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

Adam Back adam at cypherspace.org
Wed Sep 25 08:25:06 EDT 2013

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

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.)


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