Re: Encryption key longer than text to encrypt



On Wed, 03 Jan 2007 16:44:30 +0000, rossum <rossum48@xxxxxxxxxxxx> wrote:

On 3 Jan 2007 06:41:46 -0800, "Jean-François Michaud"
<cometaj@xxxxxxxxxxx> wrote:

Since it is easier in practice to use pseudo random data than to
generate random data which is then distributed as appropriate if we
have OTP in mind, would
encrypting messages à la OTP with 'pseudo random data' using a key
greater than the message make any sense to increase security,

No. The reason that an OTP is provably secure is that the keystream
is truly random. As soon as you substitute a pseudo-random keystream
the security proof fails. You no longer have an OTP, you have a
stream cypher instead.


Hi,

I always wonder about this "provably secure". I don't remember seeing
a convincing proof of security for OTP, does anyone have any references?

For example, a true random number generator could on one particular run
generate a sequence entirely of zeros. Examining the data would then
reveal the original cleartext. Yes it's extremely unlikely to happen
in practise I know (in practise it's probably much more unlikely to happen
than it should even in theory). But it's because of the chance of that
and then a whole series of progressively weaker cases that I would be
inclined to think OTP really isn't 100% secure ...

bestwishes
l



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