First of all, what is SSL pinning?

According to Wikipedia, SSL pinning is:

HTTP Public Key Pinning (HPKP) is an Internet security mechanism delivered via an HTTP header which allows HTTPS websites to resist impersonation by attackers using mis-issued or otherwise fraudulent certificates. In order to do so, it delivers a set of public keys to the client (browser), which should be the only ones trusted for connections to this domain.

Basically, SSL pinning prevents Man In The Middle attack. (You can make a Google search for learning more about this.)

Let's start implementing with Alamofire.

First of all we need to get SSL certificate from our server with:

openssl s_client -showcerts -connect yourdomain.com:443 < /dev/null | openssl x509 -outform DER > yourdomain.cer

Change the yourdomain.com with your server's address.

We got our SSL certificate.

We need a SessionManager for making request with SSL pinning:

let sessionManager: SessionManager = {
    let serverTrustPolicies: [String: ServerTrustPolicy] = ["yourdomain.com": .pinCertificates(certificates: ServerTrustPolicy.certificates(),
                                                                                                validateCertificateChain: true,
                                                                                                validateHost: true)]

    return SessionManager(serverTrustPolicyManager: ServerTrustPolicyManager(policies: serverTrustPolicies))
}()

The only thing you need to check is you should add your domain as yourdomain.com, not https://yourdomain.com.

Yes! We got the Manager!

Now we can make all our request from that Manager. If the certificate is not valid, we will get error from Alamofire.

You can test your implementation with:

sessionManager.request("https://yourdomain.com").responseString { (dataResponse) in
    switch dataResponse.result {
    case .failure(let err):
        print(err)
    case .success(let val):
        print(val)
        if let headerFields = dataResponse.response?.allHeaderFields {
            print(headerFields)
        }
    }
}

That's it. You succesfully implemented SSL Certificate pinning with Alamofire.