Everyone knows RSA , but everyone also knows that RSA is slow. Why not just use a faster operation than exponentiation?
I encrypted a public string and the flag with AES. There's no known key recovery attacks against AES, so you can't decrypt the flag.
Because of our revolutionary AES-128-OFB technology we have encrypted your user data so securely that even with the key (k=0x13371337133713371337133713371337) evil hackers can't read out the passwords!!!
I made a small, efficient block cipher. It's small because I can fit it on the page and it's efficient because it only uses the minimal amount of rounds. I didn't even try it but I'm sure it works. Here's some output with the key. Can you get the flag back out?
I heard that some common block cipher modes have lots of footguns - using none (ECB) results in the legendary ECB Penguin , while others are vulnerable to bit flipping and padding attacks, so I made my own that would never fall to such a technique.
I'm sure you'll never guess what comes next: Now do it without the key!