Comment Re:How do you know when it's decrypted? (Score 1) 186
And encrypting multiple time with the same key will, for any reasonably secure crypto system*, not increase security. I understand that from a theoretical point of view, but from a practical point of view -- how would you break an encrypted file if it is doubly encrypted, even if you knew both algorithms involved. How do you solve the problem of recognizing if you'd actually decrypted with the first key, so that you can start working with the second key?? Haven't you increased the key-space to an exponent of itself (in practical terms), and therefore created something vastly more secure?
Multiply encrypting will only increase your security if there isn't some other key that you could have used that yields the same results. For example, if we use "E(M,K)" means "Encrypt message M with key K" then encrypting twice would be E(E(M,K1),K2). The problem is, if there is some OTHER key K3 such that E(M,K3) = E(E(M,K1),K2), then you really haven't added any security by doing that.
Obligatory disclaimer: everything I know about cryptography I learned from reading Bruce Schneier's books, and they're all in storage at the moment