Go: HTTPS Request using a Client Certificate stored on a SmartCard (Windows)

Question

To perform client certificate authentication (mutual authentication) all examples I've found assume that a private key is accessible (e.g. from a file). A certificate containing private and public key is generated like this:

cert, err := tls.LoadX509KeyPair("certs/client.pem", "certs/client.key")

Now, I have to get the certificate (and private key, which as far as I know can't be extracted - signing should be done via PKCS#11) from a SmartCard. So far I was able to enumerate the certificates from the Windows certificate store:

store, err := syscall.UTF16PtrFromString("MY")
storeHandle, err := syscall.CertOpenSystemStore(0, store)
if err != nil {
    fmt.Println(syscall.GetLastError())
}

var certs []*x509.Certificate
var cert *syscall.CertContext
for {
    cert, err = syscall.CertEnumCertificatesInStore(storeHandle, cert)
    if err != nil {
        if errno, ok := err.(syscall.Errno); ok {
            if errno == CRYPT_E_NOT_FOUND {
                break
            }
        }
        fmt.Println(syscall.GetLastError())
    }
    if cert == nil {
        break
    }
    // Copy the buf, since ParseCertificate does not create its own copy.
    buf := (*[1 << 20]byte)(unsafe.Pointer(cert.EncodedCert))[:]
    buf2 := make([]byte, cert.Length)
    copy(buf2, buf)
    if c, err := x509.ParseCertificate(buf2); err == nil {
        for _, value := range c.ExtKeyUsage {
            if value == x509.ExtKeyUsageClientAuth {
                fmt.Println(c.Subject.CommonName)
                fmt.Println(c.Issuer.CommonName)
                certs = append(certs, c)
            }
        }
    }
}

The this way retrieved certificate is indeed from the SmartCard. When using it later on, the authentication fails:

cer:= tls.Certificate{Certificate: [][]byte{certs[0].Raw}, Leaf: certs[0],}

tlsConfig := &tls.Config{
    Certificates:       []tls.Certificate{cer},
    RootCAs:            caCertPool,
    InsecureSkipVerify: true,
}

transport := &http.Transport{TLSClientConfig: tlsConfig}

client := http.Client{
    Timeout:   time.Minute * 2,
    Transport: transport,
}

I guess the failure is to be expected as I didn't provide a private key.

Java (SunMSCAPI)and .NET seem to under the covers use the private key on the SmartCard, e.g. I do pretty much the same as above and the authentication "just works".

Is there any way to achieve this with Go?

Usquebaugh

Answer 1

The private key you specify for your tls.Certificate can be any object that implements crypto.Signer which, per the documentation:

is an interface for an opaque private key that can be used for signing operations. For example, an RSA key kept in a hardware module.

and is intended exactly for this kind of use.

Implementing the interface is fairly straightforward once you have access to the underlying key. thalesignite/crypto11 provides such an implementation for PKCS#11 keys, for example.

Tiler

Answer 2

You can use your vendor's PKCS11 file + crypto11 library.

package main

import (
    "crypto/tls"
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
    "github.com/ThalesIgnite/crypto11"
)

func main() {
    config := crypto11.Config{
        Path: "C:\\Windows\\System32\\vendor-pkcs11.dll",
        TokenSerial: "123456789456123",
        Pin: "123456",
    }

    context, err := crypto11.Configure(&config)
    if err != nil{
        log.Fatalln(err)
    }

    certificates, err := context.FindAllPairedCertificates()
    if err != nil{
        log.Fatalln(err)
    }

    fmt.Println("total certificates: ", len(certificates))

    cert := certificates[0]
    client := &http.Client{
        Transport: &http.Transport{
            TLSClientConfig: &tls.Config{
                Certificates: []tls.Certificate{cert},
                Renegotiation:      tls.RenegotiateOnceAsClient,
            },
        },
    }

    req, err := http.NewRequest("GET", "https://server.cryptomix.com:443/secure/", nil)
    if err != nil {
        log.Fatalln(err)
    }

    req.Header.Set("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.135 Safari/537.36")

    resp, err := client.Do(req)
    if err != nil {
        log.Fatalln(err)
    }

    fmt.Println("status code: ", resp.StatusCode)

    if resp.StatusCode == http.StatusOK {
        bodyBytes, err := ioutil.ReadAll(resp.Body)
        if err != nil {
            log.Fatal(err)
        }
        bodyString := string(bodyBytes)
        fmt.Println(bodyString)
    }
}

Christi