Re: PREFER_DH_SUITES doesn't ensure a DH key agreement

Shivam Patel <[email protected]> writes:

>I uncommented the *PREFER_DH_SUITES** line in ssl.h in order to force a DH
>key agreement rather than RSA key exchange.
>
>I found that even with *PREFER_DH_SUITES** defined in ssl.h, the client and
>server resorted to a RSA key exchange (TLS_RSA_WITH_3DES_EDE_CBC_SHA)*

I've just tried it and it's definitely doing a DH key exchange.

>Cipler suites sent in client hello message when **PREFER_DH_SUITES is defined
>(by removing the comment):
>
>TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA (0x0016)
>TLS_DHE_RSA_WITH_AES_128_CBC_SHA (0x0033)
>TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x0039)
>TLS_RSA_WITH_3DES_EDE_CBC_SHA (0x000a)
>TLS_RSA_WITH_AES_128_CBC_SHA (0x002f)
>TLS_RSA_WITH_AES_256_CBC_SHA (0x0035)
>TLS_RSA_WITH_IDEA_CBC_SHA         (0x0007)
>TLS_RSA_WITH_DES_CBC_SHA         (0x0009)
>TLS_DHE_RSA_WITH_DES_CBC_SHA (0x0015)
>
>There is a difference in the order, but the cryptlib server opts for
>TLS_RSA_WITH_3DES_EDE_CBC_SHA (0x000a) in both the cases and DH key exchange
>doesn't happen.

The only reason why it wouldn't go with the suites in the order listed is if
you've done something that prevents the DH exchange from being used, e.g. if
you use a non-signature-capable server key then it'll fall back to using the
RSA exchange.

Peter.

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