SSL/TLS Strong Encryption: How-To

Available Languages: en

+ +

+
The solution of this problem is trivial +and is left as an exercise for the reader.
+ +
-- Standard textbook cookie
+

+ +

How to solve particular security constraints for an SSL-aware +webserver is not always obvious because of the coherences between SSL, +HTTP and Apache's way of processing requests. This chapter gives +instructions on how to solve such 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 coherences.

Cipher Suites and Enforced Strong Security
Client Authentication and Access Control

Cipher Suites and Enforced Strong Security

+ +

SSLv2 only server
strong encryption only server
server gated cryptography
stronger per-directory requirements

+ +

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 details + you can find in the README.GlobalID document in the + mod_ssl distribution. In short: The server has a Global ID server + certificate, signed by a special CA certificate from Verisign which + enables strong encryption in export browsers. This works as following: + The browser connects with an export cipher, the server sends its Global + ID certificate, the browser verifies it and subsequently upgrades the + cipher suite before any HTTP communication takes place. The question + now is: How can we allow this upgrade, but enforce strong encryption. + Or in other words: Browser either have to initially connect with + strong encryption or have to upgrade to strong encryption, but are + not allowed to keep the export ciphers. The following does the trick:

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 you cannot just use a server-wide SSLCipherSuite which restricts the + ciphers to the strong variants. But mod_ssl allows you to reconfigure + the cipher suite in per-directory context and automatically forces + a renegotiation of the SSL parameters to meet the new configuration. + So, the solution is:

+ # 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

+ +

simple certificate-based client authentication
selective certificate-based client authentication
particular certificate-based client authentication
intranet vs. internet authentication

+ +

How can I authenticate clients based on certificates when I know +all my clients?

+ +

When you know your user community (i.e. a closed user group + situation), as it's the case for instance in an Intranet, you can + use plain certificate authentication. All you have 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 authenticate my clients for a particular URL based on +certificates but still allow arbitrary clients to access the remaining +parts of the server?

+ +

For this we again use the per-directory reconfiguration feature + of mod_ssl:

+ +

httpd.conf

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

+ + +

How can I authenticate only particular clients for a some URLs based +on certificates but still allow arbitrary clients to access the remaining +parts of the server?

+ +

The key is to check for various ingredients of the client certificate. + Usually this means to check the whole or part of the Distinguished + Name (DN) of the Subject. For this two methods exists: The mod_auth based variant and the SSLRequire variant. The first method is + good when the clients are of totally different type, i.e. when their + DNs have no common fields (usually the organisation, etc.). In this + case you've to establish a password database containing all + clients. The second method is better when your clients are all part of + a common hierarchy which is encoded into the DN. Then you can match + them more easily.

+ +

The first method:

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
+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

+ +

The second method:

+ +

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 a subarea on the +Intranet website for clients coming from the Internet but still allow +plain HTTP access for clients on the Intranet?

+ +

Let us assume the Intranet can be distinguished through the IP + network 192.168.1.0/24 and the subarea on the Intranet website has + the URL /subarea. Then configure the following outside + your HTTPS virtual host (so 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"
+AuthUserFile         conf/protected.passwd
+Require              valid-user
+</Directory>

+ +