HideToSee

How about some hide and seek heh? Look at this image here.

Not the most enjoyable challenge. Gives us an image called atbash.jpg, but no ciphertext yet. We actually have to use steganography techniques to extract the ciphertext from being embedded in the image, using steghide:

$ steghide extract -sf atbash.jpg

The passphrase is empty. The encrypted.txt file that is created has the following:

krxlXGU{zgyzhs_xizxp_8z0uvwwx}

Based off the filename, we can assume it's an atbash cipher, which is essentially a transposition cipher where alphabet is flipped (so A goes to Z, B goes to Y, etc).

from string import ascii_uppercase, ascii_lowercase

enc = 'krxlXGU{zgyzhs_xizxp_8z0uvwwx}'
dec = ''

for c in enc:
    if c in ascii_uppercase:
        dec += ascii_uppercase[-(ascii_uppercase.index(c)+1)]       # so index 0 transposes to -1, index 1 to -2, etc
    elif c in ascii_lowercase:
        dec += ascii_lowercase[-(ascii_lowercase.index(c)+1)]
    else:
        dec += c

print(dec)

# picoCTF{atbash_crack_8a0feddc}