i want to encrypt transparently using Rijndael. So this is, what I thought of and I would like to have an opinion from "Experts" whether this will harm encryption strenght.
edit: The 1MB chunks are just out of performance reasons. Imagine a file of 500MB to decrypt, deflate change some bytes, compress again and encrypt again. Is it really that bad a security vulnerability?
edit: The IV is never stored, but always created anew from the GUID and the Key. As far as I understand, this is the IV only for the first cipher block in CBC. (GUID XOR Key) XOR Plaintext -> ciphering -> new IV for next block.
Is it possible to deduce the IV from ciphered data, without knowing the key? If so, this IS a bad idea. Otherwise I dont see the the disadvantage.
Would this be viewed more secure? Cipher the GUID with the key and an IV of Zeros and use this as the IV for my ciphering.
Am I doing something else terribly wrong?
Are there are other obvious weak spots that I am missing?
Thanks a lot!