Golang - Mutual TLS with Identity from Keychain cannot use signer (type crypto.Signer) as type []byte in argument to tls.X509KeyPair

Question

I am trying to find a way to keychain https client certificate based authentication from macOS system. When I export the certificate and the key I can already successfully connect to my server, but that is not my goal. From the keychain it looks more difficult. I can't get the private key into the TLS Certificate Config. Maybe someone has an idea or a better way. Thanks a lot

    package main 

import (
    "crypto/tls"
    "io/ioutil"
    "log"
    "net/http"
    "os"
    "github.com/github/certstore"
    "crypto/x509"
)
 
func HttpClient() (client *http.Client) {

    // Open Keychain Certificate System Store
    store, err := certstore.Open()
    if err != nil {
        log.Println(err)
    }
    defer store.Close()

    // Get all in Identities
    idents, err := store.Identities()
    if err != nil {
        log.Println(err)
    }

    // Iterate through the identities, looking for the one we want.
    var me certstore.Identity
    for _, ident := range idents {
        defer ident.Close()

            crt, errr := ident.Certificate()
            if errr != nil {
                log.Println(errr)
            }
                if crt.Subject.CommonName == "SCEP Identity UUID12345" {
                    me = ident
                    log.Println(crt.Subject.CommonName)
                    log.Println("Org: ",crt.Subject.Organization)
                    log.Println("Signer:", ident.Signer)        

    // Everything works fine when i load the cert and key from disk.
    // x509cert, err := tls.LoadX509KeyPair("client.pem", "client.key")
    chain, _  := ident.CertificateChain()
    signer, er := me.Signer()
                if er != nil {
                log.Println("PrivateKey Error:",er)
            }

    certsnew := tls.Certificate{
        Leaf:        chain[0],
        Certificate: serializeChain(chain),
        PrivateKey:  signer,
    }


    // Test to create KeyPair but does not work 
    // Error:  cannot use signer (type crypto.Signer) as type []byte in argument to tls.X509KeyPair
    tlsKeyPair, err := tls.X509KeyPair(chain[0].Raw, signer)



    certs := []tls.Certificate{certsnew}
    
    // if len(certs) == 0 {
    //          client = &http.Client{}
    //          return
    // }
    
    tr := &http.Transport{
        TLSClientConfig: &tls.Config{Certificates: certs,
        InsecureSkipVerify: true,
        Renegotiation:      tls.RenegotiateOnceAsClient},
    }
    client = &http.Client{Transport: tr}
                }
    } 
    return
}