caesar_cipher_2

caesar cipher 2

Cryptography - 250 points

Challenge

Can you help us decrypt this message? We believe it is a form of a caesar cipher. You can find the ciphertext in /problems/caesar-cipher-2_2_d9c42f8026f320079f3d4fcbaa410615 on the shell server.

Hint

You'll have figure out the correct alphabet that was used to encrypt the ciphertext from the ascii character set

ASCII Table

Solution

A hard challenge to understand, but simple to solve once I figured it out...

Ciphertext

PICO#4&[C!ESA2?#I0H%R3?JU34?A2%N4?S%C5R%]

From the prefix of picoCTF{, we know that

P --> p
# --> C
4 --> T
& --> F
[ --> {

With this, looking at the ASCII table, all the hex codes will be shifted up by 0x20

$ python3
>>> ctext = list("PICO#4&[C!ESA2?#I0H%R3?JU34?A2%N4?S%C5R%]")
>>> ctext = list(map(lambda x: ord(x), ctext))

>>> text = list(map(lambda x: x + 0x20, ctext))
>>> ''.join(map(lambda x: chr(x), text))
'picoCTF{cAesaR_CiPhErS_juST_aREnT_sEcUrE}'

Flag

picoCTF{cAesaR_CiPhErS_juST_aREnT_sEcUrE}