Should users be allowed to reuse/recycle the same login credentials across a network for different systems? Should this be disallowed/discouraged, or are the security implications minimal? If it's frowned upon, should the usernames and passwords be unique, or is reusing usernames okay?

(This is an extension of an earlier question: Does username length/complexity/uniqueness positively impact security?)

You really can't, if you're just encrypting / decrypting text.

If you know that the encrypted string is "kdo" and the encryption method is a Caesar shift, the plaintext could just as easily be "IBM" as "HAL". You'd have to have some idea of what the plaintext "looks like". For instance, if you know the plaintext is the name of a Stanley Kubrick character, you'd have a decent idea of which one it should be.

If you have a longer string, it's much easier to narrow things down. A large text file has much fewer intelligible results than the three-character example above. But you'll still need to determine whether it's decrypted yourself.

On the other hand, if you're decrypting an entire file in some specific format (.docx, etc), you can be reasonably sure the file is decrypted if the parsing program (Word, etc) can read it.
