General cryptography question

[sec_ware]

Hi

Substitution ciphers _can_ be extremely secure. Actually, the only known
class of unbreakable ciphers is often categorised as a subsitution ciphers:
the one-time pad (or see also polyalphabetic substitutions).

The drawback certainly is that the key has to be quite long, has to be perfectly
random, and the pad should not be got lost


Modern ciphers, like AES, are block ciphers, which combine base elements,
such as substitution and transposition.

Substitution (S-box) is a mean to increase confusion, which is besides
diffusion, one of the main criteria to characterise cryptographic systems
(see Shannon[1]) in order to frustrate statistical analysis.



Oofki, I do have a question: how do you decrypt a ciphertext, which was encrypted with an irreversible algorithm?


Cheers

[1] http://netlab.cs.ucla.edu/wiki/files/shannon1949.pdf