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Using openSSL, on a TLS client, I specified a unique cipher suite with SSL_CTX_set_cipher_list().

When I use wireshark to look at the messages, I see in the Client Hello message, the cipher suites allowed are the ones I specified.

But if I look at the Extension: signature algorithms, I see that the client seems to accept a large number of algos, including some that don't seem very secure to me, like MD5.

How can I change this behavior?

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  • Sorry, I have no idea why this question was moved here. I thought security.stackexchange.com was the best place for it. This question is definitely not fitting superuser.com, because it is about programming. I am asking about the openssl API, not commandline tools.
    – Jacques
    Commented Mar 15, 2015 at 19:43

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(Found researching something else. I don't know if security, or even SO, would be better.)

In release 1.0.2 (only) there are new SSL_[CTX_]set1[_client]_sigalgs[_list] for this. I've seen no doc in any release yet, but it apparently is in "master" (i.e. the development "trunk") on the website at https://www.openssl.org/docs/manmaster/ssl/SSL_CTX_set1_sigalgs.html so presumably it will be in future releases.

In earlier versions you can set a verify callback, or just check the result chain, and reject any cert or chain using MD5, or RSA<=1024, or ECC using curves suspected of NSA backdoors, or anything else you dislike, such as CA policy that says "we hate cats".

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