SSL/TLS Strong Encryption: How-To

SSL/TLS Strong Encryption: How-To

This document 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 minimum, the following directives.

LoadModule ssl_module modules/mod_ssl.so

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:

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):

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:+EXP:+eNULL

<Location "/strong/area">
# but https://hostname/strong/area/ and below
# requires strong ciphers
SSLCipherSuite HIGH:!aNULL:!MD5
</Location>

OCSP Stapling

The Online Certificate Status Protocol (OCSP) is a mechanism for determining whether or not a server certificate has been revoked, and OCSP Stapling is a special form of this in which the server, such as httpd and mod_ssl, maintains current OCSP responses for its certificates and sends them to clients which communicate with the server. Most certificates contain the address of an OCSP responder maintained by the issuing Certificate Authority, and mod_ssl can communicate with that responder to obtain a signed response that can be sent to clients communicating with the server.

Because the client can obtain the certificate revocation status from the server, without requiring an extra connection from the client to the Certificate Authority, OCSP Stapling is the preferred way for the revocation status to be obtained. Other benefits of eliminating the communication between clients and the Certificate Authority are that the client browsing history is not exposed to the Certificate Authority and obtaining status is more reliable by not depending on potentially heavily loaded Certificate Authority servers.

Because the response obtained by the server can be reused for all clients using the same certificate during the time that the response is valid, the overhead for the server is minimal.

Once general SSL support has been configured properly, enabling OCSP Stapling generally requires only very minor modifications to the httpd configuration — the addition of these two directives:

SSLUseStapling On
SSLStaplingCache "shmcb:logs/ssl_stapling(32768)"

These directives are placed at global scope (i.e., not within a virtual host definition) wherever other global SSL configuration directives are placed, such as in conf/extra/httpd-ssl.conf for normal open source builds of httpd, /etc/apache2/mods-enabled/ssl.conf for the Ubuntu or Debian-bundled httpd, etc.

The path on the SSLStaplingCache directive (e.g., logs/) should match the one on the SSLSessionCache directive. This path is relative to ServerRoot.

This particular SSLStaplingCache directive requires mod_socache_shmcb (from the shmcb prefix on the directive's argument). This module is usually enabled already for SSLSessionCache or on behalf of some module other than mod_ssl. If you enabled an SSL session cache using a mechanism other than mod_socache_shmcb, use that alternative mechanism for SSLStaplingCache as well. For example:

SSLSessionCache "dbm:logs/ssl_scache"
SSLStaplingCache "dbm:logs/ssl_stapling"

You can use the openssl command-line program to verify that an OCSP response is sent by your server:

$ openssl s_client -connect www.example.com:443 -status -servername www.example.com
...
OCSP response: 
======================================
OCSP Response Data:
    OCSP Response Status: successful (0x0)
    Response Type: Basic OCSP Response
...
    Cert Status: Good
...

The following sections highlight the most common situations which require further modification to the configuration. Refer also to the mod_ssl reference manual.

If more than a few SSL certificates are used for the server

OCSP responses are stored in the SSL stapling cache. While the responses are typically a few hundred to a few thousand bytes in size, mod_ssl supports OCSP responses up to around 10K bytes in size. With more than a few certificates, the stapling cache size (32768 bytes in the example above) may need to be increased. Error message AH01929 will be logged in case of an error storing a response.

If the certificate does not point to an OCSP responder, or if a different address must be used

Refer to the SSLStaplingForceURL directive.

You can confirm that a server certificate points to an OCSP responder using the openssl command-line program, as follows:

$ openssl x509 -in ./www.example.com.crt -text | grep 'OCSP.*http'
OCSP - URI:http://ocsp.example.com

If the OCSP URI is provided and the web server can communicate to it directly without using a proxy, no configuration is required. Note that firewall rules that control outbound connections from the web server may need to be adjusted.

If no OCSP URI is provided, contact your Certificate Authority to determine if one is available; if so, configure it with SSLStaplingForceURL in the virtual host that uses the certificate.

If multiple SSL-enabled virtual hosts are configured and OCSP Stapling should be disabled for some

Add SSLUseStapling Off to the virtual hosts for which OCSP Stapling should be disabled.

If the OCSP responder is slow or unreliable

Several directives are available to handle timeouts and errors. Refer to the documentation for the SSLStaplingFakeTryLater, SSLStaplingResponderTimeout, and SSLStaplingReturnResponderErrors directives.

If mod_ssl logs error AH02217

AH02217: ssl_stapling_init_cert: Can't retrieve issuer certificate!

In order to support OCSP Stapling when a particular server certificate is used, the certificate chain for that certificate must be configured. If it was not configured as part of enabling SSL, the AH02217 error will be issued when stapling is enabled, and an OCSP response will not be provided for clients using the certificate.

Refer to the SSLCertificateChainFile and SSLCertificateFile for instructions for configuring the certificate chain.

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.

# 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:

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:

SSLVerifyClient      none
SSLCACertificateFile "conf/ssl.crt/ca.crt"
SSLCACertificatePath "conf/ssl.crt"

<Directory "/usr/local/apache2/htdocs/secure/area">
    SSLVerifyClient      require
    SSLVerifyDepth       5
    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.

/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:

SSLVerifyClient      none
SSLCACertificateFile "conf/ssl.crt/ca.crt"
SSLCACertificatePath "conf/ssl.crt"

<Directory "/usr/local/apache2/htdocs/secure/area">
  SSLVerifyClient      require
  SSLVerifyDepth       5
  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.

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

<Directory "/usr/local/apache2/htdocs">
    #   Outside the subarea only Intranet access is granted
    Require              ip 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
    Require              ip 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.