Key Space Reduction Using Isomorphs

Block ciphers are said to have key spaces that are large enough to prevent a brute force attack from breaking them within the lifetime of the attacker; however, messages obscured using those ciphers are regularly broken. Some are broken because of increased computer capabilities, while others are broken because of industry reliance on invalid assumptions. These assumptions include the idea that all encrypted blocks of text are equally likely to appear in a message and that only the original key can decrypt a message. In this paper, we show that information theory techniques using isomorphs can reduce the key space to a size that makes the subsequent brute force attack possible in a shorter and easily achievable time frame.