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I have a server that I want to use a wildcard cert on. It runs multiple services. I purchase a wildcard cert so that I can protect mail.something.dom, www.something.dom, im.something.dom, calendar.something.dom, addressbook.something.dom

A few months later I come into several new servers. I now have enough servers that I could set one up for each subdom that I created. I want to use my wildcard cert rather than buy an SSL cert for each one.

Is it possible if I:

  1. Copy the public and private keys for the wildcard cert from the original server.
  2. Place a copy of each of the keys in each new server.
  3. Place a copy of the wildcard cert in each new server.

If this is not the process to do this, is there a process that will work?

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The process you describe will work. Whether you will be able to enact it is a different thing: it depends on where, exactly, your private key is. In Windows systems, a private key might be marked as "non exportable", which means that Windows will not allow the export; the export is still possible (Windows is only software, it cannot do miracles) but somewhat hackish. On Linux systems, private keys are just files and files can be copied at will. If your private key generation and storage involved dedicated hardware then see the documentation of your HSM for possible options.

Possible objections against such a plan are the following:

  • There may be contractual issues. It really depends on the CA. The CA does not have technical power to prevent you from moving your private key from server to server (it is your key on your machines, they cannot spy on that), but they can still define it as a contract breach, at least theoretically. Commercial CA make money out of selling certificates, and you are buying and using a wildcard certificate precisely so that you do not have to buy a new certificate for each new server name; the commercial people at the commercial CA will understandably feel queasy at the concept, hence the possibility of some legal hindrance.

  • The value of a private key resides in its privacy. If your private key becomes known to outsiders, then you have a big problem. As a general rule, any export-transfer-import operation potentially exposes the private key; the more a private key travels, the less private it becomes. The "recommended way" is to have each server (physical machine) generate its own key pair, and never export the private key at all. What you suggests is at odds with this general principle, so beware.

  • If the private key is stolen, then the certificate will have to be revoked, and this will impact all your servers at the same time. With several distinct certificates, damage is more contained. Similarly, when the wildcard certificate expires, a renewal will have to be done, and the new certificates installed on all servers simultaneously. Depending on how many distinct servers you have, this may prove to be cumbersome.

  • You seem to suggest that each server should have it's own private key (which seems logical for me too). What is the common way to deal with that ? Should we issue as much CSR as servers or can we build a CSR from multiple keys ? – Pierre de LESPINAY Jan 6 '17 at 13:45
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It may be a licensing concern, and cost more money if you're honest

Some vendors ask you to purchase licenses to install the certificate on multiple servers. This is on the honor system, and if you ever need to add more hosts the CA vendor will gladly take your money.

Copying the certificate to other hosts is commonplace

It is technically possible to copy the certificate to multiple hosts (as long as the private key is exportable) without paying a license, but depending on the vendor, this may be dishonest.

Consider SNI technology

If your customers are using a modern web browser, you can use the same machine, and IP address for all your subdomain sites (im.something.com, calendar.something.com, addressbook.something.com). This is something you would set up on your web server and let software (web browser) handle the SNI name negotiation for you. That way you pay nothing in additional license fees, and don't have to use an additional IP address, etc.

SNI isn't a good fit if you must support older browsers that don't support this technology

  • 2
    SNI does not apply here. SNI allows using several certificates on a single physical server; this is an alternative to wildcart certs when the problem at hand is hosting several server names on one physical machine. Here, we are interested in using the same certificate on several distinct machines (which happen to have distinct names, but the certificate is wildcard so this particular problem will not occur). – Tom Leek Aug 22 '13 at 17:35
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Yes, and no.

Yes it is definitely possible. Depending on which CA you had sign it for you you may not be contractually allowed to. Some of the CAs want more money for a wildcard cert to be used on several physical devices. I don't believe that there is anything technically different between the wildcard to be used on one server for multiple names in the same domain and the wildcard to be used on multiple servers with multiple names in the same domain. (Please correct me if I am wrong, I have only ever bought one that can be used on multiple servers)

Other CAs only offer one wildcard service and that can be used on multiple devices with multiple names within the same domain.

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