implement tls client certificate authentication

the imap & smtp servers now allow logging in with tls client authentication and
the "external" sasl authentication mechanism. email clients like thunderbird,
fairemail, k9, macos mail implement it. this seems to be the most secure among
the authentication mechanism commonly implemented by clients. a useful property
is that an account can have a separate tls public key for each device/email
client.  with tls client cert auth, authentication is also bound to the tls
connection. a mitm cannot pass the credentials on to another tls connection,
similar to scram-*-plus. though part of scram-*-plus is that clients verify
that the server knows the client credentials.

for tls client auth with imap, we send a "preauth" untagged message by default.
that puts the connection in authenticated state. given the imap connection
state machine, further authentication commands are not allowed. some clients
don't recognize the preauth message, and try to authenticate anyway, which
fails. a tls public key has a config option to disable preauth, keeping new
connections in unauthenticated state, to work with such email clients.

for smtp (submission), we don't require an explicit auth command.

both for imap and smtp, we allow a client to authenticate with another
mechanism than "external". in that case, credentials are verified, and have to
be for the same account as the tls client auth, but the adress can be another
one than the login address configured with the tls public key.

only the public key is used to identify the account that is authenticating. we
ignore the rest of the certificate. expiration dates, names, constraints, etc
are not verified. no certificate authorities are involved.

users can upload their own (minimal) certificate. the account web interface
shows openssl commands you can run to generate a private key, minimal cert, and
a p12 file (the format that email clients seem to like...) containing both
private key and certificate.

the imapclient & smtpclient packages can now also use tls client auth. and so
does "mox sendmail", either with a pem file with private key and certificate,
or with just an ed25519 private key.

there are new subcommands "mox config tlspubkey ..." for
adding/removing/listing tls public keys from the cli, by the admin.

imapserver/authenticate_test.go

@@ -1,20 +1,28 @@
 package imapserver
 
 import (
+	"context"
 	"crypto/hmac"
 	"crypto/md5"
 	"crypto/sha1"
 	"crypto/sha256"
+	"crypto/tls"
 	"encoding/base64"
 	"errors"
 	"fmt"
 	"hash"
+	"net"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"golang.org/x/text/secure/precis"
 
+	"github.com/mjl-/mox/mox-"
 	"github.com/mjl-/mox/scram"
+	"github.com/mjl-/mox/store"
 )
 
