Forum Discussion

Stephen_Archer_'s avatar
Historic F5 Account
Oct 28, 2011

SHA2 / SHA256 certificates

I have a customer that wants to use SHA2 / SHA256 certificates on their website (fronted by LTM), however clients such as Windows XP SP2 are unable to verify such certificates. The customer would like to detect these clients and redirect them to a web page providing remediation advice.



I know it's possible to detect the cipher suites supported by the client: create a client SSL profile with weak ciphers allowed, then after SSL handshake completion, check the cipher suite used. It would then be possible to redirect clients using weak ciphers to the remediation page. But... if the client is unable to validate the SHA2 certificate, the SSL handshake will never complete.



The next option would be to binary scan the TCP::payload on the 'client hello' and check the presented cipher suites... however, I don't think that the list of cipher suites presented by the client tells us whether the client is able to validate a SHA2 certificate, or not. i.e. it may be possible that a client that does NOT list SHA2 / SHA256 in the list of supported cipher suites, but is still actually able to verify a SHA2 certificate.



So my question is, do I understand this issue correctly, and if I do, has anyone thought of a way to overcome it?



Thanks in advance!


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