rsa.py

#!/usr/bin/env python

import gmpy
# see http://stackoverflow.com/a/4801358/2377489
p = 43
q = 59
d = 937
plaintext = 134879475204

if __name__ == '__main__':

    n = p * q
    phi_n = (p - 1)*(q - 1)

    e = gmpy.invert(d, phi_n)

    i = 0
    # determine index i
    while 10**i < n:
        i += 1
    # set length of each block
    block_len  = i - 1
    # get the number of blocks
    num_blocks = len(str(plaintext)) / block_len
    # split plaintext
    parts = []
    for j in xrange(num_blocks):
        # starting index
        start = block_len * j
        # ending index
        end   = block_len * (j + 1)
        parts.append(int(str(plaintext)[start:end]))

    # encryption
    codetext = []
    for part in parts:
        # encrypt each part separately
        codeword = (part ** e) % n
        codetext.append(codeword)
    # merge codewords together
    codetext = ''.join([str(x) for x in codetext])
    # show codetext
    print codetext