Are you missing a call to req.setClientCertificateName?
I have APEX code where Salesforce calls out to a web service on my site. I protected it with client-side SSL. My website, the host, authorizes the client cert from Salesforce.com (vs traditional web SSL where the browser client authorizes the server cert). You can create a self-signed certificate in Salesforce Admin under Certificate and Key Management and then reference it with a call to req.setClientCertificateName. Here is some code from my production org:
HttpRequest req = new HttpRequest();
req.setMethod('POST');
req.setHeader('Host', 'www.mywebsite.com');
req.setEndpoint('https://www.mywebsite.com/post.asp');
try {
req.setClientCertificateName('Cert_For_MyWebSite');
} catch (System.CalloutException e) {
// The cert doesn't make it to the sandbox
}
req.setHeader('Connection', 'keep-alive');
req.setHeader('content-type', 'text/plain');
req.setHeader('Content-Length', body.length().format());
req.setBody(body);
Http http = new Http();
HttpResponse res = http.send(req);
System.debug(res.toString());
System.debug('STATUS:' + res.getStatus());
System.debug('STATUS_CODE:' + res.getStatusCode());
On the server (IIS 7.5) I enabled the self-signed cert with this web.config:
<configuration>
<system.webServer>
<security>
<access sslFlags="Ssl, SslNegotiateCert, SslRequireCert" />
<authentication>
<iisClientCertificateMappingAuthentication enabled="true" oneToOneCertificateMappingsEnabled="true">
<oneToOneMappings>
<!-- production salesforce -->
<add enabled="true"
userName="salesforce"
password="[enc:AesProvider:aaa...aaa:enc]"
certificate="MIIEaaa...aaa=" />
</oneToOneMappings>
</iisClientCertificateMappingAuthentication>
</authentication>
</security>
</system.webServer>
</configuration>