使用BouncyCastle读取多次签名（.xml.p7m.p7m）的XML文件的签名内容（包括错误的解决方案原型）

Question

我正在尝试读取多次签名的XML文件的签名内容。
我编写了这段代码来读取已签名的内容，并且如果文件已被一次签名，则可以正常工作：

public static MyType LoadFromSignedFile(string fileName)
{  
    using (FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read))
    {
        CmsSignedData signedFile = new CmsSignedData(fileStream);

        XmlSerializer serializer = new XmlSerializer(typeof(MyType));
        using (MemoryStream memStream = new MemoryStream())
        {
            signedFile.SignedContent.Write(memStream);
            memStream.Position = 0;
            using (StreamReader sr = new StreamReader(memStream))
                return serializer.Deserialize(sr) as MyType;
        }
    }
}  

如果文件被签名两次或更多次（反XML文档（1,1）中的错误），则反序列化将引发错误，这是可以的，因为SignedContent包含另一个SignedContent签名。

我正在尝试遍历签名者（使用GetSigners（））以在每个循环中读取签名内容，但是即使文件被签名两次，GetSigners（）也会返回1。
如果像这样“手动”完成反序列化，则效果很好：

Stream st = parser.GetSignedContent().ContentStream;
CmsSignedDataParser p = new CmsSignedDataParser(st);
Stream st2 = p.GetSignedContent().ContentStream;

using (StreamReader sr = new StreamReader(st2))
    return serializer.Deserialize(sr) as MyType;  

这是使用DataParser对象的糟糕的解决方案原型：

public static MyType LoadInvoiceFromSignedFile(string fileName)
{
    using (FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read))
    {
        CmsSignedDataParser parser = new CmsSignedDataParser(fileStream);
        XmlSerializer serializer = new XmlSerializer(typeof(MyType));

        CmsSignedData data = new CmsSignedData(new FileStream(fileName, FileMode.Open, FileAccess.Read));
        Stream internalStream = null;

        // --> try to deserialize the XML hoping there is only 1 signature
        bool isError = false;
        try
        {
            using (MemoryStream memStream = new MemoryStream())
            {
                CmsSignedData signedFile = new CmsSignedData(new FileStream(fileName, FileMode.Open, FileAccess.Read));
                signedFile.SignedContent.Write(memStream);
                memStream.Position = 0;
                using (StreamReader sr = new StreamReader(memStream))
                    return serializer.Deserialize(sr) as MyType;
            }
        }catch(Exception ex)
        {
            isError = true;
        }
        // <--

        // --> signed multiple times, trying with a loop
        if (isError)
        {
            for (int i = 0; i < data.GetSignerInfos().GetSigners().Count; i++)
            {
                internalStream = parser.GetSignedContent().ContentStream;
                try
                {
                    parser = new CmsSignedDataParser(internalStream);
                }
                catch (Exception ex)
                {
                    using (MemoryStream memStream = new MemoryStream())
                    {
                        CmsSignedData signedFile = new CmsSignedData(new FileStream(fileName, FileMode.Open, FileAccess.Read));
                        signedFile.SignedContent.Write(memStream);
                        memStream.Position = 0;
                        using (StreamReader sr = new StreamReader(memStream))
                            return serializer.Deserialize(sr) as MyType;
                    }
                }
            }

            using (StreamReader sr = new StreamReader(parser.GetSignedContent().ContentStream))
                return serializer.Deserialize(sr) as MyType;
        }

        // <--

        return null;
    }
}

CmsTypeStream既不支持seek也不支持position，因此我无法识别流的结尾（也不太了解Stream，这是导致错误的代码）。