narco/jwt/jws.go

package jwt

import (
	"encoding/base64"
	"encoding/json"
	"fmt"
)

// EncodeJWS encode a JSON Object with the serialized JWS format as
// specified in RFC 7515
func EncodeJWS(j *JOSE, v interface{}, s Signer) ([]byte, error) {
	payload, err := json.Marshal(v)
	if err != nil {
		return nil, err
	}

	if s != nil {
		j.Algorithm = s.Algorithm()
	} else {
		j.Algorithm = "none"
	}

	header, err := j.EncodeJSON()
	if err != nil {
		return nil, err
	}

	//Allocate a buffer long enough for all payload
	res := make([]byte, Base64EncodedBufferLen(len(header))+Base64EncodedBufferLen(len(payload))+2)
	base64.URLEncoding.Encode(res, header)
	lengthHeader := Base64EncodedStrippedLen(len(header))
	res[lengthHeader] = '.'
	base64.URLEncoding.Encode(res[lengthHeader+1:], payload)
	fullPayloadLength := Base64EncodedStrippedLen(len(payload)) + 1 + lengthHeader
	res[fullPayloadLength] = '.'

	if s == nil {
		// unprotected jws, not signing it
		return res[:fullPayloadLength+1], nil
	}
	signature, err := s.Sign(res[:fullPayloadLength])
	if err != nil {
		return nil, err
	}

	signedRes := make([]byte, fullPayloadLength+1+Base64EncodedBufferLen(len(signature)))
	copy(signedRes, res[:fullPayloadLength+1])
	base64.URLEncoding.Encode(signedRes[fullPayloadLength+1:], signature)
	return signedRes[:fullPayloadLength+1+Base64EncodedStrippedLen(len(signature))], nil
}

// isBase64URLEncoding returns if the byte is a character used for
// base64 URL Encoding
func isBase64URLEncoding(b byte) bool {
	if b == '-' || b == '_' {
		return true
	}

	if b < '0' || b > 'z' {
		return false
	}

	if b <= '9' {
		return true
	}

	if b < 'A' {
		return false
	}

	if b <= 'Z' {
		return true
	}

	if b < 'a' {
		return false
	}

	return true
}

// DecodeJWS decode an object encoded with the JWS serialized fromat
// as specified by RFC 7515. to avoid an attack where the unprotected
// JOSE header would contain a modified alg field, the Signer should
// also be specified.
func DecodeJWS(data []byte, v interface{}, s Signer) error {
	var headerLength, payloadLength int
	for i, c := range data {
		if isBase64URLEncoding(c) == true {
			continue
		}
		if c != '.' {
			return fmt.Errorf("jws: invalid serialization character %v in '%s'", c, data)
		}

		if headerLength == 0 {
			if i == 0 {
				return fmt.Errorf("jws: invalid emtpy JOSE header in %s", data)
			}
			headerLength = i
			continue
		}

		if payloadLength == 0 {
			if i-headerLength == 1 {
				return fmt.Errorf("jws: invalid emtpy payload in %s", data)
			}
			payloadLength = i - headerLength - 1
			continue
		}

		return fmt.Errorf("jws: invalid third '.' in %s", data)
	}
	signatureLength := len(data) - 2 - headerLength - payloadLength
	signature := make([]byte, Base64DecodedLenFromStripped(signatureLength))
	signedLength := headerLength + payloadLength + 1
	Base64Decode(signature, data[signedLength+1:])
	signature = signature[:Base64DecodedStrippedLen(signatureLength)]

	if s != nil {
		if err := s.Verify(data[:signedLength], signature); err != nil {
			return err
		}
	} else {
		if signatureLength != 0 {
			return fmt.Errorf("jws: Invalid JWS, got a signature, but none expected")
		}
	}

	//decode jose
	jose, err := DecodeJOSE(data[:headerLength])
	if err != nil {
		return err
	}

	algo := "none"
	if s != nil {
		algo = s.Algorithm()
	}

	if jose.Algorithm != algo {
		return fmt.Errorf("jws: Mismatched signing algorithm got %s, expected %s", jose.Algorithm, algo)
	}

	payload := make([]byte, Base64DecodedLenFromStripped(payloadLength))
	Base64Decode(payload, data[headerLength+1:headerLength+1+payloadLength])
	payload = payload[:Base64DecodedStrippedLen(payloadLength)]
	//data is safe, just need to decode it.
	return json.Unmarshal(payload, v)
}