Add kid into Header (#85)

jwt.go

@@ -58,12 +58,13 @@ func (t *Token) Signature() []byte {
 }
 
 // Header representa JWT header data.
-// See: https://tools.ietf.org/html/rfc7519#section-5
+// See: https://tools.ietf.org/html/rfc7519#section-5, https://tools.ietf.org/html/rfc7517
 //
 type Header struct {
 	Algorithm   Algorithm `json:"alg"`
 	Type        string    `json:"typ,omitempty"` // only "JWT" can be here
 	ContentType string    `json:"cty,omitempty"`
+	KeyID       string    `json:"kid,omitempty"`
 }
 
 // MarshalJSON implements the json.Marshaler interface.
@@ -80,6 +81,10 @@ func (h *Header) MarshalJSON() ([]byte, error) {
 		buf.WriteString(`","cty":"`)
 		buf.WriteString(h.ContentType)
 	}
+	if h.KeyID != "" {
+		buf.WriteString(`","kid":"`)
+		buf.WriteString(h.KeyID)
+	}
 	buf.WriteString(`"}`)
 
 	return buf.Bytes(), nil

jwt_test.go

@@ -58,6 +58,10 @@ func TestMarshalHeader(t *testing.T) {
 		&Header{Algorithm: RS256, Type: "JwT", ContentType: "token"},
 		`{"alg":"RS256","typ":"JwT","cty":"token"}`,
 	)
+	f(
+		&Header{Algorithm: RS256, Type: "JwT", ContentType: "token", KeyID: "test"},
+		`{"alg":"RS256","typ":"JwT","cty":"token","kid":"test"}`,
+	)
 }
 
 func TestSecurePrint(t *testing.T) {