//go:build windows

package crypto

import (
	"fmt"
	"unsafe"

	"golang.org/x/sys/windows"
)

// Unprotect calls Windows CryptUnprotectData on the given blob. entropy
// is optional secondary entropy that must match what was passed to the
// matching CryptProtectData call; pass nil if none was used. scope
// selects between the current-user and local-machine master keys.
//
// The original WinAuth wrote both User and Machine DPAPI blobs without
// extra entropy, so passing entropy=nil is what the legacy migration
// needs in practice.
func Unprotect(blob, entropy []byte, scope DPAPIScope) ([]byte, error) {
	var in windows.DataBlob
	in.Size = uint32(len(blob))
	if len(blob) > 0 {
		in.Data = &blob[0]
	}

	var entIn *windows.DataBlob
	if len(entropy) > 0 {
		entIn = &windows.DataBlob{Size: uint32(len(entropy)), Data: &entropy[0]}
	}

	var flags uint32
	if scope == DPAPIScopeLocalMachine {
		flags |= 0x4 // CRYPTPROTECT_LOCAL_MACHINE
	}

	var out windows.DataBlob
	if err := windows.CryptUnprotectData(&in, nil, entIn, 0, nil, flags, &out); err != nil {
		return nil, fmt.Errorf("dpapi: unprotect: %w", err)
	}
	defer windows.LocalFree(windows.Handle(unsafe.Pointer(out.Data)))

	if out.Size == 0 {
		return []byte{}, nil
	}
	result := make([]byte, out.Size)
	copy(result, unsafe.Slice(out.Data, out.Size))
	return result, nil
}