[Joe Forster/STA]

This is only another implementation of the Caesar cipher which is one of the earliest and most simplicistic ones. It can be deciphered easily for long encrypted messages (i.e. text files, sequences of letters), comparing the frequency of each numerical code to the frequency of each letter in the target alphabet (e.g. in English the most frequent letter is E, then come, I think, T and S etc.) to find out which code stands for which letter. But you're right, many codes for the same letter and short messages make the decryption harder. However, there are always geniuses who can crack such messages in surprisingly short times, with intuition, tricks, heuristics, trial and error and lots of calculations.

Actually, the Germans used a similar approach in the second world war but they assigned codes to words rather than letters. (The frequency is words is pretty much impossible to calculate...) Of course, the allies managed to find out lots of words but many others were still missing. At the end, they found a complete code book somewhere (on a U-boat?) that the Germans forgot to destroy. (There was an interesting documentary about this on National Geographic or Discovery Channel, try to find it.) Was this the famous Enigma?!