SSL/TLS Strong Encryption: How-To

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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
Cipher Suites and Enforcing Strong Security
Client Authentication and Access Control
Logging

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

SSLCipherSuite HIGH:!aNULL:!MD5

While with the following configuration you specify a preference for specific speed-optimized ciphers (which will be selected by mod_ssl, provided that they are supported by the client):

httpd.conf

SSLCipherSuite RC4-SHA:AES128-SHA:HIGH:!aNULL:!MD5 SSLHonorCipherOrder on

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:!aNULL:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL <Location /strong/area> # but https://hostname/strong/area/ and below # requires strong ciphers SSLCipherSuite HIGH:!aNULL:!MD5 </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.