crypto for the average programmer

James A. Donald jamesd at echeque.com
Mon Dec 12 13:08:42 EST 2005



Date sent:      	Mon, 12 Dec 2005 00:41:13 -0600
From:           	"Travis H." <solinym at gmail.com>
To:             	cryptography at metzdowd.com
Subject:        	crypto for the average programmer

> In Peter Gutmann's godzilla cryptography tutorial, he has some really
> good (though terse) advice on subtle gotchas in using DH/RSA/Elgamal.
> I learned a few no-nos, such as not sending the same message to 3
> seperate users in RSA (if using 3 as an encryption exponent).
> 
> My question is, what is the layperson supposed to do, if they must use
> crypto and can't use an off-the-shelf product?  Is there any site
> tracking such gotchas as they show up in the literature?  Are there
> APIs written specifically so that a crypto-naive programmer can safely
> use them?

It seems to me that if the only thing you use public key encryption 
for is to encrypt a single use randomly chosen symmetric key, and 
integrity bits for that key, and if you then use that symmetric key 
once and only once, to encrypt a message that already contains 
integrity checking and a unique random number, you don't need to 
worry about those issues.

Of course those issues reappear when using public keys for signature 
algorithms - so don't invent your own signature protocol.  Signatures 
are hard.


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