 
     /// Generates a one-time password using the HOTP algorithm.
     // https://tools.ietf.org/html/rfc4226#section-5
-    internal static func generatePassword(algorithm algorithm: Algorithm, digits: Int, secret: NSData, counter: UInt64) throws -> String {
+    @warn_unused_result
+    private static func generatePassword(algorithm algorithm: Algorithm, digits: Int, secret: NSData, counter: UInt64) throws -> String {
         guard validateDigits(digits) else {
             throw Error.InvalidDigits
         }

    }

    @warn_unused_result
    internal static func validatePeriod(period: NSTimeInterval) -> Bool {
        // The period must be positive and non-zero to produce a valid counter value.
        return (period > 0)
    }

    @warn_unused_result
    internal static func validateTime(timeInterval: NSTimeInterval) -> Bool {
        // The time interval must be positive to produce a valid counter value.
        return (timeInterval >= 0)
    }

    }

    @warn_unused_result
    private static func validateFactor(factor: Factor) -> Bool {
        switch factor {
        case .Counter:
            return true
