Re: [TLS] Another IRINA bug in TLS

Santiago Zanella-Beguelin <santiago@microsoft.com> Fri, 22 May 2015 17:52 UTC

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From: Santiago Zanella-Beguelin <santiago@microsoft.com>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>, "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] Another IRINA bug in TLS
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Date: Fri, 22 May 2015 17:52:29 +0000
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Archived-At: <http://mailarchive.ietf.org/arch/msg/tls/H5_wezFhj6RFp1HnI1k3za_78RA>
Subject: Re: [TLS] Another IRINA bug in TLS
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>>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?

No. We do primality tests only if the prime isn't in the cache. That happens the
first time the client connects to a server, or when the server refreshes its
parameters. That, assuming the parameters aren't already in the pre-populated
cache.

>Doesn't that make it awfully slow?

No. The typical cost of validation is just one table lookup. This works
extremely well when people use just a few thousand parameters and don't change
them often. Even if that changes in light of Logjam (I hope it does!), online
primality tests for every connection are not prohibitive. I did a quick test
using Sage and the stock is_pseudoprime: testing if a 2048-bit prime is safe
takes ~180ms in a laptop, while computing two DH operations (exponentiations)
takes ~17ms. The amortized cost using a cache is of course much lower.

> What testing do you do on unsafe primes?

We accept them if they're in the pre-populated cache. If not, we reject them
because we can't check the order of the generator and validate ephemeral keys.
Unsafe primes have other problems too, e.g. NFS pre-computation is a bit easier
than for safe primes.

--Santiago

________________________________________
From: TLS <tls-bounces@ietf.org>; on behalf of Peter Gutmann <pgut001@cs.auckland.ac.nz>;
Sent: Friday, May 22, 2015 3:54 PM
To: <tls@ietf.org>;
Subject: Re: [TLS] Another IRINA bug in TLS

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?

>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?

Peter.

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