Re: [Cfrg] password-based key exchange

One minor comment (which might be totally obvious):

> -----Original Message-----
> From: cfrg-bounces@irtf.org [mailto:cfrg-bounces@irtf.org] On Behalf
Of
> Dan Harkins
> Sent: Tuesday, December 20, 2011 4:38 PM
> To: cfrg@irtf.org
> Cc: tls@ietf.org
> Subject: [Cfrg] password-based key exchange
> 
> 
>   Hello,
> 
>   I proposed a new TLS ciphersuite using a zero knowledge proof at
> IETF 82 in Taipei. The draft is here:
> 
>        http://tools.ietf.org/html/draft-harkins-tls-pwd-01
> 
> At the TLS WG meeting I was requested to ask people on the CFRG list
> with expertise in this field to take a look at it. So here I am,
> asking.
> If someone has some spare cycles this holiday season, if you just
gotta
> get away from the relatives, or you are just feeling in the giving
> mood,
> please take a look and comment. Any analysis on this would be greatly
> appreciated. (I tried to do a formal proof but I cannot, and that's
> what's prompting this).
> 
>   I can describe the key exchange broadly here. There are subtle
> differences in the draft-- like one side keeps a salted version of the
> password and communicates the salt during a HELLO message-- that don't
> affect the exchange. It is symmetric so I can describe it from one
> side's
> perspective. The side being described is "local" and the other side is
> "peer", it goes like this:
> 
> Given: local ID, peer ID, password, an agreed-upon set of domain
> parameters defining a finite field group (it works will elliptic
> curve crypto too) and using two function:
> 
>   - E = HashToElement(d) which takes some data, d, and hashes into
>     the finite field to return an element, E.
>   - x = H(y) returns a random stream, x, given some input, y.
> 
> The key exchange works like this:
> 
> 1. PE = HashToElement(local_ID | peer_ID | password)
>    gets a "password element"
> 
>    There is an ordering defined in the draft to ensure that the IDs
>    are put in the same order on each side.
> 
> 2. generate 2 random numbers between 0 and the order of the group, q:
> 
>    0 < local_rand, local_mask < q
> 
> 3. compute a scalar and an element:
>    local_scalar = (local_rand + local_mask) mod q
>    local_element = 1/(PE^local_mask) mod p
> 
>    where p is the prime and q is the order. Send local_scalar and
>    local_element to other side
> 
> 4. receive peer_scalar and peer_element from other side
> 
> 5. compute shared key, K
> 
>    K = (PE^peer_scalar * peer_element)^local_rand mod p =
>        (PE^(peer_rand + peer_mask) * PE^(-peer_mask))^local_rand mod p
> =
>        PE^(peer_rand*local_rand) mod p

You probably should have an explicit test and rejection for K = 1
('point at infinity' for EC groups)...

> 
> 6. compute a confirmation
> 
>    local_confirm = H(K, local_scalar | local_element |
>                      peer_scalar | peer_element)
> 
>    send confirmation to peer
> 
> 7. receive peer's confirmation, calculate verifier
> 
>    verifier = H(K, peer_scalar | peer_element | local_scalar |
>                 local_element)
> 
>    if verifier equals peer's confirmation the peer is authenticated.
> 
> The peer does the same thing but from its perspective it is "local"
> and the side described above is "peer".
> 
>   Thank you in advance for any analysis that can be provided on this
> exchange.
> 
>   regards,
> 
>   Dan.
> 
> 
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