[Anti-fraud] simple (&secure??) PW-based web login (was Re: Another entry in theinternet security hall of shame....)

Ian G iang at systemics.com
Wed Sep 14 07:42:45 EDT 2005

Amir Herzberg wrote:
> For a stationary user, the extension compares _Iterations_ and confirm 
> it is at most one less than previous value of _Iterations_ used with 
> this site.

(Minor point - if relying on incrementing
Iterations, this may impact password sharing
scenarios.  Whether that's a good thing or a
bad thing depends...)

> Also, the extension keeps a table r(PK) mapping the public 
> key PK of each login site to an independant random value (we need this 
> as real random value and can't derive them from the PW, to prevent 
> dictionary attacks by fake sites).

I suspect this would not work so well in the
(common enough?) cases where a site uses a farm
of SSL boxes and certs;  a couple of sites I've
come across provide different certs every time
(although admittedly I saw this with IMAP TLS not
with browsing).

> Now, using a single PW, extension computes for a site with given PK the 
> value H(0)=h(PK, h(r(PK), PW).

What is the reason for hashing twice?  Instead of
the more obvious H(0)=h(PK, r(PK), PW) ?

(Also, you are missing a closing parenthesis there
so maybe your intent was other.)

(Somewhat challenging your assumptions here) your
design does not seem to cope with MITM.  But, it
may do if you are assuming there is an extension
that is handling the client side, and the exchange
is the setup for later transactions, not the
transaction itself:  the server can send back its
token X which needs to be further hashed in order
to gain the useful token for later:

    Y = h(..., X, PW);

where Y could be the session identifier or cookie
or something.  In this way both sides have proven
their possession of the password and hopefully
eliminated other parties from further comms.

(But I may have misunderstood something...)


The Cryptography Mailing List
Unsubscribe by sending "unsubscribe cryptography" to majordomo at metzdowd.com

More information about the cryptography mailing list