Wow, it’s been a year since my last post. Long time no see, so to speak.
Anyway, I’ve finally managed to (hopefully) get back into reversing and I want to start with something not too difficult to refresh myself. HackTheBox has always been a great place for a good challenge and it seems to still be that way.
The “crypt” part of the problem’s name is a bit of an exaggeration, but if you believe that xor and swap must be in any good crypto algorithm, then so be it. It all starts, of course, with good old xor: the checker binary “decrypts” file.bin using the complex ^0xAB method and executes the resulting binary.
The decrypted thing is also not very complicated but still does a little more than xor. The main goal of the whole algorithm is to transform the input string as follows:
- Swap eight characters in specific locations.
- Split the resulting string into two parts.
- Shuffle these parts and perform an
xoroperation on the six characters using different keys (2or3for the left and right parts respectively). - Combine the two resulting strings and compare them with the value of
RV{r15]_vcP3o]L_tazmfSTaa3s0. If they are equal, you have found the correct flag.
Given that this is a reversal exercise, you need to, well, reverse everything, including the steps you need to take to get the original flag out of that weird string. Specifically, you need to do something like this:
- Split the encrypted flag into two parts.
xorthe six characters in each part using the same keys, and shuffle the result using the reversed algorithm. “Reversed” in the sense that you need to do everything in reverse order, not that you need to fire up IDA or another disassembler.- Swap eight characters and combine the resulting strings.
The shuffle algorithm uses a hard-coded array containing the indices of the characters in the string that need to be swapped: [9, 12, 2, 10, 4, 1, 6, 3, 8, 5, 7, 11, 0, 13]. The algorithm is quite simple:
1 2 | for i in range(len(shuffle_array)): result[i] = flag[shuffle_array[i]] |
To reverse it, you would need the same array, but the values would be opposite, for example, 0 would be in ninth place, 1 in twelfth, etc.
Simple but cute for recharging forgotten skills.
The challenge can be found here.