Mod 26

We are told that the flag is encrypted with ROT13, which is a simple substitution cipher that replaces every character with the character that is 13 spaces along the alphabet. For example, the character C would be replaced by a P:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

You can see that C is the 3rd index, and P is in fact the 16th. But what if we want to encrypt the letter Y, at index 25? Well, what we do here is we loop back to the beginning; if we do this, the character 13 positions after it is in fact L!

Mathematically, we can see that the index that would be position 26 is actually looping back to position 0, so we add on the 13 and take the remainder modulo 26. We can do this easily in Python, ignoring non-letter characters:

from string import ascii_lowercase, ascii_uppercase

enc_flag = r"cvpbPGS{arkg_gvzr_V'yy_gel_2_ebhaqf_bs_ebg13_uJdSftmh}"

flag = ""

for c in enc_flag:
    if c in ascii_lowercase:
        flag += ascii_lowercase[(ascii_lowercase.index(c) + 13) % 26]
    elif c in ascii_uppercase:
        flag += ascii_uppercase[(ascii_uppercase.index(c) + 13) % 26]
    else:
        flag += c

print(flag)

# picoCTF{next_time_I'll_try_2_rounds_of_rot13_hWqFsgzu}