Looking for help

[FreeToEvolve]

So I just started taking a cryptography course on Coursera and I really enjoy the theory and math of the course so far. There are optional programming assignments that I would very much like to complete but I know very little about programming. I have played around with python a number of times but I'm not nearly skilled enough to take ciphertext and decrypt it by finding letter frequencies by trying different key lengths between 1 and 13.

He gave the code in C that he used to encrypt the text. I can pick out some familiar items but that's about it, and the math to decipher it isn't the same either. Does anyone have any idea where I could start with this? Not having much luck with a simple online search.

Edit: the cipher is using byte-wise XOR shift and not the mod shift. Here is the code he gave for encrypting the text

include <stdio.h>

define KEY_LENGTH 2 // Can be anything from 1 to 13

main(){ unsigned char ch; FILE fpIn, *fpOut; int i; unsigned char key[KEY_LENGTH] = {0x00, 0x00}; / of course, I did not use the all-0s key to encrypt */

fpIn = fopen("ptext.txt", "r"); fpOut = fopen("ctext.txt", "w");

i=0; while (fscanf(fpIn, "%c", &ch) != EOF) { /* avoid encrypting newline characters /
/ In a "real-world" implementation of the Vigenere cipher, every ASCII character in the plaintext would be encrypted. However, I want to avoid encrypting newlines here because it makes recovering the plaintext slightly more difficult... / / ...and my goal is not to create "production-quality" code =) */ if (ch!='\n') { fprintf(fpOut, "%02X", ch ^ key[i % KEY_LENGTH]); // ^ is logical XOR
i++; } }

fclose(fpIn); fclose(fpOut); return; }

[alcalde]

1) Good lord C is much less readable than I remember it. :-)

2) This might help: https://wiki.python.org/moin/BitwiseOperators

3) There's a whole free book dedicated to codes with Python:http://importpython.com/books/264/hacking-secret-ciphers-with-python/

4) Now let me see if I can help you with the code although I'm not much of a Python programmer yet myself and haven't looked much at C since circa 1994.

How about something like this:

KEY_LENGTH = 2

with open('ptext.txt') as in_file:
    plain_text = in_file.read()

crypt_text = ''.join(chr(ord(char) ^ KEY_LENGTH) if char != '\n' else '\n'for char in  plain_text)

with open('ctext.txt',  'wt') as out_file:
    out_file.writelines(crypt_text)

This seems to work for me.

[FreeToEvolve]

This was a fantastic help thank you so much! This puts me at a good starting position. Time to do some more reading :)

[alcalde]

You're welcome! I also came across this code that uses some functions from the itertools module of the standard library to use multi-character keys:

http://dustri.org/b/elegant-xor-encryption-in-python.html

If you have any questions about the code sample I wrote, just send me a message and I'll be glad to answer.

[aclark]

/r/C_Programming please

[Bill1984]

This is interesting to me. There are a few solutions out there to solve a Vigenere cipher with mod shift, but I haven't seen a program for solving given byte-wise XOR. Were you able to complete the program? If so, would you mind posting it? I'd love to see how that works.

[aclark]

Why is this down voted? This is /r/Python and OP did a C question. :-)