SSL/TLS

# SSL/TLS Strong Encryption: How-To

This documented is intended to get you started, and get a few things working. You are strongly encouraged to read the rest of the SSL documentation, and arrive at a deeper understanding of the material, before progressing to the advanced techniques.

# Basic Configuration Example

Your SSL configuration will need to contain, at a minumum, the following directives.

Listen 443
<VirtualHost *:443>
ServerName www.example.com
SSLEngine on
SSLCertificateFile /path/to/www.example.com.cert
SSLCertificateKeyFile /path/to/www.example.com.key
</VirtualHost>

# Cipher Suites and Enforcing Strong Security

- How can I create an SSL server which accepts strong encryption only?
- How can I create an SSL server which accepts all types of ciphers in general, but requires a strong cipher for access to a particular URL?

## How can I create an SSL server which accepts strong encryption only?

The following enables only the strongest ciphers:

### httpd.conf

SSLProtocol all -SSLv2
SSLCipherSuite HIGH:!ADH:!EXP:!MD5:!NULL

While with the following configuration you enable two ciphers which are resonably secure, and fast:

### httpd.conf

SSLProtocol all -SSLv2
SSLCipherSuite RC4-SHA:AES128-SHA:HIGH:!ADH:!EXP:!MD5:!NULL
SSLHonorCipherOrder on

This strongly reflects the default value of SSLCipherSuite and is the recommanded way to configure it.

## How can I create an SSL server which accepts all types of ciphers in general, but requires a strong ciphers for access to a particular URL?

Obviously, a server-wide SSLCipherSuite which restricts ciphers to the strong variants, isn't the answer here. However, mod_ssl can be reconfigured within `Location` blocks, to give a per-directory solution, and can automatically force a renegotiation of the SSL parameters to meet the new configuration. This can be done as follows:

# be liberal in general
SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL
<Location /strong/area>
# but https://hostname/strong/area/ and below
# requires strong ciphers
SSLCipherSuite HIGH:!ADH:!EXP:!MD5:!NULL
</Location>

# Client Authentication and Access Control

- How can I force clients to authenticate using certificates?
- How can I force clients to authenticate using certificates for a particular URL, but still allow arbitrary clients to access the rest of the server?
- How can I allow only clients who have certificates to access a particular URL, but allow all clients to access the rest of the server?
- How can I require HTTPS with strong ciphers, and either basic authentication or client certificates, for access to part of the Intranet website, for clients coming from the Internet?

## How can I force clients to authenticate using certificates?

When you know all of your users (eg, as is often the case on a corporate Intranet), you can require plain certificate authentication. All you need to do is to create client certificates signed by your own CA certificate (`ca.crt`) and then verify the clients against this certificate.

### httpd.conf

# require a client certificate which has to be directly
# signed by our CA certificate in ca.crt
SSLVerifyClient require
SSLVerifyDepth 1
SSLCACertificateFile conf/ssl.crt/ca.crt

## How can I force clients to authenticate using certificates for a particular URL, but still allow arbitrary clients to access the rest of the server?

To force clients to authenticate using certificates for a particular URL, you can use the per-directory reconfiguration features of mod_ssl:

### httpd.conf

SSLVerifyClient none
SSLCACertificateFile conf/ssl.crt/ca.crt
<Location /secure/area>
SSLVerifyClient require
SSLVerifyDepth 1
</Location>

## How can I allow only clients who have certificates to access a particular URL, but allow all clients to access the rest of the server?

The key to doing this is checking that part of the client certificate matches what you expect. Usually this means checking all or part of the Distinguished Name (DN), to see if it contains some known string. There are two ways to do this, using either mod_auth_basic or SSLRequire.

