picoCTF Transformation

A Reverse a day keeps the zero-days away

Problem

Problem link

First gives a file, looks like this:

灩捯䍔䙻ㄶ形楴獟楮獴㌴摟潦弸彥㜰㍢㐸㙽

.join([chr((ord(flag[i]) << 8) + ord(flag[i + 1])) for i in range(0, len(flag), 2)])

Obviously encoded wrong, that’s why appears gibberish. Just decode back, and it’s fine. Below is encoding program attached to problem description. It first splits entire flag into pairs, converts both to ASCII, multiplies first by 256 then adds second. Finally chr() becomes what we see.

Solution

Just write a program to reverse — problem uses python, so use python:

q = "灩捯䍔䙻ㄶ形楴獟楮獴㌴摟潦弸彥㜰㍢㐸㙽"
sum = []

# Convert encoding back to ASCII
for i in range(len(q)):
    sum.append(ord(q[i]))

# Brute force (since ASCII max is 126)
for i in range(len(sum)):
    for j in range(126):
        if (sum[i] - (j << 8)) <= 126 and (sum[i] - (j << 8)) >= 0:
            print(chr(j) + chr(sum[i] - (j << 8)) ,end = "")

After solving, get flag: picoCTF{16_bits_inst34d_of_8_e703b486}