mceliece reed-solomon

gistfile1.txt

def get_a(alpha, G, j):
    i = GF.fetch_int(1)
    k = G.nrows()
    if k <= j:
        return i / (i - (i/alpha) * G[0][j]/G[1][j] )
    else:
        return i / (i - (i/alpha) * (G[j][k+1]/G[1][k+1] - G[j][k]/G[1][k]) \
                                  / (G[j][k+1]/G[0][k+1] - G[j][k]/G[0][k]))

def crack(A):
    A = vector(A)
    C = codes.GeneralizedReedSolomonCode(A, k)
    D = codes.decoders.GRSBerlekampWelchDecoder(C)
    return D.decode_to_code(ciphertext)

pub = load('publickey.sobj')
ciphertext = load('ciphertext.sobj')

pubsys = pub.echelon_form()
GF = parent(pub[0][0])
n = pubsys.ncols()
k = pubsys.nrows()

for alpha in range(0, 256):
    if alpha == pub[1][j]/pub[0][j]:
        continue
    try:
        A = [0, 1] + [get_a(GF.fetch_int(alpha), pubsys, j) for j in range(2, n)]
        cc = crack(A)
        m = pub.solve_left(cc)
        print ''.join(chr(mc.integer_representation()) for mc in m)
        break
    except Exception as e:
        print e
        pass