func (TokenAuth) GenSecret(uid types.Uid, expires time.Time) (string, error) {
// [8:UID][4:expires][32:signature] == 44 bytes
buf := new(bytes.Buffer)
uidbits, _ := uid.MarshalBinary()
binary.Write(buf, binary.LittleEndian, uidbits)
binary.Write(buf, binary.LittleEndian, uint32(expires.Unix()))
hasher := hmac.New(sha256.New, hmac_salt)
hasher.Write(buf.Bytes())
binary.Write(buf, binary.LittleEndian, hasher.Sum(nil))
return base64.URLEncoding.EncodeToString(buf.Bytes()), nil
}
