So I'm trying to decrypt AES encryption with a 32-byte key. I know the first 7 bytes, and the final 19. I also have the plain text, as well as the IV and the encrypted text.
I'm currently running python code to try the combinations (for now I'm trying all the combinations of lower case letters), but I was wondering if there was the best way to approach this, instead of just brute-forcing all the combinations, because that would take a long time.
Knowing some bytes of AES's uniform random key doesn't help you recover faster on brute force. Otherwise, we would hear that years ago. So you have $7+19= 26$ bytes for AES-256 which needs 32 bytes for the key. Therefore there are $6$ bytes of the key is missing, equivalently 48 bits of the key are missing.
because that would take a long time.
Not on modern CPUs. For example; on my 7Gen i7, OpenSSL can execute
$openssl speed -evp aes-256-cbc
Doing aes-256-ecb for 3s on 16 size blocks: 119128871 aes-256-cbc's in 3.00s
This means that AES-256-NI runs with 16-byte inputs for 3 seconds. This makes $41227971$ runs on 1 second. AES in ECB has 16-byte input to process. Therefore in one second, we have $41227971 \approx 2^{25.3}$ AES calculations. This means that
| second | minute | hour | day |
|---|---|---|---|
| $\approx 2^{25.24}$ | $\approx 2^{31.14}$ | $\approx 2^{37.05}$ | $\approx 2^{41.64}$ |
This is single-core performance and needs around 84 days to finish. If you have multiple cores ( as most modern CPUs) you can use OpenMP to parallelize your search. My CPU has 8 cores, so a maximum of 8 speed-up is possible to reduce 10 days. If you have more than one computer this can be easily reduced to one day ( all you need is 84 cores to combine in OpenMP)
You need to test it before the long run. Make sure that it finds the key. You may find more than one key for a single known-plaintext pair. Make sure that you run all the space to find all of them.
And you may need your friend CPUs for some time to speed up. OpenMP is great for dynamic grow/shrink of the clients...
Note that if you don' use AES-NI, then you can have lower performance as 13356785 encryption per second. That is approximately $2.75$ times slower.
To see your CPUs performance use;
#For AES-NI
$openssl speed -evp aes-256-cbc
#For software OpenSSL AES
OPENSSL_ia32cap="~0x200000200000000" openssl speed -elapsed -evp aes-256-cbc
My CPU is listed as 41st on the OpenSSL speed pages. The top is 20 times faster than mine.
As noted by poncho, if we have some information about the key like they are lowercase ASCII then you need to search around $2^{28.2}$ key space that can be achieved in one minute on a single core.
