Subject: RE: [TLS] Some questions regarding PSK

From: [email-masked]
Date: Fri, 29 Apr 2005 10:49:30 +0300

Franke, Markus wrote:
> 
> 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.

The field is defined as "opaque psk_identity<0..2^16-1>;" so
it's not optional. If there's no hint, it's left empty (length
0). (And yes, this means that application profile specs (see
Section 5.2) can't realistically treat "no hint" and
"zero-length hint" differently.)

> 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.

RSA_PSK is based on the ordinary RSA ciphersuites that don't
sign anything either: they're based on RSA encryption, not
signatures.  (And all the handshake messages are MAC'd in the
Finished messages later).

> 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).

Ok, if we make another revision of the spec, we can mention
this explicitly.

Best regards,
Pasi

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