Download -> read file -> We've been provided with a java file
Upon some basic looking, we can see that the input has to be 18 chars - if (inp.length() != 18) { System.out.println("Input is incorrect") } .
We can also see - if (inp.equals("inagzgkpm)Wl&Tg&io")) { System.out.println("Input if the flag") } . So we know our input, and that string is 18 chars. Perfect.
We also see that - inp = shift( shift(inp) ) , so now time for the reversing.
All shift() does is gets a character in a string and then minuses i from that, and then stores it in a ret variable. So doing a bit of changing a minus to a plus, we manage to get a new string from that - "iocj~lqwu2aw2au5y\x80" (\x80 isn't an ascii char) - We can remove that \x80, as we know that it's going to be }.
I couldn't code this in python, so I turned to a online java compiler, and performed shift2() on that string, but again, taking a plus as a minus for reversing it.
That should return the flag, and you just pad it with the } that we took out earlier (\x80)
Python rev for shift
Java rev for shift2
Let's look at the source code. It gets an input, and checks if it's length is 23 first of all. If not, it automatically tells you your input is wrong.
Then, it goes in a for loop. This loop will iterate 3 times. Each time, it sets inp(variable containing the input) to transpose(inp), and then to xor(inp)
Essentially, 3 times, it transposes then xors the input.
the xor is self explanatory, it xors the input with a key.
The transposing is a little different. It has a list of numbers, then does the equivalent of this in java(if the list of numbers was called nums)
return ''.join(inp[i] for i in nums)
We can reverse this relatively simply with a python script, and get the flag.
There's a text file IceCreamManual.txt. It's actual contents aren't important, more what happens to it. The program reads all bytes from this file, then calls fillMachine on it. fillMachine basically grabs specific indexes of the file and puts them together into a byte string. I recreated this function in python and ran it to get the string lollookatthistextigotfromthemanual. The code calls 4 functions on our input, then compares the final result to that string. So we must reverse from that string back to the flag.
The 4 functions it calls, in order, are strawberry shuffle -> vanilla shuffle -> chocolate shuffle -> toppings
We'll work in reverse, going back one step each time to get the original input.
First things first: toppings. Toppings basically takes an array of bytes(bytes can be negative in java! It's weird, but largely unimportant) and every byte of the output is the respective byte of the input + the respective byte of the toppings byte array. We can simply subtract the values.
Next: chocolate shuffle
This code is a bit more complicated. For every index in the output: if an even index, AND index is > 0, set output[i] = input[i - 2] if an even index, and index IS 0, set output[i] = input[33]
if an odd index, AND index is < 32, output[i] = input[i + 2] if an odd index, and index >= 32, output[i] = input[1]
We can basically recreate the loop in python and swap around the indexes, e.g
input[i + 2] = output[i]
to reverse this step.
Next: vanilla shuffle.
This is simple. For every index in the input - if the index is even, add 1 to the byte value. If the index is odd, subtract 1. Again, easily reversible with a simple loop.
Finally, strawberry shuffle. The final thing to reverse to get the flag. The code looks a bit weird at first, but in reality all it does is reverse the string.
If we string all these things together, we get the flag. Script below.