Reversing encryption algorithm in Python

Question

Hello guys i'm having some trouble with this script. The script is supposed to basically prompt for an input and then encrypt whatever the user has input, the user can then go back and decrypt the message by typing in the encrypted message. The encryption part of the script works fine, it's just the decryption part that's giving me problems. Here is the code and ofcourse thanks in advance :D

def encrypt(key, msg):
    encryped = []
    for i, c in enumerate(msg):
        key_c = ord(key[i % len(key)])
        msg_c = ord(c)
        encryped.append(chr((msg_c + key_c) % 127))
    return ''.join(encryped)

def decrypt(key, encryped):
    msg2 = []
    for i, c in enumerate(encryped):
        key_c = ord(key[i % len(key)])
        enc_c = ord(c)
        msg.append(chr((enc_c - key_c) % 127))
    return ''.join(msg)

welcome = str(input("Press 1 to encrypt or anything else to decrypt: "))
if welcome in ['1', '1']:
    var = input("Please type in what you want to encrypt: ")
    if __name__ == '__main__':
        key = 'This is the key'
        msg = var
        encrypted = encrypt(key, msg)
        decrypted = decrypt(key, encrypted)
    print ('Please take down the key and encryption message to decrypt this message at a later stage')
    print ('This is what you want to encrypt:' , repr(msg))
    print ('This is the encryption/decryption key:', repr(key))
    print ('This is the encrypted message:', repr(encrypted))

else:
    var2 = input("Please enter your encrypted message that you would like to decrypt: ")
    if __name__ == '__main__':
        key = 'This is the key'
        msg = var2
        decrypted = decrypt(key, var2)
    print ('This is the decrypted message:', repr(decrypted))

This is what 'Hello World' looks like when encrypted: "\x1dNV`O\nkO`fD" — AbdulNaji, Dec 16 '13 at 03:03

score 0 · Answer 1 · edited May 23 '17 at 12:05

A bit simplified version:

def encrypt(key, msg):
    encrypted = []
    for i, c in enumerate(msg):
        key_c = ord(key[i % len(key)])
        msg_c = ord(c)
        encrypted.append(chr((msg_c + key_c) % 127))
    return ''.join(encrypted)

def decrypt(key, encryped):
    msg = []
    for i, c in enumerate(encryped):
        key_c = ord(key[i % len(key)])
        enc_c = ord(c)
        msg.append(chr((enc_c - key_c) % 127))
    return ''.join(msg)

if __name__ == '__main__':
    welcome = input('Press 1 to encrypt or anything else to decrypt: ')
    key = 'This is the key'
    if welcome == '1':
        msg = input('Please type in what you want to encrypt: ')
        encrypted = encrypt(key, msg)
        print('Please take down the key and encryption message to decrypt this message at a later stage')
        print('This is what you want to encrypt:', repr(msg))
        print('This is the encryption/decryption key:', repr(key))
        print('This is the encrypted message:', repr(encrypted))
    else:
        msg = input('Please enter your encrypted message that you would like to decrypt: ')
        msg = msg.encode('utf-8').decode('unicode_escape')
        decrypted = decrypt(key, msg)
        print('This is the decrypted message:', repr(decrypted))

Because your encrypted text contains the first 32 ASCII characters as escaped hex literals, on decryption input they are not interpreted as such. To process the escaped literals, msg.encode("utf-8").decode("unicode_escape") can be used (as suggested here). You might as well encode using ascii as your encrypted text contains only ASCII characters up to 127.

Reversing encryption algorithm in Python

1 Answers1