[Cryptography] People should turn on PFS in TLS (was Re: Fwd: NYTimes.com: N.S.A. Foils Much Internet Encryption)
hallam at gmail.com
Wed Sep 11 13:39:40 EDT 2013
On Tue, Sep 10, 2013 at 3:56 PM, Bill Stewart <bill.stewart at pobox.com>wrote:
> At 11:33 AM 9/6/2013, Peter Fairbrother wrote:
>> However, while the case for forward secrecy is easy to make, implementing
>> it may be a little dangerous - if NSA have broken ECDH then
>> using it only gives them plaintext they maybe didn't have before.
> I thought the normal operating mode for PFS is that there's an initial
> session key exchange (typically RSA) and authentication,
> which is used to set up an encrypted session, and within that session
> there's a DH or ECDH key exchange to set up an ephemeral session key,
> and then that session key is used for the rest of the session.
> If so, even if the NSA has broken ECDH, they presumably need to see both
> Alice and Bob's keyparts to use their break,
> which they can only do if they've cracked the outer session (possibly
> after the fact.)
> So you're not going to leak any additional plaintext by doing ECDH
> compared to sending the same plaintext without it.
One advantage of this approach is that we could use RSA for one and ECC for
the other and thus avoid most consequences of an RSA2048 break (if that is
The problem I see reviewing the list is that ECC has suddenly become
suspect and we still have doubts about the long term use of RSA.
It also have the effect of pushing the ECC IPR concerns off the CA and onto
the browser/server providers. I understand that many have already got
licenses that allow them to do what they need in that respect.
Perfect Forward Secrecy is not perfect. In fact it is no better than
regular public key. The only difference is that if the public key system is
cracked then with PFS the attacker has to break every single key exchange
and not just the keys in the certificates and if you use an RSA outer with
an ECC inner then you double the cryptanalytic cost of the attack (theory
as well as computation).
I think this is the way forward.
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