Re: [Shutup] [ietf-smtp] Compressing SMTP streams

[Aaron Zauner <[e-mail]>]

> On 06 Feb 2016, at 12:32, Martijn Grooten <[e-mail]> wrote:
> 
> On Sat, Feb 06, 2016 at 11:15:54AM +0100, Aaron Zauner wrote:
>> Do you guys have any numbers on this? I.e. what the advantage and
>> compression ratio for your average mail traffic will be? I suspect
>> compression is helpful in SMTP but it may also introduce
>> vulnerabilities in combination with TLS. CRIME wasn't the only attack
>> on compression, there's also been application layer specific attacks
>> BREACH for example (breachattack.com). A team is currently working on
>> improving these attacks in application layer protocols, circumvent
>> counter-measures in clients et cetera (from a talk at
>> RealWorldCrypto2016 -
>> https://drive.google.com/file/d/0Bzm_4XrWnl5zMkJJdHo0Rml4bXM/view?usp=sharing).
> 
> I think it's fair to say (as others have done already) that none of
> these attacks work against SMTP as they all require the attacker to
> force the client to make specific requests to the target.

Well, yes. You just can't do JavaScript via SMTP (yet) :)

> But these attacks also show that compression and encryption don't go
> well together. And crypto is hard and provides plenty of opportunities
> to mess up. For that reason, I would suggest following TLS 1.3 and not
> combine the two, as it would teach people bad habits.

There aren't that many people looking into this, with renewed research efforts I fear that these attacks may improve significantly. The authors of mentioned work on improving these attacks explicitly mentioned other application layer protocols than HTTP(S) in the Q/A after the talk. They will also be presenting new attacks at upcoming conferences. As I see it - adding compression might reduce traffic load for some providers but at the same time may introduce new vulnerabilities in the future to already poorly-secured protocols like SMTP.

Aaron