Support almost all base64 options

encoder.go

@@ -2,20 +2,21 @@ package jwt
 
 import "io"
 
-// Base64Encoder is an interface that allows to implement custom Base64 encoding
-// algorithms.
-type Base64EncodeFunc func(src []byte) string
+type Base64Encoding interface {
+	EncodeToString(src []byte) string
+	DecodeString(s string) ([]byte, error)
+}
 
-// Base64Decoder is an interface that allows to implement custom Base64 decoding
-// algorithms.
-type Base64DecodeFunc func(s string) ([]byte, error)
+type Stricter[T Base64Encoding] interface {
+	Strict() T
+}
 
-// JSONEncoder is an interface that allows to implement custom JSON encoding
-// algorithms.
+// JSONMarshalFunc is an function type that allows to implement custom JSON
+// encoding algorithms.
 type JSONMarshalFunc func(v any) ([]byte, error)
 
-// JSONUnmarshal is an interface that allows to implement custom JSON unmarshal
-// algorithms.
+// JSONUnmarshalFunc is an function type that allows to implement custom JSON
+// unmarshal algorithms.
 type JSONUnmarshalFunc func(data []byte, v any) error
 
 type JSONDecoder interface {

errors.go

@@ -22,6 +22,7 @@ var (
 	ErrTokenInvalidId            = errors.New("token has invalid id")
 	ErrTokenInvalidClaims        = errors.New("token has invalid claims")
 	ErrInvalidType               = errors.New("invalid type for claim")
+	ErrUnsupported               = errors.New("operation is unsupported")
 )
 
 // joinedError is an error type that works similar to what [errors.Join]

parser.go

@@ -26,12 +26,11 @@ type Parser struct {
 type decoders struct {
 	jsonUnmarshal  JSONUnmarshalFunc
 	jsonNewDecoder JSONNewDecoderFunc[JSONDecoder]
-	base64Decode   Base64DecodeFunc
 
-	// This field is disabled when using a custom base64 encoder.
-	decodeStrict bool
+	rawUrlBase64Encoding Base64Encoding
+	urlBase64Encoding    Base64Encoding
 
-	// This field is disabled when using a custom base64 encoder.
+	decodeStrict         bool
 	decodePaddingAllowed bool
 }
 
@@ -227,22 +226,35 @@ func (p *Parser) ParseUnverified(tokenString string, claims Claims) (token *Toke
 // take into account whether the [Parser] is configured with additional options,
 // such as [WithStrictDecoding] or [WithPaddingAllowed].
 func (p *Parser) DecodeSegment(seg string) ([]byte, error) {
-	if p.base64Decode != nil {
-		return p.base64Decode(seg)
+	var encoding Base64Encoding
+	if p.rawUrlBase64Encoding != nil {
+		encoding = p.rawUrlBase64Encoding
+	} else {
+		encoding = base64.RawURLEncoding
 	}
 
-	encoding := base64.RawURLEncoding
-
 	if p.decodePaddingAllowed {
 		if l := len(seg) % 4; l > 0 {
 			seg += strings.Repeat("=", 4-l)
 		}
-		encoding = base64.URLEncoding
+
+		if p.urlBase64Encoding != nil {
+			encoding = p.urlBase64Encoding
+		} else {
+			encoding = base64.URLEncoding
+		}
 	}
 
 	if p.decodeStrict {
-		encoding = encoding.Strict()
+		// For now we can only support the standard library here because of the
+		// current state of the type parameter system
+		stricter, ok := encoding.(Stricter[*base64.Encoding])
+		if !ok {
+			return nil, newError("strict mode is only supported in encoding/base64", ErrUnsupported)
+		}
+		encoding = stricter.Strict()
 	}
+
 	return encoding.DecodeString(seg)
 }
 

parser_option.go

@@ -142,9 +142,10 @@ func WithJSONDecoder[T JSONDecoder](f JSONUnmarshalFunc, f2 JSONNewDecoderFunc[T
 	}
 }
 
-// WithBase64Decoder supports a custom [Base64Decoder] to use in parsing the JWT.
-func WithBase64Decoder(f Base64DecodeFunc) ParserOption {
+// WithBase64Decoder supports a custom [Base64Encoding] to use in parsing the JWT.
+func WithBase64Decoder(rawURL Base64Encoding, url Base64Encoding) ParserOption {
 	return func(p *Parser) {
-		p.base64Decode = f
+		p.rawUrlBase64Encoding = rawURL
+		p.urlBase64Encoding = url
 	}
 }

parser_test.go

@@ -454,7 +454,7 @@ var jwtTestData = []struct {
 		jwt.MapClaims{"foo": "bar"},
 		true,
 		nil,
-		jwt.NewParser(jwt.WithBase64Decoder(base64.RawURLEncoding.DecodeString)),
+		jwt.NewParser(jwt.WithBase64Decoder(base64.RawURLEncoding, base64.URLEncoding)),
 		jwt.SigningMethodRS256,
 	},
 	{

token.go

@@ -39,8 +39,8 @@ type Token struct {
 }
 
 type encoders struct {
-	jsonMarshal  JSONMarshalFunc  // jsonEncoder is the custom json encoder/decoder
-	base64Encode Base64EncodeFunc // base64Encoder is the custom base64 encoder/decoder
+	jsonMarshal    JSONMarshalFunc // jsonEncoder is the custom json encoder/decoder
+	base64Encoding Base64Encoding  // base64Encoder is the custom base64 encoding
 }
 
 // New creates a new [Token] with the specified signing method and an empty map
@@ -114,12 +114,12 @@ func (t *Token) SigningString() (string, error) {
 // [TokenOption]. Therefore, this function exists as a method of [Token], rather
 // than a global function.
 func (t *Token) EncodeSegment(seg []byte) string {
-	var enc Base64EncodeFunc
-	if t.base64Encode != nil {
-		enc = t.base64Encode
+	var enc Base64Encoding
+	if t.base64Encoding != nil {
+		enc = t.base64Encoding
 	} else {
-		enc = base64.RawURLEncoding.EncodeToString
+		enc = base64.RawURLEncoding
 	}
 
-	return enc(seg)
+	return enc.EncodeToString(seg)
 }

token_option.go

@@ -10,8 +10,8 @@ func WithJSONEncoder(f JSONMarshalFunc) TokenOption {
 	}
 }
 
-func WithBase64Encoder(f Base64EncodeFunc) TokenOption {
+func WithBase64Encoder(enc Base64Encoding) TokenOption {
 	return func(token *Token) {
-		token.base64Encode = f
+		token.base64Encoding = enc
 	}
 }

token_test.go

@@ -53,7 +53,7 @@ func TestToken_SigningString(t1 *testing.T) {
 				Valid:  false,
 				Options: []jwt.TokenOption{
 					jwt.WithJSONEncoder(json.Marshal),
-					jwt.WithBase64Encoder(base64.StdEncoding.EncodeToString),
+					jwt.WithBase64Encoder(base64.StdEncoding),
 				},
 			},
 			want:    "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.e30",