patch.go

package main

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"io"
	"os"
)

func main() {
	hexkey, err := os.ReadFile("key")
	if err != nil {
		panic(err)
	}
	key := make([]byte, len(hexkey)>>1)

	if _, err := hex.Decode(key, hexkey); err != nil {
		panic(err)
	}

	newkey := make([]byte, len(key))
	if _, err := io.ReadFull(rand.Reader, newkey); err != nil {
		panic(err)
	}

	bs, err := io.ReadAll(os.Stdin)
	if err != nil {
		panic(err)
	}

	i := bytes.Index(bs, key)
	if i == -1 {
		println("key not found in binary")
		os.Exit(1)
	}

	copy(bs[i:], newkey)

	if err := os.WriteFile("patched", bs, 0o755); err != nil {
		panic(err)
	}
}