Parallel Flagging

Description

The best way to encrypt a flag is to encrypt all of it at the same time, no?

Solution

By reversing the main function it can be seen that:

• the input is padded to 256 bytes with '-'
• is divided in blocks of 32 bytes
• each block is passed to a function "kernel" with an 8 bytes key taken from env

There is no need to reverse the function kernel since it's easy to understand that it's a xor + scramble by testing with different inputs. We can obtain the key knowing that the flag must start with snakeCTF.

Once we have the xor stream just xoring the output from output.txt gives us the flag.

Here is the solver code.