Small subtle optimizations of faketls

mtglib/internal/faketls/client_hello.go

 import (
 	"crypto/hmac"
 	"crypto/sha256"
+	"crypto/subtle"
 	"encoding/binary"
 	"fmt"
 	"time"
 	}
 
 	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)
 		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:]))

mtglib/internal/faketls/init.go

 package faketls
 
-import "errors"
+import (
+	"bytes"
+	"errors"
+)
 
 const (
 	RandomLen = 32
 		0x00, 0x1d, // x25519 curve
 		0x00, 0x20, // 32 bytes of key
 	}
+	clientHelloEmptyRandom = bytes.Repeat([]byte{0}, RandomLen)
 )