use iSECPartners code to validate hostname in certificate

The problem is that if you go to a website whose certificate does not
match its hostname, it should fail.  Try this in a web browser for
https://www.kegel.com/ for example.  Your web browser will say the
certificate is for *.pair.com, not for www.kegel.com, and won't let
you visit it without clicking through a bunch of scary warnings.

However, prior to this commit, https-client was happy to fetch
https://www.kegel.com/ without complaining.  That is bad.  Now, with
this commit, it will properly complain, which is good:

pelletier@chives:~/src/libevent/sample$ ./https-client https://www.kegel.com/
Got 'MatchNotFound' for hostname 'www.kegel.com' and certificate:
/C=US/postalCode=15203/ST=Pennsylvania/L=Pittsburgh/street=Suite 210/street=2403 Sidney Street/O=pair Networks, Inc./OU=Provided by pair Networks, Inc./OU=PairWildcardSSL $250,000/CN=*.pair.com
some request failed - no idea which one though!
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
ppelletier@chives:~/src/libevent/sample$

It will still succeed for sites with an exactly-matching certificate,
such as https://github.com/ and that is also good!

However, the problem is that the iSECPartners code doesn't handle
wildcards, which means we reject https://ip.appspot.com/ even though
it is perfectly legitimate, because we don't understand the wildcard:

ppelletier@chives:~/src/libevent/sample$ ./https-client https://ip.appspot.com/
Got 'MatchNotFound' for hostname 'ip.appspot.com' and certificate:
/C=US/ST=California/L=Mountain View/O=Google Inc/CN=*.appspot.com
some request failed - no idea which one though!
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
ppelletier@chives:~/src/libevent/sample$

So, we need to fix this.  In other words, "to be continued..."

1 files changed, 56 insertions, 0 deletions

diff --git a/sample/openssl_hostname_validation.h b/sample/openssl_hostname_validation.h
new file mode 100644
index 00000000..54aa1c43
--- /dev/null
+++ b/sample/openssl_hostname_validation.h
@@ -0,0 +1,56 @@
+/* Obtained from: https://github.com/iSECPartners/ssl-conservatory */
+
+/*
+Copyright (C) 2012, iSEC Partners.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+ */
+
+/*
+ * Helper functions to perform basic hostname validation using OpenSSL.
+ *
+ * Please read "everything-you-wanted-to-know-about-openssl.pdf" before
+ * attempting to use this code. This whitepaper describes how the code works,
+ * how it should be used, and what its limitations are.
+ *
+ * Author:  Alban Diquet
+ * License: See LICENSE
+ *
+ */
+
+typedef enum {
+        MatchFound,
+        MatchNotFound,
+        NoSANPresent,
+        MalformedCertificate,
+        Error
+} HostnameValidationResult;
+
+/**
+* Validates the server's identity by looking for the expected hostname in the
+* server's certificate. As described in RFC 6125, it first tries to find a match
+* in the Subject Alternative Name extension. If the extension is not present in
+* the certificate, it checks the Common Name instead.
+*
+* Returns MatchFound if a match was found.
+* Returns MatchNotFound if no matches were found.
+* Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
+* Returns Error if there was an error.
+*/
+HostnameValidationResult validate_hostname(const char *hostname, const X509 *server_cert);