Breaking AES encryption using decrypted data
aes, encryption
Solution
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.
Problem
After a discussion about encryption, a friend of mine challenged me to crack a file he encrypted using AES with a 128bit key. I know the file was originally a GIF image, so it should start with 'GIF8'. I'm wondering if it is possible to derive the password from this knowledge in a reasonable time (ie. a week or less). Stealing the key in any way other than analyzing the encrypted file is not possible, as it defeats the point of the challenge. If so, pointers would be welcome. I failed to find a decent flow-chart-like description of how the encryption of the first block works. I remember I had one from a course at Uni, but of course, it's nowhere to be found.