When Is A Block Cipher Not A Block Cipher?
Ask most people about whether AES is a block cipher or a stream cipher, and they will typically say that it’s a block cipher. But that’s not quite true, as some of the AES modes implement with a 128-bit block, while other methods implement a stream cipher. The block cipher modes are then ECB (Electronic Code Book) and CBC (Cipher Block Chaining). These modes then require padding, in order to make sure that we fill up all the blocks. A standard padding method is CMS (Cryptographic Message Syntax), and which fill the last block with the value for the number of missing bytes. For example, if we have “hello”, we have:
68656c6c6f0b0b0b0b0b0b0b0b0b0b0b
[https://repl.it/@billbuchanan/padding]
and where 0x68 is an ‘a’. With ECB, we do not use salt, so we get the same cipher for the same plaintext and the same key:
The other method we have for a block cipher is CBC (Cipher Block Chaining) and which adds a 64-bit salt value to the cipher process. We then store the salt with the cipher so that it can be used in the decryption process:
