public static void Encrypt(string certFileName, string inFileName, string outFileName)
{
    // Разбираем сертификат получателя
    X509Certificate2 cert = new X509Certificate2(certFileName);

    ContentInfo plainContent = new ContentInfo(File.ReadAllBytes(inFileName));

    EnvelopedCms encryptedData = new EnvelopedCms(
      SubjectIdentifierType.IssuerAndSerialNumber,
      plainContent,
      new AlgorithmIdentifier(new Oid("1.2.643.7.1.2.5.1.1")));

    CmsRecipient recipient = new CmsRecipient(SubjectIdentifierType.IssuerAndSerialNumber, cert);

    encryptedData.Encrypt(recipient);

    byte[] encryptedBytes = encryptedData.Encode();
    File.WriteAllBytes(outFileName, encryptedBytes);
}

public static void Decrypt(string inFileName, string outFileName)
{
    EnvelopedCms encryptedData = new EnvelopedCms();

    encryptedData.Decode(File.ReadAllBytes(inFileName));
    encryptedData.Decrypt(encryptedData.RecipientInfos[1]);

    File.WriteAllBytes(outFileName, encryptedData.ContentInfo.Content);
}

public static void BouncyDecrypt(string inFileName, string outFileName)
{
    using (var inputFileStream = new FileStream(inFileName, FileMode.Open))
    { 
        CmsEnvelopedDataParser ep = new CmsEnvelopedDataParser(inputFileStream);
        var recInfo = GetSingleRecipient(ep);
              
        using (var outputFileStream = File.Create(outFileName))
        {
            recInfo.GetContentStream(GetPrivateKey()).ContentStream.CopyTo(outputFileStream);
        }
    }
}

private static RecipientInformation GetSingleRecipient(CmsEnvelopedDataParser parser)
{
    var recInfos = parser.GetRecipientInfos().GetRecipients();
    var enumerator = recInfos.GetEnumerator();
    enumerator.MoveNext();
    return (RecipientInformation)enumerator.Current;
}