func (cred Credentials) Equal(ccred Credentials) bool {
	if !ccred.IsValid() {
		return false
	}
	return (cred.AccessKey == ccred.AccessKey && subtle.ConstantTimeCompare([]byte(cred.SecretKey), []byte(ccred.SecretKey)) == 1 &&
		subtle.ConstantTimeCompare([]byte(cred.SessionToken), []byte(ccred.SessionToken)) == 1)
}

var timeSentinel = time.Unix(0, 0).UTC()

// ErrInvalidDuration invalid token expiry