WrapMessage (rfc822) filtered on chosen wrapping headers

[JoeShook]

I have been digging in on the SMIME features and we have worked through a couple PRS.

One of the requirements for the Direct Project is wrapping (Content-Type: message/rfc822) a mime message with only specific headers allowed. For example the Subject header would not be allowed. Once wrapped then we begin the encryption only the wrapping headers are seen.

I wrote a WrapMessage Extension method locally. My question is, would there be any intereset to add this to MimeKit or not.

public static class MimeMessageExtensions
{
    public static MimeMessage WrapMessage(this MimeMessage originalMessage, string[] headersToRetain)
    {
        var messagePart = new MessagePart("rfc822")
        {
            Message = originalMessage
        };

        using var stream = new MemoryStream();
        using var writer = new StreamWriter(stream, Encoding.UTF8, 1024);

        foreach (var header in originalMessage.Headers)
        {
            if (headersToRetain.Contains(header.Field))
            {
                writer.WriteLine($"{header.Field}: {header.Value}");
            }
        }

        writer.Flush();
        stream.Position = 0;

        var parser = new MimeParser(stream, MimeFormat.Entity);
        var message = parser.ParseMessage();

        message.Body = messagePart;

        return message;
    }
}

Thank you,
Joe

[jstedfast]

I probably don't want to have this in MimeKit because it looks fairly specific to your use-case, but if you have the specification for this process, I wouldn't mind taking a look and re-evaluating my initial thoughts.

In the meantime, this can be done much simpler:

public static class MimeMessageExtensions
{
    // Note: headersToRetain should be case-insensitive. e.g:
    // headersToRetain = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
    public static MimeMessage WrapMessage(this MimeMessage originalMessage, HashSet<string> headersToRetain)
    {
        var messagePart = new MessagePart("rfc822")
        {
            Message = originalMessage
        };

        // Create the wrapper message.
        var message = new MimeMessage();

        // Clear the default MimeMessage headers.
        message.Headers.Clear();

        // Clone only the headers we want to retain.
        foreach (var header in originalMessage.Headers)
        {
            if (headersToRetain.Contains(header.Field))
            {
                message.Headers.Add(header.Clone());
            }
        }

        // Set the wrapper message's body.
        message.Body = messagePart;

        return message;
    }
}

[JoeShook]

I had something similar to yours and benchmarking let me to mine. But, yours is better than my original. I just tested yours and the one I shared. Ran a couple times using the startrek.eml file. Pretty close. Yours is easier to read.

| Method    | Mean       | Error    | StdDev   | Gen0     | Gen1     | Gen2     | Allocated |
|---------- |-----------:|---------:|---------:|---------:|---------:|---------:|----------:|
| JoeShook  |   994.1 us | 19.85 us | 44.80 us | 199.2188 | 199.2188 | 199.2188 | 999.29 KB |
| jstedfast | 1,029.8 us | 32.41 us | 95.57 us | 199.2188 | 199.2188 | 199.2188 | 993.46 KB |


| Method    | Mean     | Error     | StdDev    | Gen0     | Gen1     | Gen2     | Allocated |
|---------- |---------:|----------:|----------:|---------:|---------:|---------:|----------:|
| JoeShook  | 1.041 ms | 0.0271 ms | 0.0774 ms | 199.2188 | 199.2188 | 199.2188 | 999.29 KB |
| jstedfast | 1.159 ms | 0.0323 ms | 0.0942 ms | 199.2188 | 199.2188 | 199.2188 | 993.46 KB |

Then I ran it against a very simple message with a text/plain with one line of text. Yours edges mine out every time.

| Method    | Mean     | Error    | StdDev   | Gen0   | Gen1   | Allocated |
|---------- |---------:|---------:|---------:|-------:|-------:|----------:|
| JoeShook  | 53.17 us | 1.066 us | 3.091 us | 2.6855 | 2.5635 |  33.57 KB |
| jstedfast | 51.70 us | 1.567 us | 4.545 us | 2.1973 | 1.9531 |  29.14 KB |

[JoeShook]

I am tied to section 2.4 of Applicability Statement for Secure Health Transport

It references RFC 8551 section 3.1 which I think is motivation to add this feature. One notable quotes in the secion is

   In order to protect outer, non-content-related message header fields
   (for instance, the "Subject", "To", "From", and "Cc" fields), the
   sending client MAY wrap a full MIME message in a message/rfc822
   wrapper in order to apply S/MIME security services to these header
   fields.

[jstedfast]

FWIW, most likely the performance slowdown is that each time a header is added to message.Headers, an event fires that the MimeMessage processes to update its own state.

I am adding a new MimeMessage .ctor that can take a list of headers so that they get added before MimeMessage sets up event handlers.

If you use the latest source code, you can now do this:

public static class MimeMessageExtensions
{
    // Note: headersToRetain should be case-insensitive. e.g:
    // headersToRetain = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
    public static MimeMessage WrapMessage(this MimeMessage originalMessage, HashSet<string> headersToRetain)
    {
        var messagePart = new MessagePart("rfc822")
        {
            Message = originalMessage
        };

        // Create the wrapper message.
        var headerList = new HeaderList();

        // Clone only the headers we want to retain.
        foreach (var header in originalMessage.Headers)
        {
            if (headersToRetain.Contains(header.Field))
            {
                headerList.Add(header.Clone());
            }
        }

        var message = new MimeMessage(headerList);

        // Set the wrapper message's body.
        message.Body = messagePart;

        return message;
    }
}

[JoeShook]

Yes this is a nice addition.

| Method    | Mean     | Error    | StdDev   | Gen0   | Gen1   | Allocated |
|---------- |---------:|---------:|---------:|-------:|-------:|----------:|
| Before    | 37.33 us | 0.785 us | 2.277 us | 2.1973 | 1.9531 |  29.14 KB |
| After     | 29.53 us | 0.843 us | 2.473 us | 1.8311 | 1.7700 |  22.48 KB |