Add OpenSSL-based S/MIME verification test

Introduce a function to locate the OpenSSL binary and use it in a new test case to verify S/MIME-signed messages. The test writes a signed message to a temporary file and uses OpenSSL to confirm its validity. This enhances test coverage for S/MIME signing functionality.

msg_test.go

@@ -18,6 +18,7 @@ import (
 	"io"
 	"net"
 	"os"
+	"os/exec"
 	"reflect"
 	"runtime"
 	"strings"
@@ -6129,7 +6130,7 @@ func TestMsg_WriteTo(t *testing.T) {
 			t.Fatalf("failed to initialize S/MIME signing: %s", err)
 		}
 		buffer := bytes.NewBuffer(nil)
-		if _, err := message.WriteTo(buffer); err != nil {
+		if _, err = message.WriteTo(buffer); err != nil {
 			t.Fatalf("failed to write message to buffer: %s", err)
 		}
 		fileContentType := "text/plain; charset=utf-8"
@@ -6767,6 +6768,35 @@ func TestMsg_WriteToTempFile(t *testing.T) {
 			t.Errorf("expected message buffer to contain Testmail, got: %s", got)
 		}
 	})
+	t.Run("WriteToTempFile S/MIME signed simple text message, verified with OpenSSL", func(t *testing.T) {
+		openSSL := getOpenSSLPath()
+		if openSSL == "" {
+			t.Skip("OpenSSL not found - skipping test")
+		}
+		message := testMessage(t)
+		keypair, err := getDummyKeyPairTLS()
+		if err != nil {
+			t.Fatalf("failed to load dummy key material: %s", err)
+		}
+		if err = message.SignWithTLSCertificate(keypair); err != nil {
+			t.Fatalf("failed to initialize S/MIME signing: %s", err)
+		}
+		msgFile, err := message.WriteToTempFile()
+		if err != nil {
+			t.Fatalf("failed to write message to buffer: %s", err)
+		}
+		t.Cleanup(func() {
+			if err := os.RemoveAll(msgFile); err != nil {
+				t.Errorf("failed to remove temp file: %s", err)
+			}
+		})
+		openSSLExec := exec.Command(openSSL, "smime", "-verify", "-noverify", "-in", msgFile)
+		out, err := openSSLExec.CombinedOutput()
+		if err != nil {
+			t.Errorf("S/MIME signing failed, expected OpenSSL to verify the message but got error: %s "+
+				"// exec output: %s", err, out)
+		}
+	})
 }
 
 func TestMsg_hasAlt(t *testing.T) {

smime_test.go

@@ -12,6 +12,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"errors"
+	"os"
 	"strings"
 	"testing"
 )
@@ -183,6 +184,21 @@ func TestGetLeafCertificate(t *testing.T) {
 	})
 }
 
+func getOpenSSLPath() string {
+	paths := []string{"/bin/openssl", "/usr/bin/openssl", "/usr/local/bin/openssl"}
+	openSSL := ""
+	for _, path := range paths {
+		if info, err := os.Stat(path); err == nil {
+			if info.IsDir() || info.Mode()&0o111 == 0 {
+				continue
+			}
+			openSSL = path
+			break
+		}
+	}
+	return openSSL
+}
+
 // getDummyRSACryptoMaterial loads a certificate (RSA), the associated private key and certificate (RSA) is loaded
 // from local disk for testing purposes
 func getDummyRSACryptoMaterial() (crypto.PrivateKey, *x509.Certificate, *x509.Certificate, error) {