c# - 将 IV 添加到 CryptoStream 的开头

Question

我正在现有文件管理程序中实现本地加密。

我可以找到很多示例代码，例如 Microsoft's ，演示了如何直接写入文件，但我需要做的是提供一个在程序其他地方使用的流:

CryptoStream GetEncryptStream(string filename)
{
    var rjndl = new RijndaelManaged();
    rjndl.KeySize = 256;
    rjndl.BlockSize = 256;
    rjndl.Mode = CipherMode.CBC;
    rjndl.Padding = PaddingMode.PKCS7;

    // Open read stream of unencrypted source fileStream:
    var fileStream = new FileStream(filename, FileMode.Open); 

    /* Get key and iv */

    var transform = rjndl.CreateEncryptor(key, iv);

    // CryptoStream in *read* mode:
    var cryptoStream = new CryptoStream(fileStream, transform, CryptoStreamMode.Read); 

    /* What can I do here to insert the unencrypted IV at the start of the
       stream so that the first X bytes returned by cryptoStream.Read are
       the IV, before the bytes of the encrypted file are returned? */

    return cryptoStream; // Return CryptoStream to be consumed elsewhere
}

我的问题在最后一行的评论中有所概述，但有一个:我如何将 IV 添加到 CryptoStream 的开头，以便它成为读取 CryptoStream 时返回的第一个 X 字节，考虑到何时控制真正开始读取流并写入文件超出了我的代码范围？

Answer 1

好的...既然您的问题很清楚，它“相当”简单...遗憾的是 .NET 不包含合并两个 Stream 的类，但我们可以轻松创建它. MergedStream 是一个只读、只进的多流合并器。

你像这样使用:

var mergedStream = new MergedStream(new Stream[] 
{
    new MemoryStream(iv),
    cryptoStream,
});

现在...当有人试图从 MergedStream 读取数据时，首先会消耗包含 IV 的 MemoryStream，然后是 cryptoStream将被消耗。

public class MergedStream : Stream
{
    private Stream[] streams;
    private int position = 0;
    private int currentStream = 0;

    public MergedStream(Stream[] streams) => this.streams = streams;
    
    public override bool CanRead => true;

    public override bool CanSeek => false;

    public override bool CanWrite => false;

    public override long Length => streams.Sum(s => s.Length);

    public override long Position 
    { 
        get => position; 
        set => throw new NotSupportedException();
    }

    public override void Flush()
    {
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        if (streams == null)
        {
            throw new ObjectDisposedException(nameof(MergedStream));
        }

        if (currentStream >= streams.Length)
        {
            return 0;
        }

        int read;

        while (true)
        {
            read = streams[currentStream].Read(buffer, offset, count);
            position += read;

            if (read != 0)
            {
                break;
            }

            currentStream++;

            if (currentStream == streams.Length)
            {
                break;
            }
        }

        return read;
    }

    public override long Seek(long offset, SeekOrigin origin)
        => throw new NotSupportedException();

    public override void SetLength(long value)
        => throw new NotSupportedException();

    public override void Write(byte[] buffer, int offset, int count)
        => throw new NotSupportedException();

    protected override void Dispose(bool disposing)
    {
        try
        {
            if (disposing && streams != null)
            {
                for (int i = 0; i < streams.Length; i++)
                {
                    streams[i].Close();
                }
            }
        }
        finally
        {
            streams = null;
        }
    }
}