 func TestAuthenticateLogin(t *testing.T) {
@@ -210,3 +218,149 @@ func TestAuthenticateCRAMMD5(t *testing.T) {
 	auth("ok", "mo\u0301x@mox.example", password1)
 	tc.close()
 }
+
+func TestAuthenticateTLSClientCert(t *testing.T) {
+	tc := startArgs(t, true, true, true, true, "mjl")
+	tc.transactf("no", "authenticate external ") // No TLS auth.
+	tc.close()
+
+	// Create a certificate, register its public key with account, and make a tls
+	// client config that sends the certificate.
+	clientCert0 := fakeCert(t, true)
+	clientConfig := tls.Config{
+		InsecureSkipVerify: true,
+		Certificates:       []tls.Certificate{clientCert0},
+	}
+
+	tlspubkey, err := store.ParseTLSPublicKeyCert(clientCert0.Certificate[0])
+	tcheck(t, err, "parse certificate")
+	tlspubkey.Account = "mjl"
+	tlspubkey.LoginAddress = "mjl@mox.example"
+	tlspubkey.NoIMAPPreauth = true
+
+	addClientCert := func() error {
+		return store.TLSPublicKeyAdd(ctxbg, &tlspubkey)
+	}
+
+	// No preauth, explicit authenticate with TLS.
+	tc = startArgsMore(t, true, true, nil, &clientConfig, false, true, true, "mjl", addClientCert)
+	if tc.client.Preauth {
+		t.Fatalf("preauthentication while not configured for tls public key")
+	}
+	tc.transactf("ok", "authenticate external ")
+	tc.close()
+
+	// External with explicit username.
+	tc = startArgsMore(t, true, true, nil, &clientConfig, false, true, true, "mjl", addClientCert)
+	if tc.client.Preauth {
+		t.Fatalf("preauthentication while not configured for tls public key")
+	}
+	tc.transactf("ok", "authenticate external %s", base64.StdEncoding.EncodeToString([]byte("mjl@mox.example")))
+	tc.close()
+
+	// No preauth, also allow other mechanisms.
+	tc = startArgsMore(t, true, true, nil, &clientConfig, false, true, true, "mjl", addClientCert)
+	tc.transactf("ok", "authenticate plain %s", base64.StdEncoding.EncodeToString([]byte("\u0000mjl@mox.example\u0000"+password0)))
+	tc.close()
+
+	// No preauth, also allow other username for same account.
+	tc = startArgsMore(t, true, true, nil, &clientConfig, false, true, true, "mjl", addClientCert)
+	tc.transactf("ok", "authenticate plain %s", base64.StdEncoding.EncodeToString([]byte("\u0000móx@mox.example\u0000"+password0)))
+	tc.close()
+
+	// No preauth, other mechanism must be for same account.
+	acc, err := store.OpenAccount(pkglog, "other")
+	tcheck(t, err, "open account")
+	err = acc.SetPassword(pkglog, "test1234")
+	tcheck(t, err, "set password")
+	tc = startArgsMore(t, true, true, nil, &clientConfig, false, true, true, "mjl", addClientCert)
+	tc.transactf("no", "authenticate plain %s", base64.StdEncoding.EncodeToString([]byte("\u0000other@mox.example\u0000test1234")))
+	tc.close()
+
+	// Starttls and external auth.
+	tc = startArgsMore(t, true, false, nil, &clientConfig, false, true, true, "mjl", addClientCert)
+	tc.client.Starttls(&clientConfig)
+	tc.transactf("ok", "authenticate external =")
+	tc.close()
+
+	tlspubkey.NoIMAPPreauth = false
+	err = store.TLSPublicKeyUpdate(ctxbg, &tlspubkey)
+	tcheck(t, err, "update tls public key")
+
+	// With preauth, no authenticate command needed/allowed.
+	// Already set up tls session ticket cache, for next test.
+	serverConfig := tls.Config{
+		Certificates: []tls.Certificate{fakeCert(t, false)},
+	}
+	ctx, cancel := context.WithCancel(ctxbg)
+	defer cancel()
+	mox.StartTLSSessionTicketKeyRefresher(ctx, pkglog, &serverConfig)
+	clientConfig.ClientSessionCache = tls.NewLRUClientSessionCache(10)
+	tc = startArgsMore(t, true, true, &serverConfig, &clientConfig, false, true, true, "mjl", addClientCert)
+	if !tc.client.Preauth {
+		t.Fatalf("not preauthentication while configured for tls public key")
+	}
+	cs := tc.conn.(*tls.Conn).ConnectionState()
+	if cs.DidResume {
+		t.Fatalf("tls connection was resumed")
+	}
+	tc.transactf("no", "authenticate external ") // Not allowed, already in authenticated state.
+	tc.close()
+
+	// Authentication works with TLS resumption.
+	tc = startArgsMore(t, true, true, &serverConfig, &clientConfig, false, true, true, "mjl", addClientCert)
+	if !tc.client.Preauth {
+		t.Fatalf("not preauthentication while configured for tls public key")
+	}
+	cs = tc.conn.(*tls.Conn).ConnectionState()
+	if !cs.DidResume {
+		t.Fatalf("tls connection was not resumed")
+	}
+	// Check that operations that require an account work.
+	tc.client.Enable("imap4rev2")
+	received, err := time.Parse(time.RFC3339, "2022-11-16T10:01:00+01:00")
+	tc.check(err, "parse time")
+	tc.client.Append("inbox", nil, &received, []byte(exampleMsg))
+	tc.client.Select("inbox")
+	tc.close()
+
+	// Authentication with unknown key should fail.
+	// todo: less duplication, change startArgs so this can be merged into it.
+	err = store.Close()
+	tcheck(t, err, "store close")
+	os.RemoveAll("../testdata/imap/data")
+	err = store.Init(ctxbg)
+	tcheck(t, err, "store init")
+	mox.Context = ctxbg
+	mox.ConfigStaticPath = filepath.FromSlash("../testdata/imap/mox.conf")
+	mox.MustLoadConfig(true, false)
+	switchStop := store.Switchboard()
+	defer switchStop()
+
+	serverConn, clientConn := net.Pipe()
+	defer clientConn.Close()
+
+	done := make(chan struct{})
+	defer func() { <-done }()
+	connCounter++
+	cid := connCounter
+	go func() {
+		defer serverConn.Close()
+		serve("test", cid, &serverConfig, serverConn, true, false)
+		close(done)
+	}()
+
+	clientConfig.ClientSessionCache = nil
+	clientConn = tls.Client(clientConn, &clientConfig)
+	// note: It's not enough to do a handshake and check if that was successful. If the
+	// client cert is not acceptable, we only learn after the handshake, when the first
+	// data messages are exchanged.
+	buf := make([]byte, 100)
+	_, err = clientConn.Read(buf)
+	if err == nil {
+		t.Fatalf("tls handshake with unknown client certificate succeeded")
+	}
+	if alert, ok := mox.AsTLSAlert(err); !ok || alert != 42 {
+		t.Fatalf("got err %#v, expected tls 'bad certificate' alert", err)
+	}
+}