tamuctf-crypto3.md

Challenge name: :)

Difficulty: easy

Description:

Look at what I found! XUBdTFdScw5XCVRGTglJXEpMSFpOQE5AVVxJBRpLT10aYBpIVwlbCVZATl1WTBpaTkBOQFVcSQdH

Solution:

What we see is like base64, so we decode it.

import base64

c = 'XUBdTFdScw5XCVRGTglJXEpMSFpOQE5AVVxJBRpLT10aYBpIVwlbCVZATl1WTBpaTkBOQFVcSQdH'
c = base64.b64decode(c)

Output

']@]LWRs\x0eW\tTFN\tI\\JLHZN@N@U\\I\x05\x1aKO]\x1a`\x1aHW\t[\tV@N]VL\x1aZN@N@U\\I\x07G'

So we got something meaningless. Here we need an encryption system that change hidden characters to word. The difficulty is easy, so it should be an easy one. I guess it's a repeating-XOR. We know flag starts with 'gigem{', so lets XOR 6 starting bytes with this string.

def xorbyte(s, key):
    key = key * (len(s) // len(key) + 1)
    output = ''
    for i in range(len(s)):
        output += chr(ord(s[i]) ^ ord(key[i]))
    return output
    
xorbyte(c[:6], 'gigem{'

Output

':):):)'

Repeats of ':)', it seems we got the key! Lets XOR all bytes to ':)'.

xorbyte(c, ':)')

Output

"gigem{I'm not superstitious, but I am a little stitious.}"