Paul Hoffman wrote: > > > > >Without a rationale for when the extension is useful, it is impossible > >for implementers to know when use of this extension is warranted or not. > > The environment I described in the earlier thread is TLS with > Diffie-Hellman. I thought that saying that was sufficient, but I guess > it wasn't. > > In Diffie-Hellman key establishment with static keys, even if the PRNG > of one side is bad, the resulting pre-master secret is still sound. > Neither side knows whether or not the PRNG of the other side is bad, so > each side wants to supply sufficient randomness for the master secret > even if the other side's PRNG is bad. If a side with a bad PRNG adds its > own input, it doesn't hurt the randomness of the result, but a side with > a good PRNG can bring up the amount of randomness. > > I did not want to list this as the justification because there may be > other reasons to use the extension, and I don't want readers to think > that this is the only one. For example, future types of TLS key > establishment might have similar properties as static-static > Diffie-Hellman. > > Does that help?
This will need a lot of big caveats. Static DH requires DH-certificates, something which is within [unusual,esoteric,not-implemented] for the installed base of TLS and CAs. And if your low-entropy device creates the static DH keypair itself then you are still screwed, even with this extension. In the past there have been serious TLS security problems related to lack-of-entropy for one of the communicating entities. In 1995 a weakness in the TLS encryption was found in the Netscape browser based on a lack of entropy in the randomness that was used for generation of the RSA premaster secret. http://seclists.org/bugtraq/1995/Sep/62 In 2008, there was an bad change made to Debian's OpenSSL distribution which totally crippled its cryptographic number generator. http://www.debian.org/security/2008/dsa-1571 The lack of real entropy creates security problems for (examples): - generation of shared secrets (session keys) for symmetric cryptography or keyed hashes/MACs - generation of asymmetric cryptographic keys (RSA,DSA,DH,EC*,...), e.g. for use in credentials, host credentials, certificates & certificate requests - client-side generation of TLS pre-master secrets for RSA-ciphersuites - key exchange with ephemeral Diffie Helman - DSA-signatures so if this extension has any value, it must be tailored to that specific purpose. Currently, it leaves _very_ dangerous impressions with readers and implementors. -Martin _______________________________________________ Ietf mailing list Ietf@ietf.org https://www.ietf.org/mailman/listinfo/ietf
