From 54a7c6a2a5321eb10473aac0e2edcd85b4523efb Mon Sep 17 00:00:00 2001
From: 9seconds <nineseconds@yandex.ru>
Date: Tue, 6 Apr 2021 11:09:37 +0300
Subject: [PATCH] Small subtle optimizations of faketls

---
 mtglib/internal/faketls/client_hello.go | 12 ++++--------
 mtglib/internal/faketls/init.go         |  6 +++++-
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/mtglib/internal/faketls/client_hello.go b/mtglib/internal/faketls/client_hello.go
index 62d3278..51e1fe5 100644
--- a/mtglib/internal/faketls/client_hello.go
+++ b/mtglib/internal/faketls/client_hello.go
@@ -3,6 +3,7 @@ package faketls
 import (
 	"crypto/hmac"
 	"crypto/sha256"
+	"crypto/subtle"
 	"encoding/binary"
 	"fmt"
 	"time"
@@ -39,10 +40,7 @@ func ParseClientHello(secret, handshake []byte) (ClientHello, error) {
 	}
 
 	copy(hello.Random[:], handshake[ClientHelloRandomOffset:])
-
-	for i := ClientHelloRandomOffset; i < ClientHelloRandomOffset+RandomLen; i++ {
-		handshake[i] = 0
-	}
+	copy(handshake[ClientHelloRandomOffset:], clientHelloEmptyRandom)
 
 	rec := record.AcquireRecord()
 	defer record.ReleaseRecord(rec)
@@ -62,10 +60,8 @@ func ParseClientHello(secret, handshake []byte) (ClientHello, error) {
 		computedRandom[i] ^= hello.Random[i]
 	}
 
-	for i := 0; i < RandomLen-4; i++ {
-		if computedRandom[i] != 0 {
-			return hello, ErrBadDigest
-		}
+	if subtle.ConstantTimeCompare(clientHelloEmptyRandom[:RandomLen-4], computedRandom[:RandomLen-4]) != 1 {
+		return hello, ErrBadDigest
 	}
 
 	timestamp := int64(binary.LittleEndian.Uint32(computedRandom[RandomLen-4:]))
diff --git a/mtglib/internal/faketls/init.go b/mtglib/internal/faketls/init.go
index dfa269b..5bbf6f3 100644
--- a/mtglib/internal/faketls/init.go
+++ b/mtglib/internal/faketls/init.go
@@ -1,6 +1,9 @@
 package faketls
 
-import "errors"
+import (
+	"bytes"
+	"errors"
+)
 
 const (
 	RandomLen = 32
@@ -34,4 +37,5 @@ var (
 		0x00, 0x1d, // x25519 curve
 		0x00, 0x20, // 32 bytes of key
 	}
+	clientHelloEmptyRandom = bytes.Repeat([]byte{0}, RandomLen)
 )