golang-stdlib RNG migration for Windows

os_windows.go

_BCryptGenRandom stdFunction

	var bcryptdll = []byte("Bcrypt.dll\000")
	bcrypt := windowsLoadSystemLib(bcryptdll)
	if bcrypt == 0 {
		throw("Bcrypt.dll not found")
	}
	_BCryptGenRandom = windowsFindfunc(bcrypt, []byte("BCryptGenRandom\000"))


//go:nosplit
func getRandomData(r []byte) {
	n := 0
	const BCRYPT_USE_SYSTEM_PREFERRED_RNG = 0x00000002
	if stdcall4(_BCryptGenRandom, 0, uintptr(unsafe.Pointer(&r[0])), uintptr(len(r)), uintptr(BCRYPT_USE_SYSTEM_PREFERRED_RNG))&0xff != 0 {
		n = len(r)
	}
	extendRandom(r, n)
}

rand_windows.go

// Copyright 2010 The Go & AZ-X Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Windows cryptographically secure pseudorandom number
// generator.

package rand

import (
	"os"
	"syscall"
	"unsafe"
)

func init() { Reader = &rngReader{} }

type rngReader struct{}

var (
	modcng                          = syscall.NewLazyDLL("Bcrypt.dll")
	procBCryptGenRandom             = modcng.NewProc("BCryptGenRandom")
)

func bCryptGenRandom(buf *uint8, bytes uint32) (err error) {
	const BCRYPT_USE_SYSTEM_PREFERRED_RNG = 0x00000002
	_, _, e := syscall.Syscall6(procBCryptGenRandom.Addr(), 4, 0, uintptr(unsafe.Pointer(buf)), uintptr(bytes), uintptr(BCRYPT_USE_SYSTEM_PREFERRED_RNG), 0, 0)
	switch e {
	case 0: err = nil
	default: err = e
	}
	return
}

func (r *rngReader) Read(b []byte) (n int, err error) {
	inputLen := uint32(len(b))

	if inputLen == 0 {
		return 0, nil
	}

	err = bCryptGenRandom(&b[0], inputLen)
	if err != nil {
		return 0, os.NewSyscallError("bCryptGenRandom", err)
	}
	return int(inputLen), nil
}