Question

Imagine I have this:

$cdata = AES_256($data, $pass);

AES_256 implements the AES algorithm.

If I know the content of $cdata and the content of $data and also have
the AES_256() code, can I reverse engineer and find $pass?

Answer 1

Simple answer: NO.

This has been tested, and mentioned in the Wiki link.

A related-key attack can break up to 9
rounds of 256-bit AES. A
chosen-plaintext attack can break 8
rounds of 192- and 256-bit AES, and 7
rounds of 128-bit AES, although the
workload is impractical at 2¹²⁸ –
2¹¹⁹.

Or put it another way: you have a better chance of being struck by lighting… on the same day you win the Lottery, than breaking it!