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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.txt> 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
[http://dovecot.org/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
[http://dovecot.org/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.
Certificate Authorities
-----------------------
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: get it from an external CA,
or create your own CA.
The clients have a built-in list of trusted CAs, so getting it from one of
those CAs will have the advantage of the certificate working without any client
configuration. There are already-trusted-CAs where you can buy the certificate,
and there are already-trusted-CAs where you can get your certificate for free.
On the oher hand, if you create your own CA, you'll have to install the CA
certificate to all the clients (see <SSL.CertificateClientImporting.txt>).
So the two options end up being three:
* a) Get it from an external CA (which already have the public keys installed
on the clients, so the clients trust the CA)
* a.1) Purchase the certificate.
* a.2) Get a free certificate.
* b) Create your own CA (in this case you'll have to add the CA public keys
into the clients, as you are bot trusted by default)
If you choose "a.2)", there is letsencrypt.org [https://letsencrypt.org/] where
you need to do some technical effort to demonstrate to a robot that you own the
domains for which the certificate is issued. Let's encrypt is an initiative of
the Internet Security Research Group (ISRG), with board members from Cisco,
Mozilla and the University of Michingan among others and technical advisors
from Akamai, Google, Electronic Frontier Foundation, Internet Sociaty and
independents among others. Let's encrypt CA is trusted by default by many
clients. If you can control the entries in your DNS, you'll be able to
demonstrate to the robot that "you are you" for domain-based identities. There
is this specific conversation about letsencrypt + dovecot here
[https://community.letsencrypt.org/t/simple-guide-using-lets-encrypt-ssl-certs-with-dovecot/2921].
If you choose "b)", there are multiple different tools for managing your own
CA. The simplest way is to use a CA managing tool as gnoMint
[http://gnomint.sourceforge.net/] or TinyCA [http://tinyca.sm-zone.net/].
However, if you need to tailor the properties of the CA, you always can use
OpenSSL, very much customizable, but however a bit cumbersome.
(This file was created from the wiki on 2019-06-19 12:42)
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