picoCTF 2025 – Cryptography • Easy
We can observe that the message starts with 4 lowercases letter followed by three upper cases letters, and there is a string in between curly braces.
cvpbPGS{abg_gbb_onq_bs_n_ceboyrz}This looks like the pattern of a flag but with the letters substitued. It is in fact a substitution cipher.
We observe that each letter of the string 'cvpbPGS' has distance 13 from the letter in the same position of the string 'picoCTF'.
From this we deduce that this is a shift cipher with key = 13. We apply the inverse shift to all the message to recover the flag.
A shift cipher that has 'k = 13' is called ROT13. The peculiarity of this cipher is that the encryption and decryption function are identical as applying two shift of 13 positions is equivalent to apply a shift of 26 position which is no shift at all.