SSL/TLS Strong Encryption: How-To
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
- 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 strongest ciphers:
httpd.conf
SSLProtocol all -SSLv2
SSLCipherSuite HIGH:!aNULL:!MD5
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:!aNULL:!MD5
</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>