[openssl-users] DTLS handshake in WebRTC

[Suman Paul]

The last bit of information makes my life a little hard.

In DTLS-SRTP usage, the DTLS server must present it's server fingerprint in SDP before the client support ciphersuites are known, how can a DTLS server support clients that may support only RSA or ECDSA?

Suman

> On Mar 1, 2017, at 4:01 PM, Matt Caswell <matt at openssl.org> wrote:
> 
> 
> 
> On 01/03/17 23:52, Suman Paul wrote:
>> What I have seen in my trials with s_server and s_client is that if run
>> s_server with an ECDSA cert/key and I specify one RSA and one ECDSA
>> cipher with the -cipher option, then s_client can only connect to it
>> using the ECDSA cipher. I have been unsuccessful in connecting to this
>> server using a RSA cipher. RSA cipher fail shows up at the s_server as 
>> 
>> 140480482967256:error:1408A0C1:SSL routines:ssl3_get_client_hello:no
>> shared cipher:s3_srvr.c:1417:
>> 
>> Your thoughts on this?
> 
> Yes, this is expected. The ciphersuite selection is limited by the
> available server certificate(s). That is different to the client
> certificate which is independent of the ciphersuite.
> 
> Matt
> 
> 
>> 
>> Suman
>> 
>>> On Mar 1, 2017, at 1:51 AM, Matt Caswell <matt at openssl.org <mailto:matt at openssl.org>
>>> <mailto:matt at openssl.org <mailto:matt at openssl.org>>> wrote:
>>> 
>>> 
>>> 
>>> On 01/03/17 09:39, Suman Paul wrote:
>>>> Sorry, I meant to say when the client sends its certificate, firefox in
>>>> this case, it has a key of type ECDSA. How does a key of this type work
>>>> when the  cipher selected is of type RSA?
>>> 
>>> Ah, right - you are using client auth. The choice of client certificate
>>> has nothing to do with the underlying ciphersuite - it is chosen
>>> independently. When client auth is in use you should see the server
>>> sending a CertificateRequest message to the client. That
>>> CertificateRequest contains within it the list of acceptable certificate
>>> types.
>>> 
>>> Matt
>>> 
>>>> 
>>>> Suman
>>>>> On Mar 1, 2017, at 1:33 AM, Matt Caswell <matt at openssl.org <mailto:matt at openssl.org>
>>>>> <mailto:matt at openssl.org <mailto:matt at openssl.org>>
>>>>> <mailto:matt at openssl.org>> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> On 01/03/17 05:55, Suman Paul wrote:
>>>>>> I have been looking at WebRTC DTLS handshake and don’t understand the
>>>>>> logic of how it works.
>>>>>> 
>>>>>> My Firefox client has support for both RSA and ECDSA ciphers while my
>>>>>> DTLS server only supports DHE-RSA-AES128-SHA and has a RSA key. I see
>>>>>> that Firefox sends a ECDSA key during client hello. What ends up
>>>>>> happening is that DHE-RSA-AES128-SHA is selected. I would have
>>>>>> expected the negotiation to fail due to there being no common
>>>>>> ciphers.
>>>>>> 
>>>>>> I also verified this behavior using the OpenSSL s_server and s_client
>>>>>> utilities. Seems to me that as long as s_server has a cert and key of
>>>>>> the type of cipher I enforce with ‘-cipher’ option the negotiation
>>>>>> succeeds irrespective of the type of key the s_client (provided that
>>>>>> cipher is also supported by the client).
>>>>> 
>>>>> Your terminology is slightly confusing. No keys are sent in the
>>>>> ClientHello at all. You should see a list of all the ciphersuites that
>>>>> the client supports being sent in the ClientHello and then the server
>>>>> should respond with a ServerHello which picks a ciphersuite from that
>>>>> list.
>>>>> 
>>>>> Matt
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>>>> 
>>>> 
>>>> 
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>> 
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