crypto.py 文件源码

def _nfold(str, nbytes):
    # Convert str to a string of length nbytes using the RFC 3961 nfold
    # operation.

    # Rotate the bytes in str to the right by nbits bits.
    def rotate_right(str, nbits):
        nbytes, remain = (nbits//8) % len(str), nbits % 8
        return ''.join(chr((ord(str[i-nbytes]) >> remain) |
                           ((ord(str[i-nbytes-1]) << (8-remain)) & 0xff))
                       for i in xrange(len(str)))

    # Add equal-length strings together with end-around carry.
    def add_ones_complement(str1, str2):
        n = len(str1)
        v = [ord(a) + ord(b) for a, b in zip(str1, str2)]
        # Propagate carry bits to the left until there aren't any left.
        while any(x & ~0xff for x in v):
            v = [(v[i-n+1]>>8) + (v[i]&0xff) for i in xrange(n)]
        return ''.join(chr(x) for x in v)

    # Concatenate copies of str to produce the least common multiple
    # of len(str) and nbytes, rotating each copy of str to the right
    # by 13 bits times its list position.  Decompose the concatenation
    # into slices of length nbytes, and add them together as
    # big-endian ones' complement integers.
    slen = len(str)
    lcm = nbytes * slen / gcd(nbytes, slen)
    bigstr = ''.join((rotate_right(str, 13 * i) for i in xrange(lcm / slen)))
    slices = (bigstr[p:p+nbytes] for p in xrange(0, lcm, nbytes))
    return reduce(add_ones_complement, slices)