control/controlclient: sign RegisterRequest (#1549)

control/controlclient: sign RegisterRequest

Some customers wish to verify eligibility for devices to join their
tailnets using machine identity certificates. TLS client certs could
potentially fulfill this role but the initial customer for this feature
has technical requirements that prevent their use. Instead, the
certificate is loaded from the Windows local machine certificate store
and uses its RSA public key to sign the RegisterRequest message.

There is room to improve the flexibility of this feature in future and
it is currently only tested on Windows (although Darwin theoretically
works too), but this offers a reasonable starting place for now.

Updates tailscale/coral#6

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

control/controlclient/direct.go

@@ -351,12 +351,14 @@ func (c *Direct) doLogin(ctx context.Context, t *tailcfg.Oauth2Token, flags Logi
 		err = errors.New("hostinfo: BackendLogID missing")
 		return regen, url, err
 	}
+	now := time.Now().Round(time.Second)
 	request := tailcfg.RegisterRequest{
 		Version:    1,
 		OldNodeKey: tailcfg.NodeKey(oldNodeKey),
 		NodeKey:    tailcfg.NodeKey(tryingNewKey.Public()),
 		Hostinfo:   hostinfo,
 		Followup:   url,
+		Timestamp:  &now,
 	}
 	c.logf("RegisterReq: onode=%v node=%v fup=%v",
 		request.OldNodeKey.ShortString(),
@@ -365,6 +367,20 @@ func (c *Direct) doLogin(ctx context.Context, t *tailcfg.Oauth2Token, flags Logi
 	request.Auth.Provider = persist.Provider
 	request.Auth.LoginName = persist.LoginName
 	request.Auth.AuthKey = authKey
+	err = signRegisterRequest(&request, c.serverURL, c.serverKey, c.machinePrivKey.Public())
+	if err != nil {
+		// If signing failed, clear all related fields
+		request.SignatureType = tailcfg.SignatureNone
+		request.Timestamp = nil
+		request.DeviceCert = nil
+		request.Signature = nil
+
+		// Don't log the common error types. Signatures are not usually enabled,
+		// so these are expected.
+		if err != errCertificateNotConfigured && err != errNoCertStore {
+			c.logf("RegisterReq sign error: %v", err)
+		}
+	}
 	bodyData, err := encode(request, &serverKey, &c.machinePrivKey)
 	if err != nil {
 		return regen, url, err