Question

So, I’m brand new to creating a https-compatible site.

I’m currently working with a client with whom I developed a custom Facebook tab for; the files are currently hosted on my server which I have not purchased a security certificate for.

My client has a security certificate for one of their websites, which I do not have access to. My client sent me a text file with a combination of letters and numbers, and I have absolutely no idea what I’m supposed to do with it.

Anybody have any clue how I’m supposed to use it?

Answer 1

In short, you (probably) can’t use it for that. But we need to check to be sure.

Background

As you know, SSL is used to secure the communication between two systems, one a server and the other a client (well, for the purposes of this communication link anyway). For the code that sits on initiating, client end of the communication channel to know that it’s talking to the right server, it needs not just to have secure communication but also the identity of the server. (Without that, DNS spoofing or any number of IP-level tricks would be utterly massive problems.) This is where certificates come in.

Servers have a cryptographic identity (a public/private key-pair) that they use as part of the boot-strapping of the SSL connection which proves who they are. The public part of that is told to anyone who asks, and the server proves that it has private part through the fact that it can do the key-pair based cryptography (basically, that’s mathematical magic, a.k.a. number theory). Then, all the client has to do to know whether to trust the connection is to work out whether they trust the identity stamped into the public key. This can either be by having been previously told directly “trust this certificate” or by the fact that it was digitally signed by someone it trusts (which is how the Certificate Authority system works).

A certificate is basically the public key of a key-pair, at least one digital signature, plus additional information. Examples of the additional information that could be there are the name of the host for which this is a certificate, the period of time for which the certificate is valid, who the administrative contact is, or where to go to find out whether the certificate has been withdrawn early. There are many other options.

What to do with a bare certificate?

With a bare certificate (in PEM format, as you say) all you can do is add it to your collection of trusted certificates or look at the information encoded within the certificate. So we’ll start by looking at the information. For that, we use the openssl program (which has a horrible command line interface):

openssl x509 -in thecert.pem -text -noout

That will splurge a whole bunch of information out. The most important part is the “Subject” field; what or who is this certificate talking about? Since this is about HTTPS (which imposes a few extra constraints of its own) we should check whether that contains a hostname of some kind, and what host it is talking about.

Now you have the information to be able to figure out what’s going on.

• If the whole certificate matches up (especially the digital signature) with what you’ve already got deployed on your own HTTPS-enabled server, then your customer has just sent you back something you already have. Ho hum.

• If the hostname is for a machine that you control and your customer doesn’t (e.g., your development server) then your customer has just tried to get a certificate on your behalf. That’s a bit of a no-no, but I advise taking it well — especially if you’ve not yet set up HTTPS. For the purposes of testing, you can get your own single-host certificate (that signs a public key where you’ve generated the private key yourself) for next to nothing. It’s also a reasonable expense to bill your customer.

• If the hostname is for the machine where the customer has told you they want to deploy your code in production, then they’ve just given you something you don’t really need. I suppose it might be relevant for client code that wants to connect to the deployment server, but that’s not as useful as all that; certificates expire, stuff moves round, and all sorts of things happen in production that can mean that it is useful to issue a new server certificate. Having to push updates to all the deployed clients just because someone accidentally deleted the server certificate without keeping a backup (a more common thing than you might wish) would Truly Suck. Thus, the deployment host certificate is not something you should need.

• If its none of these, and it’s a long lived certificate (check the Validity field from the information you printed out before) then it might actually be the certificate of a back end service that you’re supposed to talk to. Or the certificate of a private CA that signs all the certificates of the back-end services that you talk to. (Are you doing this? I don’t know, and I don’t know your app, but it’s quite possible.) In this case you would add the certificate to the list of trusted certificates in your code (the exact way depends on how your code handles SSL) and this is the only use I can think of for a certificate at the stage you’re at.

  Trouble is, I don’t think (on the basis of what you write) that it’s all that likely. Talk to your customer; security is something where you want to get it right, and use and trust of certificates is key to that.

If it’s truly none of the above, talk to the customer and say you’re a bit confused. I know I am in this case!