Re: [TLS] Elliptic Curve J-PAKE

Hi Hugo,

Thanks for your comments.

Just to clarify the difference between SPAKE2 and J-PAKE - The proof of SPAKE2 depends on the assumption of a trusted setup: the discrete logarithm between the two group generators must be unknown by anyone. If a powerful adversary (3 letter agency) gathers sufficient resources and time (say 1 year) to break one instance of discrete logarithm, it will be a class attack, breaking all instances of SPAKE2 without anyone knowing it. By contrast, they can only break one session in J-PAKE, since by design the randomness is refreshed in every session rather than being built into a static setup. This explain why J-PAKE requires more computation than SPAKE2. Hope it clarifies.

Regards,
Feng

From: TLS <tls-bounces@ietf.org<mailto:tls-bounces@ietf.org>> on behalf of Hugo Krawczyk <hugo@ee.technion.ac.il<mailto:hugo@ee.technion.ac.il>>
Date: Wednesday, 27 March 2019 at 02:49
To: Hannes Tschofenig <Hannes.Tschofenig@arm.com<mailto:Hannes.Tschofenig@arm.com>>
Cc: "tls@ietf.org<mailto:tls@ietf.org>" <tls@ietf.org<mailto:tls@ietf.org>>
Subject: Re: [TLS] Elliptic Curve J-PAKE

Hi Hannes,

J-PAKE is a symmetric PAKE. Both parties store the same password. It is not suitable for most client-server scenarios where using J-PAKE would mean that an attacker that breaks into the server simply steals all plaintext passwords. OPAQUE is an asymmetric (or augmented) PAKE where user remembers a password (and nothing else, including no public key of the server) while the server stores a one-way image of the password. Security requires that if the server is compromised, the attacker needs to run an offline dictionary attack for each user in the database to find the password.

If what you need is a symmetric PAKE then there are better candidates than J-PAKE such as SPAKE2 described in draft-irtf-cfrg-spake2-08. SPAKE2 is *much* more efficient than J-PAKE and while both J-PAKE and SPAKE2 have proofs of security, SPAKE2 is proven in a stronger security model relative to J-PAKE.

I am not aware of any advantage of J-PAKE over SPAKE2 - but I may be missing something. Maybe the PAKE presentation in cfrg will clarify these issues further.

Hugo

On Tue, Mar 26, 2019 at 1:03 PM Hannes Tschofenig <Hannes.Tschofenig@arm.com<mailto:Hannes..Tschofenig@arm.com>> wrote:
Hi all,

in context of the OPAQUE talk by Nick today at the TLS WG meeting I mentioned that the Thread Group has used the Elliptic Curve J-PAKE for IoT device onboarding.
Here is the draft written for TLS 1.2:
https://tools.ietf.org/html/draft-cragie-tls-ecjpake-01

The mechanism is described in https://tools.ietf.org/html/rfc8236

@Nick & Richard: Have a look at it and see whether it fits your needs.

Ciao
Hannes

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