cifra.py

def cifra(a, b, key, num_bytes=2):
    assert len(a) == len(b)

    l = len(a)

    #padding
    if len(a)%num_bytes:
        a += chr(0)*((-len(a))%num_bytes)
        b += chr(0)*((-len(b))%num_bytes)

    assert len(a)%num_bytes == 0
    assert len(b)%num_bytes == 0

    a_chunks = [a[x:x+num_bytes] for x in range(0,len(a),num_bytes)]
    b_chunks = [b[x:x+num_bytes] for x in range(0,len(b),num_bytes)]

    ret = ""
    for pos in range(len(a_chunks)):
        a_chunk_n = str2n(a_chunks[pos])
        b_chunk_n = str2n(b_chunks[pos])
        n = (256**num_bytes - 1) & ((a_chunk_n^b_chunk_n) + key*(pos+1))
        ns = n2str(n, num_bytes)
        ret += ns

    #quita padding
    ret = ret[:l]

    return ret