Breaking AES encryption using decrypted data

wvdschel, while I certainly wish you good luck, consider that if you solve this problem you'll be probably entitled to a Ph.D in computer science or mathematics. AES was designed to be extremely difficult to break (i.e. in the exponential order of the amount of bits) even if you know some minor details about the encrypted file.

Any attack that can lower the complexity from about 2 to the power of the bit-length of the key somewhat will be a great breakthrough. In the past, such attacks on DES (that merely lowered its strength by a few times) won their authors wide acclaim.

Read up on linear cryptanalysis of AES.

Think about this: If you could derive the password by just knowing the first cleartext-letters, how many encrypted messages would be worthless? How many letters/emails start with "Hello", how many of them have a standard (and known) signature (especially in companies). They would be all flawed. And in protocols you know a lot of cleartext-information, too. Encryption would be worthless.