Encrypt a lowercase string into another lowercase string with no spaces

encrypt_a_lowercase_string.py

#!/usr/bin/env python
"""
encrypt and descript lowercase strings without using
but lowercase strings
"""
from string import lowercase
from random import choice

MAX_GARBAGE_LENGTH = 5


def encrypt(string):
    offset = len(string) % 26
    chars = []
    # Get an offset based on the string length
    for char in string:
        modified = ord(char) - offset
        if modified < 97:
            modified += 26
        # Then get the inverse of the character set
        modified = 97 * 2 - modified
        if modified < 97:
            modified += 26
        chars.append(chr(modified))
    # Reverse the string to make things worse
    chars.reverse()
    shifted_and_inverted = ''.join(chars)
    # Throw in garbage so that no sane person could
    # deduce what's going on
    messy = []
    for char in shifted_and_inverted:
        garbage_header = choice(lowercase)
        messy.append(garbage_header)
        for _ in range(ord(garbage_header) % MAX_GARBAGE_LENGTH):
            messy.append(choice(lowercase))
        messy.append(char)
    return ''.join(messy)


def decrypt(string):

    # Remove the garbage
    good_chars = []
    next_bad_char = 0
    next_good_char = 0
    for i, char in enumerate(string):
        if next_bad_char == i:
            bad_chars = ord(char) % MAX_GARBAGE_LENGTH
            next_good_char = i + 1 + bad_chars
            next_bad_char = i + 2 + bad_chars
        if next_good_char == i:
            good_chars.append(char)
    # Reverse the chars
    good_chars.reverse()
    # Now invert
    chars = []
    for char in good_chars:
        inverted = 97 * 2 - ord(char)
        if inverted < 97:
            inverted += 26
        chars.append(chr(inverted))
    # Now correct for the offset
    offset = len(chars) % 26
    for i, char in enumerate(chars):
        modified = ord(char) + offset
        if modified > 122:
            modified -= 26
        chars[i] = chr(modified)
    return ''.join(chars)


def main():
    # text = 'davidmortonisonecrazyprogrammingfiend'
    text = lowercase
    encrypted = encrypt(text)
    print text
    print encrypted
    print decrypt(encrypted)
    print len(text)
    

if __name__ == '__main__':
    main()