Add support for CWT Claims & Type in Protected Headers (#189)

Signed-off-by: steve lasker <[email protected]>
Co-authored-by: Orie Steele <[email protected]>

cwt.go

@@ -0,0 +1,20 @@
+package cose
+
+// https://www.iana.org/assignments/cwt/cwt.xhtml#claims-registry
+const (
+	CWTClaimIssuer         int64 = 1
+	CWTClaimSubject        int64 = 2
+	CWTClaimAudience       int64 = 3
+	CWTClaimExpirationTime int64 = 4
+	CWTClaimNotBefore      int64 = 5
+	CWTClaimIssuedAt       int64 = 6
+	CWTClaimCWTID          int64 = 7
+	CWTClaimConfirmation   int64 = 8
+	CWTClaimScope          int64 = 9
+
+	// TODO: the rest upon request
+)
+
+// CWTClaims contains parameters that are to be cryptographically
+// protected.
+type CWTClaims map[any]any

cwt_test.go

@@ -0,0 +1,82 @@
+package cose_test
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"fmt"
+
+	"github.com/veraison/go-cose"
+)
+
+// This example demonstrates signing and verifying COSE_Sign1 signatures.
+func ExampleCWTMessage() {
+	// create message to be signed
+	msgToSign := cose.NewSign1Message()
+	msgToSign.Payload = []byte("hello world")
+	msgToSign.Headers.Protected.SetAlgorithm(cose.AlgorithmES512)
+
+	msgToSign.Headers.Protected.SetType("application/cwt")
+	claims := cose.CWTClaims{
+		cose.CWTClaimIssuer:  "issuer.example",
+		cose.CWTClaimSubject: "subject.example",
+	}
+	msgToSign.Headers.Protected.SetCWTClaims(claims)
+
+	msgToSign.Headers.Unprotected[cose.HeaderLabelKeyID] = []byte("1")
+
+	// 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
+	err = msgToSign.Sign(rand.Reader, nil, signer)
+	if err != nil {
+		panic(err)
+	}
+	sig, err := msgToSign.MarshalCBOR()
+	// uncomment to review EDN
+	// coseSign1Diagnostic, err := cbor.Diagnose(sig)
+	// fmt.Println(coseSign1Diagnostic)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("message signed")
+
+	// create a verifier from a trusted public key
+	publicKey := privateKey.Public()
+	verifier, err := cose.NewVerifier(cose.AlgorithmES512, publicKey)
+	if err != nil {
+		panic(err)
+	}
+
+	// verify message
+	var msgToVerify cose.Sign1Message
+	err = msgToVerify.UnmarshalCBOR(sig)
+	if err != nil {
+		panic(err)
+	}
+	err = msgToVerify.Verify(nil, verifier)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("message verified")
+
+	// tamper the message and verification should fail
+	msgToVerify.Payload = []byte("foobar")
+	err = msgToVerify.Verify(nil, verifier)
+	if err != cose.ErrVerification {
+		panic(err)
+	}
+	fmt.Println("verification error as expected")
+	// Output:
+	// message signed
+	// message verified
+	// verification error as expected
+}

headers.go

@@ -23,6 +23,8 @@ const (
 	HeaderLabelCounterSignature0   int64 = 9
 	HeaderLabelCounterSignatureV2  int64 = 11
 	HeaderLabelCounterSignature0V2 int64 = 12
+	HeaderLabelCWTClaims           int64 = 15
+	HeaderLabelType                int64 = 16
 	HeaderLabelX5Bag               int64 = 32
 	HeaderLabelX5Chain             int64 = 33
 	HeaderLabelX5T                 int64 = 34
@@ -97,11 +99,35 @@ func (h *ProtectedHeader) UnmarshalCBOR(data []byte) error {
 	return nil
 }
 
-// SetAlgorithm sets the algorithm value to the algorithm header.
+// SetAlgorithm sets the algorithm value of the protected header.
 func (h ProtectedHeader) SetAlgorithm(alg Algorithm) {
 	h[HeaderLabelAlgorithm] = alg
 }
 
+// SetType sets the type of the cose object in the protected header.
+func (h ProtectedHeader) SetType(typ any) (any, error) {
+	if !canTstr(typ) && !canUint(typ) {
+		return typ, errors.New("header parameter: type: require tstr / uint type")
+	}
+	h[HeaderLabelType] = typ
+	return typ, nil
+}
+
+// SetCWTClaims sets the CWT Claims value of the protected header.
+func (h ProtectedHeader) SetCWTClaims(claims CWTClaims) (CWTClaims, error) {
+	iss, hasIss := claims[1]
+	if hasIss && !canTstr(iss) {
+		return claims, errors.New("cwt claim: iss: require tstr")
+	}
+	sub, hasSub := claims[2]
+	if hasSub && !canTstr(sub) {
+		return claims, errors.New("cwt claim: sub: require tstr")
+	}
+	// TODO: validate claims, other claims
+	h[HeaderLabelCWTClaims] = claims
+	return claims, nil
+}
+
 // Algorithm gets the algorithm value from the algorithm header.
 func (h ProtectedHeader) Algorithm() (Algorithm, error) {
 	value, ok := h[HeaderLabelAlgorithm]
@@ -460,8 +486,8 @@ func validateHeaderParameters(h map[any]any, protected bool) error {
 		// Reference: https://datatracker.ietf.org/doc/html/rfc8152#section-3.1
 		switch label {
 		case HeaderLabelAlgorithm:
-			_, is_alg := value.(Algorithm)
-			if !is_alg && !canInt(value) && !canTstr(value) {
+			_, isAlg := value.(Algorithm)
+			if !isAlg && !canInt(value) && !canTstr(value) {
 				return errors.New("header parameter: alg: require int / tstr type")
 			}
 		case HeaderLabelCritical:
@@ -471,12 +497,31 @@ func validateHeaderParameters(h map[any]any, protected bool) error {
 			if err := ensureCritical(value, h); err != nil {
 				return fmt.Errorf("header parameter: crit: %w", err)
 			}
+		case HeaderLabelType:
+			isTstr := canTstr(value)
+			if !isTstr && !canUint(value) {
+				return errors.New("header parameter: type: require tstr / uint type")
+			}
+			if isTstr {
+				v := value.(string)
+				if len(v) == 0 {
+					return errors.New("header parameter: type: require non-empty string")
+				}
+				if v[0] == ' ' || v[len(v)-1] == ' ' {
+					return errors.New("header parameter: type: require no leading/trailing whitespace")
+				}
+				// Basic check that the content type is of form type/subtype.
+				// We don't check the precise definition though (RFC 6838 Section 4.2).
+				if strings.Count(v, "/") != 1 {
+					return errors.New("header parameter: type: require text of form type/subtype")
+				}
+			}
 		case HeaderLabelContentType:
-			is_tstr := canTstr(value)
-			if !is_tstr && !canUint(value) {
+			isTstr := canTstr(value)
+			if !isTstr && !canUint(value) {
 				return errors.New("header parameter: content type: require tstr / uint type")
 			}
-			if is_tstr {
+			if isTstr {
 				v := value.(string)
 				if len(v) == 0 {
 					return errors.New("header parameter: content type: require non-empty string")