Subject: [TLS] Some questions regarding PSK

From: "Franke, Markus" <[email-masked]>
Date: Thu, 28 Apr 2005 19:54:20 +0200

Hi all.

I have following questions regarding the PSK ciphersuites:

1. In chapter 3 is stated "the ServerKeyExchange message is always
sent even if no PSK identity hint is provided". But how to not provide 
a identity hint? 
- Is it then empty (length = 00 00)? If so, what is then the difference 
between an empty identity hint in pure PSK ciphersuites?
- Or is it an optional field? Then it may be better to flip the
order of the fields psk_identity_hint and public.

2. The ServerKeyExchange message in RSA_PSK is - in opposite to EXPORT_RSA
or DHE_RSA cipheruistes - not signed by the server's private key 
corresponding to the server certificate? Why? 
- For performance reasons on client side (no signature validation)?
- Or because it is not considered to be necessary since 
proof-of-private-key-possession is done by successfully decrypting the 
pre_master_secret?
- Or to prevent a RSA certificate that is to be used for signature
and encryption at the same time?
Maybe it's worth to spend some words on that.

3. Maybe it should be explicitly mentioned that for RSA_PSK ciphersuites
the keyEncipherment bit has to be set (see chapter "7.4.2. Server
certificate"
in RFC 2246).

Regards,
Markus

_______________________________________________
TLS mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/tls

---

From: [email-masked] (Peter Gutmann)
Date: Fri, 29 Apr 2005 15:50:09 +1200

>sent even if no PSK identity hint is provided". But how to not provide

I'd already complained about this weird requirement some time ago ("a 
message whose only purpose is to indicate that the page is intentionally 
left blank"), I thought this was fixed in more recent drafts but it 
seems like it's still there.  The same with the other comments, a few
months after the above I sent another comment:

  I have no idea what an "identity hint" could possibly be (the spec says 
  it's optional, but I'd have no idea what to put in there in any case).  
  If the field is used for something, what's it supposed to be used for?  
  If not, perhaps it should be dropped.

  (Hmm, and I'll repeat my earlier gripe that apart from its use to convey 
   this  unknown quantity, there's no need to send the server keyex at all.  
   In other  words in 99.99% of cases it's just going to be code and message 
   bloat).

Can we finally get this fixed, or how many more times does it have to be
brought up as a problem with the spec?  The solution is to make the
ServerKeyExchange optional (it's not needed in 99.99% of cases), and explain 
what the identity hint is and how it's used.

Peter.