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. 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 that 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 certificate 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 Web browsers, and computers, that support SSL 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:

  1. Create a private and public encryption key pair.

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

  3. 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.

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

  5. 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.

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. 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.

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.

  1. First, create the directories to hold the CA certificate and related files:

    sudo mkdir /etc/ssl/CA
    sudo mkdir /etc/ssl/newcerts
  2. 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
  3. 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
  4. 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.

  5. Now install the root certificate and key:

    sudo mv cakey.pem /etc/ssl/private/
    sudo mv cacert.pem /etc/ssl/certs/
  6. 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.

  7. 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.

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

  8. 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.