cmd/tailscale/cli: start making cert output support pkcs12 (p12) output

If the --key-file output filename ends in ".pfx" or ".p12", use pkcs12
format.

This might not be working entirely correctly yet but might be enough for
others to help out or experiment.

Updates #2928
Updates #5011

Change-Id: I62eb0eeaa293b9fd5e27b97b9bc476c23dd27cf6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cert.go

@@ -7,7 +7,9 @@ package cli
 import (
 	"bytes"
 	"context"
+	"crypto/rand"
 	"crypto/tls"
+	"crypto/x509"
 	"errors"
 	"flag"
 	"fmt"
@@ -17,6 +19,7 @@ import (
 	"strings"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
+	"software.sslmate.com/src/go-pkcs12"
 	"tailscale.com/atomicfile"
 	"tailscale.com/ipn"
 	"tailscale.com/version"
@@ -130,17 +133,25 @@ func runCert(ctx context.Context, args []string) error {
 			}
 		}
 	}
-	if certArgs.keyFile != "" {
-		keyChanged, err := writeIfChanged(certArgs.keyFile, keyPEM, 0600)
+	if dst := certArgs.keyFile; dst != "" {
+		contents := keyPEM
+		if isPKCS12(dst) {
+			var err error
+			contents, err = convertToPKCS12(certPEM, keyPEM)
+			if err != nil {
+				return err
+			}
+		}
+		keyChanged, err := writeIfChanged(dst, contents, 0600)
 		if err != nil {
 			return err
 		}
 		if certArgs.keyFile != "-" {
 			macWarn()
 			if keyChanged {
-				printf("Wrote private key to %v\n", certArgs.keyFile)
+				printf("Wrote private key to %v\n", dst)
 			} else {
-				printf("Private key unchanged at %v\n", certArgs.keyFile)
+				printf("Private key unchanged at %v\n", dst)
 			}
 		}
 	}
@@ -160,3 +171,29 @@ func writeIfChanged(filename string, contents []byte, mode os.FileMode) (changed
 	}
 	return true, nil
 }
+
+func isPKCS12(dst string) bool {
+	return strings.HasSuffix(dst, ".p12") || strings.HasSuffix(dst, ".pfx")
+}
+
+func convertToPKCS12(certPEM, keyPEM []byte) ([]byte, error) {
+	cert, err := tls.X509KeyPair(certPEM, keyPEM)
+	if err != nil {
+		return nil, err
+	}
+	var certs []*x509.Certificate
+	for _, c := range cert.Certificate {
+		cert, err := x509.ParseCertificate(c)
+		if err != nil {
+			return nil, err
+		}
+		certs = append(certs, cert)
+	}
+	if len(certs) == 0 {
+		return nil, errors.New("no certs")
+	}
+	// TODO(bradfitz): I'm not sure this is right yet. The goal was to make this
+	// work for https://github.com/tailscale/tailscale/issues/2928 but I'm still
+	// fighting Windows.
+	return pkcs12.Encode(rand.Reader, cert.PrivateKey, certs[0], certs[1:], "" /* no password */)
+}

cmd/tailscale/depaware.txt

@@ -37,6 +37,8 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         nhooyr.io/websocket                                          from tailscale.com/derp/derphttp+
         nhooyr.io/websocket/internal/errd                            from nhooyr.io/websocket
         nhooyr.io/websocket/internal/xsync                           from nhooyr.io/websocket
+        software.sslmate.com/src/go-pkcs12                           from tailscale.com/cmd/tailscale/cli
+        software.sslmate.com/src/go-pkcs12/internal/rc2              from software.sslmate.com/src/go-pkcs12
         tailscale.com                                                from tailscale.com/version
         tailscale.com/atomicfile                                     from tailscale.com/ipn+
         tailscale.com/client/tailscale                               from tailscale.com/cmd/tailscale/cli+
@@ -119,6 +121,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         golang.org/x/crypto/hkdf                                     from crypto/tls+
         golang.org/x/crypto/nacl/box                                 from tailscale.com/types/key
         golang.org/x/crypto/nacl/secretbox                           from golang.org/x/crypto/nacl/box
+        golang.org/x/crypto/pbkdf2                                   from software.sslmate.com/src/go-pkcs12
         golang.org/x/crypto/salsa20/salsa                            from golang.org/x/crypto/nacl/box+
         golang.org/x/net/bpf                                         from github.com/mdlayher/netlink+
         golang.org/x/net/dns/dnsmessage                              from net+