify(external, verifier)
+}
+
+// Sign1Untagged signs an UntaggedSign1Message using the provided Signer.
+//
+// This method is a wrapper of `UntaggedSign1Message.Sign()`.
+//
+// Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-4.4
+func Sign1Untagged(rand io.Reader, signer Signer, headers Headers, payload []byte, external []byte) ([]byte, error) {
+	msg := UntaggedSign1Message{
+		Headers: headers,
+		Payload: payload,
+	}
+	err := msg.Sign(rand, external, signer)
+	if err != nil {
+		return nil, err
+	}
+	return msg.MarshalCBOR()
+}

sign1_test.go

@@ -978,3 +978,114 @@ func TestSign1Message_toBeSigned(t *testing.T) {
 		})
 	}
 }
+
+func TestUntaggedSign1Message_MarshalCBOR(t *testing.T) {
+	tests := []struct {
+		name    string
+		m       *UntaggedSign1Message
+		want    []byte
+		wantErr string
+	}{
+		{
+			name: "valid message",
+			m: &UntaggedSign1Message{
+				Headers: Headers{
+					Protected: ProtectedHeader{
+						HeaderLabelAlgorithm: AlgorithmES256,
+					},
+					Unprotected: UnprotectedHeader{
+						HeaderLabelContentType: 42,
+					},
+				},
+				Payload:   []byte("foo"),
+				Signature: []byte("bar"),
+			},
+			want: []byte{
+				0x84,
+				0x43, 0xa1, 0x01, 0x26, // protected
+				0xa1, 0x03, 0x18, 0x2a, // unprotected
+				0x43, 0x66, 0x6f, 0x6f, // payload
+				0x43, 0x62, 0x61, 0x72, // signature
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := tt.m.MarshalCBOR()
+
+			if err != nil && (err.Error() != tt.wantErr) {
+				t.Errorf("UntaggedSign1Message.MarshalCBOR() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			} else if err == nil && (tt.wantErr != "") {
+				t.Errorf("UntaggedSign1Message.MarshalCBOR() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("UntaggedSign1Message.MarshalCBOR() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestUntaggedSign1Message_UnmarshalCBOR(t *testing.T) {
+	// test others
+	tests := []struct {
+		name    string
+		data    []byte
+		want    UntaggedSign1Message
+		wantErr string
+	}{
+		{
+			name: "valid message",
+			data: []byte{
+				0x84,
+				0x43, 0xa1, 0x01, 0x26, // protected
+				0xa1, 0x03, 0x18, 0x2a, // unprotected
+				0x43, 0x66, 0x6f, 0x6f, // payload
+				0x43, 0x62, 0x61, 0x72, // signature
+			},
+			want: UntaggedSign1Message{
+				Headers: Headers{
+					RawProtected: []byte{0x43, 0xa1, 0x01, 0x26},
+					Protected: ProtectedHeader{
+						HeaderLabelAlgorithm: AlgorithmES256,
+					},
+					RawUnprotected: []byte{0xa1, 0x03, 0x18, 0x2a},
+					Unprotected: UnprotectedHeader{
+						HeaderLabelContentType: int64(42),
+					},
+				},
+				Payload:   []byte("foo"),
+				Signature: []byte("bar"),
+			},
+		},
+		{
+			name: "tagged message",
+			data: []byte{
+				0xd2, // tag
+				0x84,
+				0x43, 0xa1, 0x01, 0x26, // protected
+				0xa1, 0x03, 0x18, 0x2a, // unprotected
+				0x43, 0x66, 0x6f, 0x6f, // payload
+				0x43, 0x62, 0x61, 0x72, // signature
+			},
+			wantErr: "cbor: invalid COSE_Sign1 object",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var got UntaggedSign1Message
+			err := got.UnmarshalCBOR(tt.data)
+			if (err != nil) && (err.Error() != tt.wantErr) {
+				t.Errorf("Sign1Message.UnmarshalCBOR() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			} else if err == nil && (tt.wantErr != "") {
+				t.Errorf("Sign1Message.UnmarshalCBOR() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("Sign1Message.UnmarshalCBOR() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}