Perform client side certificate check

SessionDelegate.swift

import Foundation

public struct IdentityAndTrust {
    public var identityRef:SecIdentity
    public var trust:SecTrust
    public var certArray:NSArray
}

public func extractIdentity(certData:NSData, certPassword:String) -> IdentityAndTrust {
    
    var identityAndTrust:IdentityAndTrust!
    var securityError:OSStatus = errSecSuccess
    
    var items: CFArray?
    let certOptions: Dictionary = [ kSecImportExportPassphrase as String : certPassword ];
     // import certificate to read its entries
    securityError = SecPKCS12Import(certData, certOptions as CFDictionary, &items);
    if securityError == errSecSuccess {
        
        let certItems:CFArray = items as CFArray!;
        let certItemsArray:Array = certItems as Array
        let dict:AnyObject? = certItemsArray.first;
        
        if let certEntry:Dictionary = dict as? Dictionary<String, AnyObject> {
            
            // grab the identity
            let identityPointer:AnyObject? = certEntry["identity"];
            let secIdentityRef:SecIdentity = identityPointer as! SecIdentity!;
            
            // grab the trust
            let trustPointer:AnyObject? = certEntry["trust"];
            let trustRef:SecTrust = trustPointer as! SecTrust;
            
            // grab the certificate chain
            var certRef: SecCertificate?
            SecIdentityCopyCertificate(secIdentityRef, &certRef);
            let certArray:NSMutableArray = NSMutableArray();
            certArray.add(certRef as SecCertificate!);
            
            identityAndTrust = IdentityAndTrust(identityRef: secIdentityRef, trust: trustRef, certArray: certArray);
        }
    }
    
    return identityAndTrust;
}



public class SessionDelegate : NSObject, URLSessionDelegate {
    
    public func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) {
        if let localCertPath = Bundle.main.url(forResource: "{YOUR_CERT_NAME}", withExtension: "{YOUR_CERT_EXTENSION}"),
            let localCertData = try?  Data(contentsOf: localCertPath)
        {
            
            let identityAndTrust:IdentityAndTrust = extractIdentity(certData: localCertData as NSData, certPassword: "{YOUR_CERT_PWD}")
            
            if challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodClientCertificate {
                
                let urlCredential:URLCredential = URLCredential(
                    identity: identityAndTrust.identityRef,
                    certificates: identityAndTrust.certArray as [AnyObject],
                    persistence: URLCredential.Persistence.forSession);
                
                completionHandler(URLSession.AuthChallengeDisposition.useCredential, urlCredential);
                
                return
            }
        }
        
        challenge.sender?.cancel(challenge)
        completionHandler(URLSession.AuthChallengeDisposition.rejectProtectionSpace, nil)
    }
    
}

Please attach sample ViewController File

How to call this file in view VC or Button click

where is trustRef used in didReceiveChallenge?

If you have more than one certificate (SecCertificate) use this:
I used it for MBWAY authentication.

    
    struct IdentityAndTrust {
        var identityRef: SecIdentity
        var trust: SecTrust
        var certificates: [SecCertificate]
    }
    
    func extractIdentity(certData: NSData, certPassword: String) -> IdentityAndTrust? {
        
        var identityAndTrust: IdentityAndTrust?
        var securityStatus: OSStatus = errSecSuccess
        
        var items: CFArray?
        let certOptions: Dictionary = [kSecImportExportPassphrase as String : certPassword]
        securityStatus = SecPKCS12Import(certData, certOptions as CFDictionary, &items)
        if securityStatus == errSecSuccess {
            let certificateItems: CFArray = items! as CFArray
            let certItemsArray: Array = certificateItems as Array
            let dict: AnyObject? = certItemsArray.first
            
            if let certificateDict: Dictionary = dict as? Dictionary<String, AnyObject> {
                
                // get the identity
                let identityPointer: AnyObject? = certificateDict["identity"]
                let secIdentityRef: SecIdentity = identityPointer as! SecIdentity
                
                // get the trust
                let trustPointer: AnyObject? = certificateDict["trust"]
                let trustRef: SecTrust = trustPointer as! SecTrust
                
                // get the certificate chain
                var certRef: SecCertificate? // <- write on
                SecIdentityCopyCertificate(secIdentityRef, &certRef)
                var certificateArray = [SecCertificate]()
                certificateArray.append(certRef! as SecCertificate)
                
                let count = SecTrustGetCertificateCount(trustRef)
                if count > 1 {
                    for i in 1..<count {
                        if let cert = SecTrustGetCertificateAtIndex(trustRef, i) {
                            certificateArray.append(cert)
                        }
                    }
                }
                
                identityAndTrust = IdentityAndTrust(identityRef: secIdentityRef, trust: trustRef, certificates: certificateArray)
            }
        }
        
        return identityAndTrust
    }