SSL-Does a server certificate bind to a specific machine?

Question

Recently we created a server with tomcat and we also add SSL support for this little server. For SSL support, we need a certificate which issued by a third issuer like Entrust, Thawte etc.

A colleague said to me that the certificate is binding to a specific machine. That's once we got the issued certificate, then this cert can't be used in another machine.

I doubt this completely because the CSR doesn't contain any info of the machine. Is that true?

Thanks

Solution

The certificate isn't necessarily bound to a particular machine. To be able to "use a certificate" on a machine, you need two things: the certificate itself, and its private key. You should have generated the private key along with the CSR (depending on which tools you've used).

Some systems don't allow you to re-extract the private key (e.g. Windows has an option to import a private key in a way you can no longer export it, but as far as I understand, this can be bypassed if you have sufficient access rights on that machine). In cases where you're using a smart card or hardware token, the private key may be generated there in such a way that you can't extract it (in this case, moving the token to the new machine would make sense if necessary).

The other part is the certificate and its name. The host name(s) in the certificate (which is often also found in the CSR, although that's ultimately not necessary), should be the host name(s) of this machine, as seen by the clients trying to connect to it (see RFC 2818 Section 3.1 for detail on host name verification for HTTPS). As such, although the certificate itself isn't tied to a particular machine in terms of hardware, it will be tied to this host name (which allows you to change the hardware for this machine or its IP address for example).

Comments

[zgcharley]

So that's mean the certificate is tied to a host name. Then how can i see the host name in the cert? Is the CN? Another question is does SSL client always requires host name checking?(I don't think so)

[Bruno]

If there are DNS subject alternative names, the host name can be any of them. Otherwise, it's the CN in the Subject DN. SSL clients should always check the host name (some don't, but that makes them vulnerable to MITM attacks -- not verifying the name should be treated as a security bug).

[zgcharley]

Thanks for your explanation!