Re: [TLS] [Cfrg] Review of Dragonfly PAKE

Watson Ladd <watsonbladd@gmail.com> Wed, 11 December 2013 01:54 UTC

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Date: Tue, 10 Dec 2013 17:54:32 -0800
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From: Watson Ladd <watsonbladd@gmail.com>
To: Trevor Perrin <trevp@trevp.net>
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Subject: Re: [TLS] [Cfrg] Review of Dragonfly PAKE
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On Tue, Dec 10, 2013 at 2:30 PM, Trevor Perrin <trevp@trevp.net>; wrote:
> Dear CFRG (cc: TLS),
>
> Here's a review of the Dragonfly Password-Authenticated Key Exchange (PAKE) from
> draft-irtf-cfrg-dragonfly-02 [CFRGDRAFT].
>
> Overview
> ----
> The Dragonfly PAKE is built on SPEKE with an "obfuscation" applied to
> the exchange of Diffie-Hellman values.  The obfuscation lacks formal
> analysis and serves no obvious purpose, but may be an attempt to avoid
> the SPEKE patent [IPSEC].  Dragonfly has security weaknesses due to
> use of a variable-time algorithm to map a password to an EC point
> [STRUIK], and lack of "augmented" PAKE properties.
>
> Obfuscating the SPEKE DH exchange
> ----
> SPEKE is an old and well-known PAKE [JABLON].  In SPEKE each party
> uses a shared password to derive a Diffie-Hellman generator.  The
> generator is then used for a Diffie-Hellman exchange.
>
> SPEKE is patented until 2017 [SPEKEPATENT].  Alternatives without
> current patents incude [DH-EKE] and [J-PAKE].  Alternatives with
> royalty-free terms include [SRP] and [AUGPAKE].
>
> Dragonfly uses the SPEKE approach but obfuscates the exchange of DH
> values.  In particular, given:
>
>   g : DH generator (calculated by hashing the password)
>   a : Alice's DH private key
>
> In SPEKE, Alice sends g^a.
>
> In Dragonfly, Alice generates a mask m, and sends (a+m, g^-m).  Bob
> uses these values and g to reconstruct g^a.  [ g^a = g^(a+m) * g^-m ]
>
> This obfuscation adds computation and bandwidth costs.  It's not clear
> whether it adds any security benefit.  It's not clear how Dragonfly's
> security relates to SPEKE; whether the SPEKE security proof from
> [MACKENZIE] still applies; or whether another security proof could be
> created.
>
Note Bena: The proof does not address resistance to offline attack,
and uses an assumption that
isn't know to be equivalent to anything.