JWT Authentication in Golang

jwt.go

// Return JWT header using HS256 algorithm
func generateHeader() (s string) {
  header := `{"alg": "HS256", "typ": "JWT"}`
  encoded := b64.StdEncoding.EncodeToString([]byte(header))
  return strings.TrimRight(encoded, "=")
}

// Return JWT payload with some some example data e.g. expiration and username
func generatePayload(username string) (s string) {
  exp := int(time.Now().Add(expThreshold).Unix())
  payload := `{"exp": "` + toString(expiration) + `", "username": "` + username + `"}`
  encoded := b64.StdEncoding.EncodeToString([]byte(payload))
  return strings.TrimRight(encoded, "=")
}

// Return JWT signature using SHA256 including salt
func generateSignature(secret string) (s string) {
  hasher := sha256.New()
  _, _ = hasher.Write([]byte(secret + salt))
  hash := base64.URLEncoding.EncodeToString(hasher.Sum(nil))
  return strings.TrimRight(hash, "=")
}

// Return complete JWT token with header, payload and signature
func generateToken(user User) (s string) {
  header := generateHeader()
  payload := generatePayload(user.Username)
  return header + "." + payload + "." + user.Password
}

// Checks whether received token is valid
func authenticateToken(token string) (b bool) {
  // Add padding for comparison
  components := strings.Split(token, ".")
  if i := len(components[1]) % 4; i != 0 {
    components[1] += strings.Repeat("=", 4-i)
  }

  // Decode payload data to get user info
  p, _ := b64.StdEncoding.DecodeString(components[1])
}