The mod_auth_basic method is generally required when the certificates are completely arbitrary, or when their DNs have no common fields (usually the organisation, etc.). In this case, you should establish a password database containing all clients allowed, as follows:

### httpd.conf

    SSLVerifyClient      none
    <Directory /usr/local/apache2/htdocs/secure/area>

    SSLVerifyClient      require
    SSLVerifyDepth       5
    SSLCACertificateFile conf/ssl.crt/ca.crt
    SSLCACertificatePath conf/ssl.crt
    SSLOptions           +FakeBasicAuth
    SSLRequireSSL
    AuthName             "Snake Oil Authentication"
    AuthType             Basic
    AuthBasicProvider    file
    AuthUserFile         /usr/local/apache2/conf/httpd.passwd
    Require              valid-user
    </Directory>

The password used in this example is the DES encrypted string "password". See the SSLOptions docs for more information.

### httpd.passwd

    /C=DE/L=Munich/O=Snake Oil, Ltd./OU=Staff/CN=Foo:xxj31ZMTZzkVA
    /C=US/L=S.F./O=Snake Oil, Ltd./OU=CA/CN=Bar:xxj31ZMTZzkVA
    /C=US/L=L.A./O=Snake Oil, Ltd./OU=Dev/CN=Quux:xxj31ZMTZzkVA

When your clients are all part of a common hierarchy, which is encoded into the DN, you can match them more easily using SSLRequire, as follows:

### httpd.conf

    SSLVerifyClient      none
    <Directory /usr/local/apache2/htdocs/secure/area>

      SSLVerifyClient      require
      SSLVerifyDepth       5
      SSLCACertificateFile conf/ssl.crt/ca.crt
      SSLCACertificatePath conf/ssl.crt
      SSLOptions           +FakeBasicAuth
      SSLRequireSSL
      SSLRequire       %{SSL_CLIENT_S_DN_O}  eq "Snake Oil, Ltd." \
                   and %{SSL_CLIENT_S_DN_OU} in {"Staff", "CA", "Dev"}
    </Directory>

## How can I require HTTPS with strong ciphers, and either basic authentication or client certificates, for access to part of the Intranet website, for clients coming from the Internet? I still want to allow plain HTTP access for clients on the Intranet.

These examples presume that clients on the Intranet have IPs in the range 192.168.1.0/24, and that the part of the Intranet website you want to allow internet access to is `/usr/local/apache2/htdocs/subarea`. This configuration should remain outside of your HTTPS virtual host, so that it applies to both HTTPS and HTTP.

### httpd.conf

    SSLCACertificateFile conf/ssl.crt/company-ca.crt

    <Directory /usr/local/apache2/htdocs>
    #   Outside the subarea only Intranet access is granted
    Order                deny,allow
    Deny                 from all
    Allow                from 192.168.1.0/24
    </Directory>

    <Directory /usr/local/apache2/htdocs/subarea>
    #   Inside the subarea any Intranet access is allowed
    #   but from the Internet only HTTPS + Strong-Cipher + Password
    #   or the alternative HTTPS + Strong-Cipher + Client-Certificate

    #   If HTTPS is used, make sure a strong cipher is used.
    #   Additionally allow client certs as alternative to basic auth.
    SSLVerifyClient      optional
    SSLVerifyDepth       1
    SSLOptions           +FakeBasicAuth +StrictRequire
    SSLRequire           %{SSL_CIPHER_USEKEYSIZE} >= 128

    #   Force clients from the Internet to use HTTPS
    RewriteEngine        on
    RewriteCond          %{REMOTE_ADDR} !^192\.168\.1\.[0-9]+$
    RewriteCond          %{HTTPS} !=on
    RewriteRule          .* - [F]

    #   Allow Network Access and/or Basic Auth
    Satisfy              any

    #   Network Access Control
    Order                deny,allow
    Deny                 from all
    Allow                192.168.1.0/24

    #   HTTP Basic Authentication
    AuthType             basic
    AuthName             "Protected Intranet Area"
    AuthBasicProvider    file
    AuthUserFile         conf/protected.passwd
    Require              valid-user
    </Directory>

# Logging

mod_ssl can log extremely verbose debugging information to the error log, when its LogLevel is set to the higher trace levels. On the other hand, on a very busy server, level `info` may already be too much. Remember that you can configure the LogLevel per module to suite your needs.