Re: [TLS] signature-algorithms extension in ServerHello

[Michael D'Errico <[e-mail]>]

A client should always include the signature-algorithms extension in
the ClientHello, whether it is attempting to resume a session or not
for the reasons you stated.

The spec forbids a server from ever including it in the ServerHello.

The wording you quoted is just trying to emphasize that it is not
included when a session is resumed.  This is a requirement given in
section 7.4.1.4:

    In general, the specification of each extension type needs to
    describe the effect of the extension both during full handshake and
    session resumption....

Mike




Xuelei Fan wrote:
>  Hi,
> 
> In the end of section 7.4.1.4.1, RFC5246 (TLS 1.2), it says that
>     Servers MUST NOT send this extension. ...
>     When performing session resumption, this extension is not included
> in Server Hello, and the server ignores the extension in Client Hello
> (if present).
> 
> "this extension is not included in Server Hello" is a little confusing
> to me. No matter full handshake or session resumption, this extension
> MUST NOT be included in the ServerHello message, right?
> 
> And I don't think it is a type of "this extension is not included in
> Client Hello", because when requesting a session resumption, a client
> normally cannot expect whether the server will do a full handshake or a
> abbreviated handshake, so the client need to always include this
> extension in ClientHello for the preference signature algorithms.
> Otherwise, the server may be able  to response with a full handshake and
> use not-that-strong algorithms, such as (sha1, rsa), while the client
> may only want to SHA-2 hash functions.
> 
> Thanks,
> Xuelei Fan