This guess guess 123 game is also known as Cows and Bulls or Mastermind. It is a very old game which is starts from UK in middle of 20th century. You can choose how many digits you want to guess and whether it is allowed to have repeated digits.

For example:
The generated secret number is "5143". The first attempt to guess the number was "5216". The result of this first attempt will be "1A1B". The "5" is in correct position and this is "1A", the "1" in guess number matches the second digit "1" in secret number, but it is "1B".

You have ten time attempts and get all digits correct to beat the boss.