You can also have /loc/certs
run mkcert.pl
then copy certs/server* /etc/postfix/sasl

Certificates
One of the most common forms of cryptography today is public-key cryptography. 
Public-key cryptography utilizes a public key and a private key. The system 
works by encrypting information using the public key. The information can then 
only be decrypted using the private key.

A common use for public-key cryptography is encrypting application traffic using
a Secure Socket Layer (SSL) or Transport Layer Security (TLS) connection. One 
example: configuring Apache to provide HTTPS, the HTTP protocol over SSL/TLS. 
This allows a way to encrypt traffic using a protocol that does not itself 
provide encryption.

A certificate is a method used to distribute a public key and other information
about a server and the organization who is responsible for it. Certificates can
be digitally signed by a Certification Authority, or CA. A CA is a trusted third
party that has confirmed that the information contained in the certificate is 
accurate.



Types of Certificates

To set up a secure server using public-key cryptography, in most cases, you send
your certificate request (including your public key), proof of your company’s 
identity, and payment to a CA. The CA verifies the certificate request and your
identity, and then sends back a certificate for your secure server. 
Alternatively, you can create your own self-signed certificate.

Note
self-signed certificates should not be used in most production environments.

Continuing the HTTPS example, a CA-signed certificate provides two important 
capabilities that a self-signed certificate does not: 

Browsers (usually) automatically recognize the CA signature and allow a 
secure connection to be made without prompting the user.
When a CA issues a signed certificate, it is guaranteeing the identity of 
the organization that is providing the web pages to the browser.

Most of the software supporting SSL/TLS have a list of CAs whose certificates 
they automatically accept. If a browser encounters a certificate whose 
authorizing CA is not in the list, the browser asks the user to either accept or
decline the connection. Also, other applications may generate an error message 
when using a self-signed certificate.

The process of getting a certificate from a CA is fairly easy. A quick overview
is as follows:

Create a private and public encryption key pair.

Create a certificate signing request based on the public key. The 
certificate request contains information about your server and the company 
hosting it.

Send the certificate request, along with documents proving your identity, to
a CA. We cannot tell you which certificate authority to choose. Your decision 
may be based on your past experiences, or on the experiences of your friends or
colleagues, or purely on monetary factors.
Once you have decided upon a CA, you need to follow the instructions they 
provide on how to obtain a certificate from them.

When the CA is satisfied that you are indeed who you claim to be, they send
you a digital certificate.

Install this certificate on your secure server, and configure the 
appropriate applications to use the certificate.



Generating a Certificate Signing Request (CSR)
Whether you are getting a certificate from a CA or generating your own 
self-signed certificate, the first step is to generate a key.

If the certificate will be used by service daemons, such as Apache, Postfix, 
Dovecot, etc., a key without a passphrase is often appropriate. Not having a 
passphrase allows the services to start without manual intervention, usually the
preferred way to start a daemon.

This section will cover generating a key with a passphrase, and one without. The
non-passphrase key will then be used to generate a certificate that can be used
with various service daemons.

Warning
Running your secure service without a passphrase is convenient because you will not 
need to enter the passphrase every time you start your secure service.  But it 
is insecure and a compromise of the key means a compromise of the server as well.


To generate the keys for the Certificate Signing Request (CSR) run the following
command from a terminal prompt:
	openssl genrsa -des3 -out server.key 2048

	Generating RSA private key, 2048 bit long modulus
..........................++++++
.......++++++
e is 65537 (0x10001)
Enter pass phrase for server.key:

You can now enter your passphrase. For best security, it should at least contain
eight characters. The minimum length when specifying -des3 is four characters. 
As a best practice it should include numbers and/or punctuation and not be a 
word in a dictionary. Also remember that your passphrase is case-sensitive.

Re-type the passphrase to verify. Once you have re-typed it correctly, the 
server key is generated and stored in the server.key file.

Now create the insecure key, the one without a passphrase, and shuffle the key 
names:
	openssl rsa -in server.key -out server.key.insecure
mv server.key server.key.secure
mv server.key.insecure server.key

The insecure key is now named server.key, and you can use this file to generate
the CSR without passphrase.

