Jacob Valenta - 3 years ago 179
Python Question

# Custom Python Encryption algorithm

Hey, I have been working on this for a while, and I can remebr my brother stepped me through this very same alogorithm.

Basicly, it just adds the ascii values of both the characters from the key, and the phrase.

I can encrypt it with this:

``````def encrypt(key, string):
encoded = ''
for i in range(len(string)):
key_c = ord(key[i % len(key)])
string_c = ord(string[i % len(string)])
encoded += chr((key_c + string_c) % 127)
return encoded
``````

But I can't seem to remember what we did as far as decrypting. Its difficult to revers a mod :P
Any ideas?

That's simple, let's see how it works. First of all, the encrypted message is obtained by subtracting key.

``````enc = msg + key (mod 127)
``````

How can we obtain the original message? That's easy, subtract key in both side

``````enc - key = msg + key - key (mod 127)
``````

And here we get:

``````enc - key = msg (mod 127)
``````

For more details, please reference to Modular arithmetic, I think it should belong one of group/field/ring. I'm not expert in math, for deeper theoretical knowledge, you can found them in Number theory. Here is the refined code:

``````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):
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__':
key = 'This_is_my_awsome_secret_key'
msg = 'Hello world'
encrypted = encrypt(key, msg)
decrypted = decrypt(key, encrypted)

print 'Message:', repr(msg)
print 'Key:', repr(key)
print 'Encrypted:', repr(encrypted)
print 'Decrypted:', repr(decrypted)
``````

Output

``````Message: 'Hello world'
Key: 'This_is_my_awsome_secret_key'
Encrypted: '\x1dNV`O\nkO`fD'
Decrypted: 'Hello world'
``````
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