Python/ciphers/rot13.py

from __future__ import print_function
def dencrypt(s, n):
    out = ''
    for c in s:
        if c >= 'A' and c <= 'Z':
            out += chr(ord('A') + (ord(c) - ord('A') + n) % 26)
        elif c >= 'a' and c <= 'z':
            out += chr(ord('a') + (ord(c) - ord('a') + n) % 26)
        else:
            out += c
    return out


def main():
    s0 = 'HELLO'

    s1 = dencrypt(s0, 13)
    print(s1)  # URYYB

    s2 = dencrypt(s1, 13)
    print(s2)  # HELLO


if __name__ == '__main__':
    main()