SSL/TLS Strong Encryption: How-To
The solution to this problem is trivialand is left as an exercise for the reader.
-- Standard textbook cookie
How to solve particular security problems for an SSL-awarewebserver is not always obvious because of the interactions between SSL,HTTP and Apache's way of processing requests. This chapter givesinstructions on how to solve some typical situations. Treat it as a firststep to find out the final solution, but always try to understand the stuff before you use it. Nothing is worse than using a security solutionwithout 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 encryptiononly?
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 encryptiononly, 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 ciphersin general, but requires a strong ciphers for access to a particularURL?
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 aparticular 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 aparticular 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 requireSSLVerifyDepth 5SSLCACertificateFile conf/ssl.crt/ca.crtSSLCACertificatePath conf/ssl.crtSSLOptions +FakeBasicAuthSSLRequireSSLAuthName "Snake Oil Authentication"AuthType BasicAuthBasicProvider fileAuthUserFile /usr/local/apache2/conf/httpd.passwdRequire 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 basicauthentication or client certificates, for access to part of theIntranet website, for clients coming from the Internet? I still want to allowplain 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 grantedOrder deny,allowDeny from allAllow 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 optionalSSLVerifyDepth 1SSLOptions +FakeBasicAuth +StrictRequireSSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128# Force clients from the Internet to use HTTPSRewriteEngine onRewriteCond %{REMOTE_ADDR} !^192.168.1.[0-9]+$RewriteCond %{HTTPS} !=onRewriteRule .* - [F]# Allow Network Access and/or Basic AuthSatisfy any# Network Access ControlOrder deny,allowDeny from allAllow 192.168.1.0/24# HTTP Basic AuthenticationAuthType basicAuthName "Protected Intranet Area"AuthBasicProvider fileAuthUserFile conf/protected.passwdRequire valid-user</Directory>
