Question

If I am using a block cipher such as AES which has a block size of 128 bits, what do I do if my data is not an even multiple of 128 bits? I am working with packets of data and do not want to change the size of my packet when encrypting it, yet my data is not an even multiple of 128?

Does the AES block cipher allow handling of a final block that is short without changing the size of my message once encrypted?

Answer 1

That kind of detail depends on the chaining mode which you use. The chaining mode is what defines how many times you invoke the AES primitive. and on what, for a given input message. The simplest chaining mode consists in simply splitting the input data into successive 16-byte blocks and encrypting each of them independently; this is called ECB (as “Electronic Code Book”) and it is known to have weaknesses (namely, if two input blocks are identical, something which is frequent in “real life” data, then the two corresponding output blocks will be equal to each other as well, and anybody can see that).

Some chaining modes enlarge the data, i.e. the encrypted message will be slightly larger than the input message. Other chaining modes (e.g. CTR) do not. Almost all secure chaining modes require handling an “initial value”, which is a piece of data (usually the same size than a block) which needs not be secret, but must be known to both the sender and the receiver, and must be distinct for each message. Some modes (e.g. CBC) require a uniformly random IV, whereas some other modes will be happy with a simple counter. It is customary to send the IV along with the encrypted message. You could also derive the IV from the secret key itself with a hash function.

These things are tricky, and it is difficult to know whether you did it right: security cannot be tested; a weak cryptosystem compiles and runs just like any other application. Designing your own cryptographic protocol is not recommended. At all. Employing robust primitives is no guarantee that the result will be secure.