To create the CSR, run the following command at a terminal prompt:
	openssl req -new -key server.key -out server.csr

It will prompt you enter the passphrase. If you enter the correct passphrase, it
will prompt you to enter Company Name, Site Name, Email Id, etc. Once you enter
all these details, your CSR will be created and it will be stored in the server.
csr file.

You can now submit this CSR file to a CA for processing. The CA will use this 
CSR file and issue the certificate. On the other hand, you can create 
self-signed certificate using this CSR.



Creating a Self-Signed Certificate

To create the self-signed certificate, run the following command at a terminal 
prompt:
	openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt

The above command will prompt you to enter the passphrase. Once you enter the 
correct passphrase, your certificate will be created and it will be stored in 
the server.crt file.

Warning
If your secure server is to be used in a production environment, you probably 
need a CA-signed certificate. It is not recommended to use self-signed certificate.




Installing the Certificate

You can install the key file server.key and certificate file server.crt, or the
certificate file issued by your CA, by running following commands at a terminal
prompt:
	sudo cp server.crt /etc/ssl/certs
sudo cp server.key /etc/ssl/private

Now simply configure any applications, with the ability to use public-key 
cryptography, to use the certificate and key files. For example, Apache can 
provide HTTPS, Dovecot can provide IMAPS and POP3S, etc.



Certification Authority

If the services on your network require more than a few self-signed certificates
it may be worth the additional effort to setup your own internal Certification 
Authority (CA). Using certificates signed by your own CA, allows the various 
services using the certificates to easily trust other services using 
certificates issued from the same CA.

First, create the directories to hold the CA certificate and related files:
	sudo mkdir /etc/ssl/CA
sudo mkdir /etc/ssl/newcerts

The CA needs a few additional files to operate, one to keep track of the last 
serial number used by the CA, each certificate must have a unique serial number,
and another file to record which certificates have been issued:
	sudo sh -c "echo '01' > /etc/ssl/CA/serial"
sudo touch /etc/ssl/CA/index.txt

The third file is a CA configuration file. Though not strictly necessary, it is
very convenient when issuing multiple certificates. Edit /etc/ssl/openssl.cnf, 
and in the [ CA_default ] change:
	dir             = /etc/ssl              # Where everything is kept
database        = $dir/CA/index.txt     # database index file.
certificate     = $dir/certs/cacert.pem # The CA certificate
serial          = $dir/CA/serial        # The current serial number
private_key     = $dir/private/cakey.pem# The private key 

Next, create the self-signed root certificate:
	openssl req -new -x509 -extensions v3_ca -keyout cakey.pem -out cacert.pem 
-days 3650

You will then be asked to enter the details about the certificate.

Now install the root certificate and key:
	sudo mv cakey.pem /etc/ssl/private/
sudo mv cacert.pem /etc/ssl/certs/

You are now ready to start signing certificates. The first item needed is a 
Certificate Signing Request (CSR), see Generating a Certificate Signing Request
(CSR) for details. Once you have a CSR, enter the following to generate a 
certificate signed by the CA:
	sudo openssl ca -in server.csr -config /etc/ssl/openssl.cnf

After entering the password for the CA key, you will be prompted to sign the 
certificate, and again to commit the new certificate. You should then see a 
somewhat large amount of output related to the certificate creation.

There should now be a new file, /etc/ssl/newcerts/01.pem, containing the same 
output. Copy and paste everything beginning with the line: 
-----BEGIN CERTIFICATE----- and continuing through the line: 
----END CERTIFICATE----- lines to a file named after the hostname of the server
where the certificate will be installed. For example mail.example.com.crt, is a
nice descriptive name.

Subsequent certificates will be named 02.pem, 03.pem, etc.

Note
Replace mail.example.com.crt with your own descriptive name.

Finally, copy the new certificate to the host that needs it, and configure the 
appropriate applications to use it. The default location to install certificates
is /etc/ssl/certs. This enables multiple services to use the same certificate 
without overly complicated file permissions.

For applications that can be configured to use a CA certificate, you should also
copy the /etc/ssl/certs/cacert.pem file to the /etc/ssl/certs/ directory on each
server.



References

The Wikipedia HTTPS page has more information regarding HTTPS.
For more information on OpenSSL see the OpenSSL Home Page.
Also, O’Reilly’s Network Security with OpenSSL is a good in-depth reference.