Re: [TLS] signature-algorithms extension in ServerHello

Xuelei Fan <xuelei.fan@oracle.com> Thu, 30 September 2010 11:27 UTC

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Date: Thu, 30 Sep 2010 19:26:42 +0800
From: Xuelei Fan <xuelei.fan@oracle.com>
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To: "Michael D'Errico" <mike-list@pobox.com>
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Subject: Re: [TLS] signature-algorithms extension in ServerHello
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 Thanks for the clarification.

Xuelei

On 9/30/2010 2:49 PM, Michael D'Errico wrote:
> 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