How to solve "Could not establish trust relationship for the SSL/TLS secure channel with authority"

As a workaround you could add a handler to the ServicePointManager's ServerCertificateValidationCallback on the client side:

System.Net.ServicePointManager.ServerCertificateValidationCallback +=
    (se, cert, chain, sslerror) =>
        {
            return true;
        };

but be aware that this is not a good practice as it completely ignores the server certificate and tells the service point manager that whatever certificate is fine which can seriously compromise client security. You could refine this and do some custom checking (for certificate name, hash etc). at least you can circumvent problems during development when using test certificates.

When I have this problem it is because the client.config had its endpoints like:

 https://myserver/myservice.svc 

but the certificate was expecting

 https://myserver.mydomain.com/myservice.svc

Changing the endpoints to match the FQDN of the server resolves my problem. I know this is not the only cause of this problem.

Your problem arises because you're using a self signed key. The client does not trust this key, nor does the key itself provide a chain to validate or a certificate revocation list.

You have a few options - you can

1. turn off certificate validation on the client (bad move, man in the middle attacks abound)

2. use makecert to create a root CA and create certificates from that (ok move, but there is still no CRL)

3. create an internal root CA using Windows Certificate Server or other PKI solution then trust that root cert (a bit of a pain to manage)

4. purchase an SSL certificate from one of the trusted CAs (expensive)