[Cryptography] forward-secrecy >=2048-bit in legacy browser/servers? (Re: RSA equivalent key length/strength)

Peter Fairbrother zenadsl6186 at zen.co.uk
Wed Sep 25 19:37:18 EDT 2013

On 25/09/13 13:25, Adam Back wrote:
> On Wed, Sep 25, 2013 at 11:59:50PM +1200, Peter Gutmann wrote:
>> Something that can "sign a new RSA-2048 sub-certificate" is called a
>> CA.  For
>> a browser, it'll have to be a trusted CA.  What I was asking you to
>> explain is
>> how the browsers are going to deal with over half a billion (source:
>> Netcraft
>> web server survey) new CAs in the ecosystem when "websites sign a new
>> RSA-2048
>> sub-certificate".
> This is all ugly stuff, and probably < 3072 bit RSA/DH keys should be
> deprecated in any new standard, but for the legacy work-around senario to
> try to improve things while that is happening:
> Is there a possibility with RSA-RSA ciphersuite to have a certified RSA
> signing key, but that key is used to sign an RS key negotiation?
> At least that was how the export ciphersuites worked (1024+ bit RSA auth,
> 512-bit export-grade key negotation).  And that could even be weakly
> forward
> secret in that the 512bit RSA key could be per session.  I imagine that
> ciphersuite is widely disabled at this point.
> But wasnt there also a step-up certificate that allowed stronger keys if
> the
> right certificate bits were set (for approved export use like banking.)
> Would setting that bit in all certificates allow some legacy
> server/browsers
> to get forward secrecy via large, temporary key negotiation only RSA keys?
> (You have to wonder if the 1024-bit max DH standard and code limits was bit
> of earlier sabotage in itself.)

A couple of points: all the big CAs will give you a new certificate with 
a new key for free (but revocation is your baby) - while it isn't 
something they do, can't they issue say two years worth of one-day certs 
for perhaps a little more than the price of a two-year cert?

In the UK we have a law called RIPA, part of which allows Plod to demand 
keys. They can demand keys used for encryption and for key setup - but 
they can't demand keys used only for authentication. I don't think they 
routinely demand keys from TLS/SSL webservers.

The point is that in an ordinary TLS session the RSA key is used for 
both secrecy and authentication - in any future TLS these functions 
should be split.

Also, Dan Boneh was talking at this years RSA cryptographers track about 
putting some sort of quantum-computer-resistant PK into browsers - maybe 
something like that should go into TLS2 as well?

You need to get the browser makers - Apple, Google, Microsoft, Mozilla - 
and the webservers - Apache, Microsoft, nginx - together and get them to 
agree "we must all implement this" before writing the RFC.

-- Peter Fairbrother

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