Source code for pcr.rc4

# Copyright (c) 2013 Stefano Palazzo <stefano.palazzo@gmail.com>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

''' RC4 stream cipher '''


def key_schedule(key):
    s, j = list(range(256)), 0
    for i in range(256):
        j = (j + s[i] + key[i % len(key)]) % 256
        s[i], s[j] = s[j], s[i]
    return s


def key_stream(s):
    i, j = 0, 0
    while True:
        i = (i + 1) % 256
        j = (j + s[i]) % 256
        s[i], s[j] = s[j], s[i]
        yield s[(s[i] + s[j]) % 256]