> -----Original Message-----
> From: Gregory Nicholls [mailto:[EMAIL PROTECTED]]
> The data I need to encrypt is your basic bit of user data going from
> system A to system B. Now given that the whole point of encrypting this
> is to prevent someone from reading the data, is there any reason why I
> can't select a random bit out of each user message, perform some
> incantation on it and use this to seed the PRNG ??
How do you select a "random bit" of a message if you don't have a source of
randomness in the first place? From a cryptographic perspective, you'd best
assume an attacker knows which bit of the message you're using, even without
knowing what the message contains.
Also, note that a crypto PRNG seed should ideally contain considerable
entropy. Are these messages themselves unpredictable enough to provide
sufficient seed material?
> I mean, if someone can predict the contents of the data well enough to
work
> out what my seed is, surely they've defeated the purpose of the encryption
> anyway .. true ???
That's a very big assumption. An attacker could know enough about the
structure or probable contents of the message to greatly reduce the actual
amount of entropy in the seed, without being able to guess the message.
(Perhaps details in the message are important, but the message is known to
always be English text.) An attacker might be able to inject a
known-plaintext attack and defeat the PRNG seeding entirely. (Bob is a
legitimate user of your system, but he's secretly trying to crack it as
well, so he can read other people's data.)
Of course, there are many ways to sanitize the sampled data to reduce the
attacker's effective knowledge about it - hashing it, for example. That's
certainly better than the original Netscape time+pid+ppid approach.
Ultimately it's a question of your threat model. Does your model try to
account for serious attacks by knowledgeable people with lots of resources?
If not, how you seed your PRNG isn't something to lose a lot of sleep over.
If so, then you'd be better off hiring crypto and security experts to vet
your application.
Michael Wojcik [EMAIL PROTECTED]
MERANT
Department of English, Miami University
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