SSL certificate creation
Self-signed SSL certificates
Self-signed SSL certificates are the easiest way to get your SSL server working. However unless you take some action to prevent it, this is at the cost of security:
- The first time the client connects to the server, it sees the certificate and asks the user whether to trust it. The user of course doesn't really bother verifying the certificate's fingerprint, so a man-in-the-middle attack can easily bypass all the SSL security, steal the user's password and so on.
- If the client was lucky enough not to get attacked the first time it connected, the following connections will be secure as long as the client had permanently saved the certificate. Some clients do this, while others have to be manually configured to accept the certificate.
The only way to be fully secure is to import the SSL certificate to client's (or operating system's) list of trusted CA certificates prior to first connection. See SSL/CertificateClientImporting how to do it for different clients.
Self-signed certificate creation
Dovecot includes a script to build self-signed SSL certificates using OpenSSL. In the source distribution this exists in doc/mkcert.sh. Binary installations usually create the certificate automatically when installing Dovecot and don't include the script.
The SSL certificate's configuration is taken from doc/dovecot-openssl.cnf file. Modify the file before running mkcert.sh. Especially important field is the CN (Common Name) field, which should contain your server's host name. The clients will verify that the CN matches the connected host name, otherwise they'll say the certificate is invalid. It's also possible to use wildcards (eg. *.domain.com) in the host name. They should work with most clients.
By default the certificate is created to /etc/ssl/certs/dovecot.pem and the private key file is created to /etc/ssl/private/dovecot.pem. Also by default the certificate will expire in 365 days. If you wish to change any of these, modify the mkcert.sh script.
The correct way to use SSL is to have each SSL certificate signed by an Certificate Authority (CA). The client has a list of trusted Certificate Authorities, so whenever it sees a new SSL certificate signed by a trusted CA, it will automatically trust the new certificate without asking the user any questions.
There are two ways to get a CA signed certificate: buy it, or create your own CA. The clients have a built-in list of trusted CAs, so buying from one of those CAs will have the advantage of the certificate working without any client configuration. If you create your own CA, you'll have to install the CA certificate to all the clients (see SSL/CertificateClientImporting).
There are multiple different tools for managing your own CA. The simplest way is to use a CA managing tool as gnoMint or TinyCA. However, if you need to tailor the properties of the CA, you always can use OpenSSL, very much customizable, but however a bit cumbersome.