Quote Originally Posted by 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
Why would they need to be perfectly random? If you used something like a book for a key, how would that be worse than using the same length string of random characters? The chance of someone guessing that you used that book as a key would be fairly slim, so i wouldn't even consider that as a factor in the security, unless it was an extremely common book that has something to do with the contents or something in that nature. And even if they did guess it, they'd still need the exact same document, with all the same spaces and everything.