C# 将 SecureString 安全地转换为 UTF-8 byte[]

Question

我正在尝试将 SecureString 转换为 byte[] 的形式，我可以将其固定在 GC 中，并以 UTF-8 格式编码。我已经成功地做到了这一点，但是使用了 UTF-16（默认编码），但是如果 GC 没有机会在某处创建数据的托管副本（数据需要保持安全）。

这是我目前所掌握的（上下文：计算 SecureString 的哈希值的算法）

public static byte[] Hash(this SecureString secureString, HashAlgorithm hashAlgorithm)
{
  IntPtr bstr = Marshal.SecureStringToBSTR(secureString);
  int length = Marshal.ReadInt32(bstr, -4);
  var utf16Bytes = new byte[length];
  GCHandle utf16BytesPin = GCHandle.Alloc(utf16Bytes, GCHandleType.Pinned);
  byte[] utf8Bytes = null;

  try
  {
    Marshal.Copy(bstr, utf16Bytes, 0, length);
    Marshal.ZeroFreeBSTR(bstr);
    // At this point I have the UTF-16 byte[] perfectly.
    // The next line works at converting the encoding, but it does nothing
    // to protect the data from being spread throughout memory.
    utf8Bytes = Encoding.Convert(Encoding.Unicode, Encoding.UTF8, utf16Bytes);
    return hashAlgorithm.ComputeHash(utf8Bytes);
  }
  finally
  {
    if (utf8Bytes != null)
    {
      for (var i = 0; i < utf8Bytes.Length; i++)
      { 
        utf8Bytes[i] = 0;
      }
    }
    for (var i = 0; i < utf16Bytes.Length; i++)
    { 
      utf16Bytes[i] = 0;
    }
    utf16BytesPin.Free();
  }
}

进行此转换的最佳方法是什么？我是在正确的位置尝试进行转换，还是应该以某种方式更早进行转换？通过完全跳过 UTF-16 byte[] 步骤可以提高内存效率吗？

Answer 1

您是否考虑过在获取哈希后调用 GC.Collect()？

根据MSDN on GC.Collect:

Forces an immediate garbage collection of all generations. Use this method to try to reclaim all memory that is inaccessible. It performs a blocking garbage collection of all generations.

All objects, regardless of how long they have been in memory, are considered for collection; however, objects that are referenced in managed code are not collected. Use this method to force the system to try to reclaim the maximum amount of available memory.

根据我在您的代码中看到的情况，它不应保留对转换中使用的对象的任何引用。都应该由GC收集和处理。

Answer 2

我找到了一种按照我想要的方式执行此操作的方法。我这里的代码还没有完成（在失败的情况下需要更好的异常处理和内存管理），但这里是：

[DllImport("kernel32.dll")]
static extern void RtlZeroMemory(IntPtr dst, int length);

public unsafe static byte[] HashNew(this SecureString secureString, HashAlgorithm hashAlgorithm)
{
  IntPtr bstr = Marshal.SecureStringToBSTR(secureString);
  int maxUtf8BytesCount = Encoding.UTF8.GetMaxByteCount(secureString.Length);
  IntPtr utf8Buffer = Marshal.AllocHGlobal(maxUtf8BytesCount);

  // Here's the magic:
  char* utf16CharsPtr = (char*)bstr.ToPointer();
  byte* utf8BytesPtr  = (byte*)utf8Buffer.ToPointer();
  int utf8BytesCount = Encoding.UTF8.GetBytes(utf16CharsPtr, secureString.Length, utf8BytesPtr, maxUtf8BytesCount);

  Marshal.ZeroFreeBSTR(bstr);
  var utf8Bytes = new byte[utf8BytesCount];
  GCHandle utf8BytesPin = GCHandle.Alloc(utf8Bytes, GCHandleType.Pinned);
  Marshal.Copy(utf8Buffer, utf8Bytes, 0, utf8BytesCount);
  RtlZeroMemory(utf8Buffer, utf8BytesCount);
  Marshal.FreeHGlobal(utf8Buffer);
  try
  {
    return hashAlgorithm.ComputeHash(utf8Bytes);
  }
  finally
  {
    for (int i = 0; i < utf8Bytes.Length; i++)
    {
      utf8Bytes[i] = 0;
    }
    utf8BytesPin.Free();
  }
}

它依赖于获取指向原始 UTF-16 字符串和 UTF-8 缓冲区的指针，然后使用 Encoding.UTF8.GetBytes(Char*, Int32, Byte*, Int32) 将转换保持在非托管内存中。

C# 将 SecureString 安全地转换为 UTF-8 byte[]

C# convert SecureString to UTF-8 byte[] securely

c#

character-encoding

marshalling

securestring