crackme-py

encode_decode_rot47.py

#!/bin/python3
'''Encode or decode ROT47'''
# Any spaces are left unchanged.


def rot47(string):
    '''Encode/decode string.'''
    enc_dec = ''  # string encoded/decoded to/from ROT47
    for char in string:
        ascii_code = ord(char)  # convert character to ASCII code number
        if 33 <= ascii_code <= 79:
            enc_dec += chr(ascii_code + 47)
        elif 80 <= ascii_code <= 126:
            enc_dec += chr(ascii_code - 47)
        else:
            enc_dec += char
    return enc_dec


if __name__ == '__main__':
    print(rot47(input('Enter string: ')))

picoCTF: crackme-py

Get Python Script

$ curl https://mercury.picoctf.net/static/be2ba466c6154e42c756bf737ddcecc3/crackme.py -O

Decode a Variable

In order to find the picoCTF flag, decode the value of the bezos_cc_secret from crackme.py using the ROT47 cipher.

Encoder/Decoder Script

rot47(s) both encodes and decodes ROT47, while any spaces will be left unchanged.

def rot47(string):
    enc_dec = ''
    for char in string:
        ascii_code = ord(char)
        if 33 <= ascii_code <= 79:
            enc_dec += chr(ascii_code + 47)
        elif 80 <= ascii_code <= 126:
            enc_dec += chr(ascii_code - 47)
        else:
            enc_dec += char
    return enc_dec


if __name__ == '__main__':
    print(rot47(input('Enter string: ')))

$ python3 encode_decode_rot47.py
Enter string: A:4@r%uL`M-^M0c0AbcM-MFE0cdhb52g2N
picoCTF{1|\/|_4_p34|\|ut_4593da8a}

Simplified Chained Comparisons

pylint suggests the use of arbitrarily chained comparisons.

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).

33 <= ascii_code <= 79
80 <= ascii_code <= 126

References

• nick3499/encode_decode_rot47.py
• ROT-47 Cipher
• ASCII Code - The extended ASCII table
• 6.10. Comparisons