Fixes issue #40: extraneous zero padding on serialized exponent

encoding.go

@@ -20,6 +20,7 @@ import (
 	"bytes"
 	"compress/flate"
 	"encoding/base64"
+	"encoding/binary"
 	"encoding/json"
 	"io"
 	"math/big"
@@ -131,6 +132,14 @@ func newBuffer(data []byte) *byteBuffer {
 	}
 }
 
+func newBufferFromInt(num uint64) *byteBuffer {
+	data := make([]byte, 8)
+	binary.BigEndian.PutUint64(data, num)
+	return &byteBuffer{
+		data: data,
+	}
+}
+
 func (b *byteBuffer) MarshalJSON() ([]byte, error) {
 	return json.Marshal(b.base64())
 }
@@ -175,3 +184,17 @@ func (b byteBuffer) bigInt() *big.Int {
 func (b byteBuffer) toInt() int {
 	return int(b.bigInt().Int64())
 }
+
+func (b byteBuffer) trim() *byteBuffer {
+	i := 0
+	for _, b := range b.data {
+		if b == 0 {
+			i++
+		} else {
+			break
+		}
+	}
+	return &byteBuffer{
+		data: b.data[i:],
+	}
+}

encoding_test.go

@@ -106,3 +106,15 @@ func TestInvalidCompression(t *testing.T) {
 		t.Error("should not accept invalid data")
 	}
 }
+
+func TestByteBufferTrim(t *testing.T) {
+	buf := newBufferFromInt(1).trim()
+	if len(buf.data) != 1 || buf.data[0] != 1 {
+		t.Error("Trimmed byte buffer for integer '1' should contain [0x01]")
+	}
+
+	buf = newBufferFromInt(65537).trim()
+	if len(buf.data) != 3 || !bytes.Equal(buf.data, []byte{0x01, 0x00, 0x01}) {
+		t.Error("Trimmed byte buffer for integer '1' should contain [0x01, 0x00, 0x01]")
+	}
+}

jwk.go

@@ -20,7 +20,6 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rsa"
-	"encoding/binary"
 	"encoding/json"
 	"fmt"
 	"math/big"
@@ -128,13 +127,10 @@ func (key rawJsonWebKey) rsaPublicKey() (*rsa.PublicKey, error) {
 }
 
 func fromRsaPublicKey(pub *rsa.PublicKey) *rawJsonWebKey {
-	e := make([]byte, 4)
-	binary.BigEndian.PutUint32(e, uint32(pub.E))
-
 	return &rawJsonWebKey{
 		Kty: "RSA",
 		N:   newBuffer(pub.N.Bytes()),
-		E:   newBuffer(e),
+		E:   newBufferFromInt(uint64(pub.E)).trim(),
 	}
 }
 

jwk_test.go

@@ -18,11 +18,11 @@ package jose
 
 import (
 	"bytes"
-	"fmt"
-	"encoding/json"
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rsa"
+	"encoding/json"
+	"fmt"
 	"math/big"
 	"reflect"
 	"testing"
@@ -178,7 +178,7 @@ func TestMarshalNonPointer(t *testing.T) {
 		t.Error(fmt.Sprintf("Error marshalling JSON: %v", err))
 		return
 	}
-	expected := "{\"Key\":{\"kty\":\"RSA\",\"n\":\"vd7rZIoTLEe-z1_8G1FcXSw9CQFEJgV4g9V277sER7yx5Qjz_Pkf2YVth6wwwFJEmzc0hoKY-MMYFNwBE4hQHw\",\"e\":\"AAEAAQ\"}}"
+	expected := "{\"Key\":{\"kty\":\"RSA\",\"n\":\"vd7rZIoTLEe-z1_8G1FcXSw9CQFEJgV4g9V277sER7yx5Qjz_Pkf2YVth6wwwFJEmzc0hoKY-MMYFNwBE4hQHw\",\"e\":\"AQAB\"}}"
 	if string(out) != expected {
 		t.Error("Failed to marshal embedded non-pointer JWK properly:", string(out))
 	}