Re: [TLS] Another IRINA bug in TLS

Jeffrey Walton <noloader@gmail.com> Fri, 22 May 2015 17:05 UTC

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Date: Fri, 22 May 2015 13:05:31 -0400
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From: Jeffrey Walton <noloader@gmail.com>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
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Subject: Re: [TLS] Another IRINA bug in TLS
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On Fri, May 22, 2015 at 10:54 AM, Peter Gutmann
<pgut001@cs.auckland.ac.nz>; wrote:
> Santiago Zanella-Beguelin <santiago@microsoft.com>; writes:
>
>>Regarding validation of DHE parameters and how reasonable it is in practice,
>>I wanted to make some publicity for miTLS (http://www.mitls.org/).
>>
>>A miTLS client maintains a table of known trusted parameters, including the
>>subgroup order for parameters with non-safe primes. When receiving unknown
>>parameters from a server, it tests the primality of p and (p-1)/2 to check if
>>p is a safe prime (and caches a positive result in the table).
>
> So you do a full primality test (Miller-Rabin or whatever) for each connect?
> Doesn't that make it awfully slow?

The software under my purview does.

I don't allow the developers to use any parameters without validation.
That includes crypto parameters. But its for a systems that handles
medium and high value data; and not low value data where web browsers
and their use of TLS would be appropriate.

I don't think browsers will ever be able to handle anything other than
low value data, so I probably won't have to worry about web browsers
and what do (or don't) validate before use.

>>If the modulus is prime but not safe, miTLS checks if it is in the table of
>>trusted parameters and otherwise rejects it. We distribute miTLS with a pre-
>>populated table of parameters which we tested thoroughly (and we welcome
>>people to test further). Most of the time we hit an entry in the table.
>
> What testing do you do on unsafe primes?

GnuTLS with its Lim-Lee primes causes me a lot of problems because
they cannot be validated.

If we encounter them, then developers are *not* allowed to apply a
secret to them. The user has to generate a new, verifiable key and
then add that key to their keyring.

Jeff