Verify and decrypt p7m-mime

p7m-verify.cs

Hi!

I have a data Content-Type: application/pkcs7-mime
Here is openssl command verifies and print the decoded content in console:

openssl smime -verify -in request.p7m -inform PEM -nointern -certfile yamoney.pem -CAfile yamoney.pem

It works fine. But I want to use MimeKit and ApplicationPkcs7Mime to simplify the usage in my application pipeline.
Parsing yamoney.pem I have a X509CertificateStore.

using (var fs = File.OpenRead(@"request.p7m"))
{
    var mime = new ApplicationPkcs7Mime(SecureMimeType.SignedData, fs); // This throws...
    ...

There is a request file in a git (request.p7m).
And pem - it is not secret.

P7mVerifyXml.cs

using System;
using System.IO;
using System.Text;

using Org.BouncyCastle.Asn1.Cms;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.X509;

using MimeKit;
using MimeKit.IO;
using MimeKit.Cryptography;

namespace P7mVerify 
{
    class Program
    {
        public const string RequestPkcs7File = @"request.p7m";
        public const string CertsFile = @"yamoney.pem";
        
        private byte[] GetRawRequest()
        {
            using (var reader = File.OpenText(RequestPkcs7File))
            {
                var pem = new PemReader(reader);
                var contentInfo = (ContentInfo) pem.ReadObject();
                return contentInfo.GetEncoded();
            }
        }
        
        public static void Main ()
        {
            using (var pem = File.OpenRead(CertsFile))
            {
                using (var ctx = PemSecureMimeContext.Create(pem))
                {
                    Stream stream = null;
                    try
                    {
                        using (var content = new MemoryStream(GetRawRequest(), false))
                        {
                            var signatures = ctx.Verify(content, out stream);
                            foreach (var signature in signatures)
                            {
                                try
                                {
                                    if (!signature.Verify())
                                    {
                                        Console.WriteLine("Bad signature from {0}",
                                            signature.SignerCertificate.Email);
                                    }
                                    else
                                    {
                                        Console.WriteLine("Verified. Sender: {0}", signature.SignerCertificate.Email);
                                    }
                                }
                                catch (DigitalSignatureVerifyException ex)
                                {
                                    Console.WriteLine("Failed to verify signature: {0}", ex);
                                }
                            }
                            XDocument xResponse = XDocument.Load(stream);
                            // show decoded xml
                            Console.WriteLine(xResponse.ToString(SaveOptions.None));
                        }
                    }
                    catch (Exception e)
                    {
                        // something goes wrong
                    }
                    finally
                    {
                        stream?.Dispose();
                    }
                }
            }
        }
    }
}

PemSecureMimeContext.cs

using System;
using System.Collections.Generic;
using System.IO;
using MimeKit.Cryptography;
using Org.BouncyCastle.Cms;
using Org.BouncyCastle.X509;

namespace P7mVerify
{
    public class PemSecureMimeContext : TemporarySecureMimeContext
    {
        public static PemSecureMimeContext Create(Stream s)
        {
            var ctx = new PemSecureMimeContext();
            ctx.ImportUnique(s);
            return ctx;
        }

        readonly HashSet<X509Certificate> unique = new HashSet<X509Certificate>();

        public DigitalSignatureCollection Verify(Stream signedData, out Stream stream)
        {
            if (signedData == null)
                throw new ArgumentNullException("signedData");

            var parser = new CmsSignedDataParser(signedData);
            var signed = parser.GetSignedContent();

            stream = new MemoryStream();
            signed.ContentStream.CopyTo(stream, 4096);
            stream.Position = 0;

            return GetDigitalSignatures(parser);
        }

	/// <summary>
	/// Imports the certificate(s) from the specified stream.
	/// </summary>
	/// <remarks>
	/// Imports the certificate(s) from the specified stream.
	/// </remarks>
	/// <param name="stream">The stream to import.</param>
	/// <exception cref="System.ArgumentNullException">
	/// <paramref name="stream"/> is <c>null</c>.
	/// </exception>
	/// <exception cref="System.IO.IOException">
	/// An error occurred reading the stream.
	/// </exception>
	public void ImportUnique(Stream stream)
        {
            if (stream == null)
                throw new ArgumentNullException("stream");

            var parser = new X509CertificateParser();

            foreach (X509Certificate certificate in parser.ReadCertificates(stream))
            {
                if (unique.Add(certificate))
                    Import(certificate);
            }
        }
    }
}

request.p7m

-----BEGIN PKCS7-----
MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAaCA
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-----END PKCS7-----

yamoney.pem

subject=/C=RU/L=Saint-Petersburg/O=Yandex.Money/OU=IT/CN=PaymentCenter/emailAddress=systemadministrators@yamoney.ru
issuer=/C=RU/O=PS Yandex.Money/CN=Yandex Money Issuing CA
-----BEGIN CERTIFICATE-----
MIIE2TCCBEKgAwIBAgIKeSU5HgACAAA1mzANBgkqhkiG9w0BAQUFADBJMQswCQYD
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-----END CERTIFICATE-----

subject=/C=RU/O=PS Yandex.Money/CN=Yandex Money Issuing CA
issuer=/C=RU/O=PS Yandex.Money/CN=Yandex Money Root CA
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----

subject=/C=RU/O=PS Yandex.Money/CN=Yandex Money Root CA
issuer=/C=RU/O=PS Yandex.Money/CN=Yandex Money Root CA
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----

Brilliant! I modified PemSecureMimeContext with static Create function and ability to avoid duplicate X509 certs and added simple program for this stuff P7mVerifyXml.cs which uses this crypto context and shows verification message and output request XML.
But, yes, I made method GetDigitalSignatures protected to compile this.