The inner workings of the cryptosystems are completely known by the attacker, the only secret is the key.
Classic crypto:
 | Simple Substitution Cipher |
| Double Transposition Cipher - Changing the rows |
| One-Time Pad - The key is chosen at random, same length as the message, the key can only be used once. STREAM CIPHER |
| Project Venona (one time pad, can be an error in the method generating the pads) |
| Codebook Cipher - Words and their corresponding codewords. BLOCK CIPHER |
1929: Gentlemen do not read each other`s mail
Confusion: (simple substitution and a one-time pad)
Obscure the relationship between the plaintext and the ciphertext.
Diffusion: (double transposition)
Supposed to spread the plaintext statistics through the ciphertext.
Diffusion alone is not enough to be probable secure. One-time pad has both. If you use codebook and permutation then you use both.
Stream cipher:
Better in hardware
Block cipher:
Better in software