How to make S/MIME certificates available to mail recipients

Question

I would like to use S/MIME encryption with our emails. I think that I understand how things are working and I am able to send encrypted mails between people when they have all the necessary certificates installed.

But in real life, when starting the show, people might not have my S/MIME certificate installed on their devices. How do they get them?

Option 1) Do I have to send the certificates to them using a standard email and send the certificate as an attachment? (For example, Microsoft Outlook does not allow that by default...)

Option 2) Is my public key automatically appended to my mails as soon as I have them installed? (But then, I would receive many, many emails with public key attachments...)

Option 3) Is my public key automatically appended to my mails if they are encrypted (using my private key)? This would be the easiest solution in my opinion. But is this the case?

If none of the above is true, how do the recipients get my certificate so that they can decrypt my mails?

Answer 1

There are generally two ways of making one's S/MIME certificates with the public key available to other email users.

1. Signed emails

Once you have set up S/MIME in an email client, you would usually configure it to sign all emails outgoing emails. Most - if not all - mail clients will automatically embed the S/MIME certificate in that S/MIME signature.

The recipient receives the signed email and can import the embedded S/MIME certificate. Some mail clients can be configured to do this automatically, but it should always be doable manually. Once the S/MIME certificate has been imported, the recipient can start sending encrypted email to the certificate owner.

2. Directories

S/MIME certificates can be published in LDAP based directories, and mail clients can be configured to automatically retrieve them from the directory.

Through this method, it becomes unnecessary to exchange signed emails first before encrypted communication can be established. However, it requires both email clients to be configured to look up the proper directory. This method is often used for users that are within the same organizations. Sometimes several organizations share a common directory to enable and simplify encrypted communication between their users.

However, there is not really something like public S/MIME directories as there are PGP key servers for PGP encryption. I know of the Open Keys initiative (ldap.openkeys.de), and the Zertificon Global Trustpoint (ldap.globaltrustpoint.com), but I doubt that they are used a lot.

PKI and Trust

Their respective Public Key Infrastructure (PKI) are the main difference between email encryption with S/MIME and PGP. S/MIME relies on root certificates as trust anchors, while PGP uses a distributed trust methodology ('web of trust'), where every user decides on more or less their own if they can trust a key or not.

Answer 2

if they are encrypted (using my private key)?

Messages are not encrypted using your private key (or any private key at all). They're only decrypted using one.

(They are, however, signed using your private key – but that's a separate feature in S/MIME and is not to be confused with encryption! The contents of a signed message may be packaged in a way that looks similar to an encrypted message, but that does not mean any of it is actually encrypted.)

how do the recipients get my certificate so that they can decrypt my mails?

Recipients do not need your certificate to decrypt your mails. They only need their own certificate – or rather, the private key that goes with it.

Instead, you need the recipient's certificate so that you can encrypt the message for that particular recipient.

So it does not make sense for you to send anything together with the message (certainly not a private key – the whole point of public-key systems is that the private key is never sent anywhere, ever); instead the recipient has to send you their certificate first.

You might be confusing S/MIME encryption with signing, which is opposite in many ways: you use your own private key to create a digital signature, and the recipient indeed needs your certificate so that they could verify that signature. (But not necessarily to read the message – they can just open it unverified if they wish. Indeed most people receiving a signed message don't verify it.)

When signing messages, your own certificate can indeed be included within the signed data; I believe most mail apps do so by default with S/MIME. This works under the assumption that your certificate has been issued by a CA that the recipient "trusts". (With OpenPGP it's also an option but not as common, due to the different nature of OpenPGP certificate verification.)

This is also one way that you can collect certificates for encryption – the other person sends you a signed "Hello!", you verify it, and hopefully, your mail client stores the person's certificate in the address book. (At least some mail clients have an option to do this, manually or automatically.)