Encryption with Python?

[Robert Kern]

Anthra Norell wrote:
> I rolled my own for relatively short sequences, like passwords. The key is
> an integer. To decrypt use the negative encryption key. I consider the
> encryption unbreakable, as it is indistinguishable from a random sequence.
> 
> Frederic
> 
> ###
> 
> def crypt (sequence, key):
>    import random
>    sign = (key > 0) * 2 - 1
>    random.seed (abs (key * sign))
>    s = ''
>    for i in xrange (len (sequence)):
>       r = random.randint (0, 255)
>       s += chr ((ord (sequence [i]) + r * sign) % 256)
>    return s

The mind boggles.

You do realize that if I have two ciphertexts encrypted with the same 
key, I can subtract them? Then I have a sequence, that while not 
immediately readable, is just a straightforward combination of the two 
plaintexts without any encryption.

This function is also vulnerable to a chosen-plaintext attack. The 
underlying PRNG is definitely not suitable for cryptographic 
applications. The documentation even says so!

http://docs.python.org/lib/module-random.html
"However, being completely deterministic, it is not suitable for all 
purposes, and is completely unsuitable for cryptographic purposes."

Do yourself a favor and don't try to roll your own cryptographic 
functions. Do everyone else a favor and don't call something 
"unbreakable" unless you actually have the domain expertise to make that 
determination.

And do read _Practical Cryptography_.

-- 
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
  Are the graves of dreams allowed to die."
   -- Richard Harter