[OTR-dev] Socialist millionaire efficiency on J2ME platforms

[Vladimir]

On 02/03/2010 17:11, Ian Goldberg wrote:
> On Tue, Mar 02, 2010 at 01:44:05PM +0000, Vladimir wrote:
>> Hello,
>>
>> I'm currently using fingerprints to identify clients using my
>> application on their mobile phones. The application uses J2ME.
> Cool; which application is this?
>
>> I am  interested in using SMP but I doubt it will be possible because
>> of the  computationally intense calculations (power in particular).
>> The example  in the documentation talks about its uses on a
>> BlackBerry, but the aim  of my software is to be used on less powerful
>> devices too. What effects  on performance will SMP have on a less
>> powerful processor?
> In regular OTR conversation, 2 modexps (the expensive operation) are
> done every time one of the parties creates a new temporary encryption
> key, which is generally done approximately every message.
My protocol is different from OTR in that respect. It uses public key 
encryption to exchange a symmetric key, which is used for the duration 
of the conversation. Both clients (A and B) have to generate a pair for 
every application startup. If A wants to speak to B, then A encrypts a 
freshly generated symmetric key using B's public key. Along with the 
encrypted symmetric key, A sends a hash fingerprint of both public keys 
to B. Then A and B have to contact each other to confirm the 
fingerprint. By confirming the fingerprint, we know that no MITM attack 
has taken place, since the keys used for encrypting them are the correct 
ones. In a way A says "I encrypted the symmetric key using this public 
key, is that ok?".
> In SMP, each side does about 9 modexps, but only once per person you
> talk to.  So I'd say SMP should take way less power than the rest of the
> conversation.
I'm not familiar with the modexps measurements but it seems to me that 
using 1536-bit primes is still more than generating a single RSA 
key-pair, which is enough of a burden right now. Please correct me if 
I'm wrong.

The application is not released yet, it is a project I'm working on at 
the moment for my dissertation. I'm glad to see interest in it.

Thanks
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