When using AES (or probably most any cipher), it is bad practice to reuse an initialization vector (IV) for a given key. For example, suppose I encrypt a chunk of data with a given IV using cipher block chaining (CBC) mode. For the next chunk of data, the IV should be changed (e.g., the nonce might be incremented or something). I'm wondering, though, (and mostly out of curiosity) how much of a security risk it is if the same IV is used if it can be guaranteed that the first four bytes of the chunks are incrementing. In other words, suppose two chunks of data to be encrypted are:
0x00000000someotherdatafollowsforsomenumberofblocks
0x00000001someotherdatathatmaydifferormaynotfollows
If the same IV is used for both chunks of data, how much information would be leaked?
In this particular case, it's probably OK (but don't do it, anyway). The "effective IV" is your first encrypted block, which is guaranteed to be different for each message (as long as the nonce truly never repeats under the same key), because the block cipher operation is a bijection. It's also not predictable, as long as you change the key at the same time as you change the "IV", since even with fully predictable input the attacker should not be able to predict the output of the block cipher (block cipher behaves as a pseudo-random function).
It is, however, very fragile. Someone who is maintaining this protocol long after you've moved on to greener pastures might well not understand that the security depends heavily on that non-repeating nonce, and could "optimise" it out. Is sending that single extra block each message for a real IV really an overhead you can't afford?