From f7cbe0b1e84a47ad850303dc24adcb0f3e954777 Mon Sep 17 00:00:00 2001
From: Samuel Hawksby-Robinson <samuel@samyoul.com>
Date: Sat, 6 Aug 2022 14:26:16 +0100
Subject: [PATCH] Get server cert, run validation on cert

---
 server/client.go              | 56 +++++++++++++++++++++++++++++++++++
 server/server.go              | 10 +++++++
 server/server_pairing_test.go |  3 ++
 3 files changed, 69 insertions(+)

diff --git a/server/client.go b/server/client.go
index 7952f4fb4..3e8e6100c 100644
--- a/server/client.go
+++ b/server/client.go
@@ -3,10 +3,13 @@ package server
 import (
 	"bytes"
 	"crypto/ecdsa"
+	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
+	"encoding/asn1"
 	"fmt"
 	"io/ioutil"
+	"math/big"
 	"net/http"
 	"net/url"
 )
@@ -19,6 +22,7 @@ type PairingClient struct {
 	certPEM     []byte
 	privateKey  *ecdsa.PrivateKey
 	serverMode  Mode
+	serverCert  *x509.Certificate
 }
 
 func NewPairingClient(c *ConnectionParams, config *PairingPayloadManagerConfig) (*PairingClient, error) {
@@ -94,3 +98,55 @@ func (c *PairingClient) receiveAccountData() error {
 
 	return c.PayloadManager.Receive(payload)
 }
+
+func verifyCertSig(cert *x509.Certificate) (bool, error) {
+	var esig struct {
+		R, S *big.Int
+	}
+	if _, err := asn1.Unmarshal(cert.Signature, &esig); err != nil {
+		return false, err
+	}
+
+	hash := sha256.New()
+	hash.Write(cert.RawTBSCertificate)
+
+	verified := ecdsa.Verify(cert.PublicKey.(*ecdsa.PublicKey), hash.Sum(nil), esig.R, esig.S)
+	return verified, nil
+}
+
+func (c *PairingClient) getServerCert() error {
+	conf := &tls.Config{
+		InsecureSkipVerify: true, // Only skip verify to get the server's TLS cert. DO NOT skip for any other reason.
+	}
+
+	conn, err := tls.Dial("tcp", c.baseAddress.Host, conf)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	certs := conn.ConnectionState().PeerCertificates
+	if len(certs) != 1 {
+		return fmt.Errorf("expected 1 TLS certificate, received '%d'", len(certs))
+	}
+
+	certKey, ok := certs[0].PublicKey.(*ecdsa.PublicKey)
+	if !ok {
+		return fmt.Errorf("unexpected public key type, expected ecdsa.PublicKey")
+	}
+
+	if certKey.Equal(c.privateKey) {
+		return fmt.Errorf("server certificate MUST match the given public key")
+	}
+
+	verified, err := verifyCertSig(certs[0])
+	if err != nil {
+		return err
+	}
+	if !verified {
+		return fmt.Errorf("server certificate signature MUST verify")
+	}
+
+	c.serverCert = certs[0]
+	return nil
+}
diff --git a/server/server.go b/server/server.go
index 1295b0a19..dafd013c0 100644
--- a/server/server.go
+++ b/server/server.go
@@ -117,6 +117,16 @@ func (s *Server) SetHandlers(handlers HandlerPatternMap) {
 	s.handlers = handlers
 }
 
+func (s *Server) AddHandlers(handlers HandlerPatternMap) {
+	if s.handlers == nil {
+		s.handlers = make(HandlerPatternMap)
+	}
+
+	for name := range handlers {
+		s.handlers[name] = handlers[name]
+	}
+}
+
 func (s *Server) MakeBaseURL() *url.URL {
 	return &url.URL{
 		Scheme: "https",
diff --git a/server/server_pairing_test.go b/server/server_pairing_test.go
index 5895dc46d..180381174 100644
--- a/server/server_pairing_test.go
+++ b/server/server_pairing_test.go
@@ -59,6 +59,9 @@ func (s *PairingServerSuite) TestPairingServer_StartPairing() {
 		c, err := NewPairingClient(ccp, nil)
 		s.Require().NoError(err)
 
+		err = c.getServerCert()
+		s.Require().NoError(err)
+
 		// Replace PairingClient.PayloadManager with a MockEncryptOnlyPayloadManager
 		c.PayloadManager, err = NewMockEncryptOnlyPayloadManager(s.EphemeralPK)
 		s.Require().NoError(err)