Encryption with Python?

[Anthra Norell]

Thanks a lot for the feedback. This is certainly a great learning
experience. It's a fascinating topic too. Without wishing to annoy, I'd be
interested in knowing more. I insert questions below.

----- Original Message -----
From: "Robert Kern" <rkern at ucsd.edu>
To: <python-list at python.org>
Sent: Saturday, May 07, 2005 11:11 AM
Subject: Re: Encryption with Python?


> 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.

How do you know the same key was used?
What if the messages aren't the same length?
How does one reconstruct either or both of two messages from their
difference?

> 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!

What is a plain-text attack? Is it you give me a plain text. I encrypt it
using the same keys and hand you the result? Would I comply with the
request?

> 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."

The logic escapes me if it is understood to mean that cryptography, in order
to be usable, has to be indeterministic. If, however, it is meant to suggest
that it is the reversible, direct one-on-one relation between all the
characters of a plain text and its encryption that falls short of
state-of-the-art technology, I'd have no problem with that. That doesn't
mean, however, that an exercise is required to measure up to
state-of-the-art standards in order to be taken seriously. I do invent my
own solutions for simple problems. I find it more challenging than stalking
other people's solutions how ever superior and typically it also saves me
time. It is not my ambition to excel in any field, nor to flaunt my
erudition. It is my ambition to satisfy the requirements of a given task.
And this, I think, my solution does. I can be wrong and if so, I'd
appreciate being proven wrong. I have now received well-meaning advice for
which I am grateful. But I haven't been proven wrong. I claim that my
solution satisfies the requirements of the task at hand and challenge anyone
to prove the contrary. You can meet the challenge by deciphering the
following tiny message, knowing now the encryption method, which in practice
would not be known and, as it seems, would hardly be suspected by
deciphering experts for its amateurishness.

> 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.

Here's the challenge. Prove this breakable

'\x10\x88d\x1d\xba\xa1\xdcK\x05w\x02/s\xa7Q0\xeb8\xb6Gx\xef\xcb\x1e=\xf5\x7f
\x9bI\xcb(\x87>\xa5\x04\xc1soF\xfd\xc6\xc6\xd9|\x971\xdb\xcdT\tw#\x86a\xdc\x
b8P\xfb=n\xda\x80\x9f\x84m\x12\x98\x98\xca=o\x0b\x8e\x08O\xb7\x0b\x04SC\x96\
xc7\xab*\x0b\x996\x06\x86\x83(\x8dQ\x9eG\x8f$\xb2x)\xa9fv\x0c1B\x9b\r\xde\xf
fc\x08'

When you give up we may try a plain-text attack. Okay?

> And do read _Practical Cryptography_.

I certainly will.

> --
> 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
>
> --
> http://mail.python.org/mailman/listinfo/python-list