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SSL/TLS Strong Encryption: How-To

The solution to this problem is trivial and is left as an exercise for the reader.

-- Standard textbook cookie

How to solve particular security problems for an SSL-aware webserver is not always obvious because of the interactions between SSL, HTTP and Apache's way of processing requests. This chapter gives instructions on how to solve some typical situations. Treat it as a first step to find out the final solution, but always try to understand the stuff before you use it. Nothing is worse than using a security solution without knowing its restrictions and how it interacts with other systems.

Cipher Suites and Enforcing Strong Security

How can I create a real SSLv2-only server?

The following creates an SSL server which speaks only the SSLv2 protocol and its ciphers.

httpd.conf

SSLProtocol -all +SSLv2 SSLCipherSuite SSLv2:+HIGH:+MEDIUM:+LOW:+EXP

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

The following enables only the seven strongest ciphers:

httpd.conf

SSLProtocol all SSLCipherSuite HIGH:MEDIUM

How can I create an SSL server which accepts strong encryption only, but allows export browsers to upgrade to stronger encryption?

This facility is called Server Gated Cryptography (SGC) and requires a Global ID server certificate, signed by a special CA certificate from Verisign. This enables strong encryption in 'export' versions of browsers, which traditionally could not support it (because of US export restrictions).

When a browser connects with an export cipher, the server sends its Global ID certificate. The browser verifies this, and can then upgrade its cipher suite before any HTTP communication takes place. The problem lies in allowing browsers to upgrade in this fashion, but still requiring strong encryption. In other words, we want browsers to either start a connection with strong encryption, or to start with export ciphers but upgrade to strong encryption before beginning HTTP communication.

This can be done as follows:

httpd.conf

# allow all ciphers for the initial handshake, # so export browsers can upgrade via SGC facility SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL <Directory /usr/local/apache2/htdocs> # but finally deny all browsers which haven't upgraded SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128 </Directory>

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:MEDIUM </Location>

Client Authentication and Access Control

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

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

<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.

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
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.

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>