​
// Sign the stream and save it to another stream.
private static Stream SignRequestContent(MemoryStream streamToSign, X509Certificate2 personalCertificate)
{
    var contentInfo = new ContentInfo(streamToSign.ToArray());
    var signedCms = new SignedCms(contentInfo);
    var cmsSigner = new CmsSigner(personalCertificate);
    signedCms.ComputeSignature(cmsSigner);
    return new MemoryStream(signedCms.Encode());
}

​
public static Stream VerifyAndDecodeResponseContent(MemoryStream streamToVerifyAndDecode, string bureauResponsiblePersonCertificateSerialNumber)
{
    var signedCms = new SignedCms();
    signedCms.Decode(streamToVerifyAndDecode.ToArray());
    if (signedCms.SignerInfos.Count == 0)
    {
        return null;
    }
    var isSignatureValid = false;
    SignerInfoEnumerator enumerator = signedCms.SignerInfos.GetEnumerator();
    while (enumerator.MoveNext())
    {
        SignerInfo current = enumerator.Current;
        try
        {
            current.CheckSignature(new X509Certificate2Collection(GetCertificate(bureauResponsiblePersonCertificateSerialNumber)), false);
            isSignatureValid = true;
            signedCms.RemoveSignature(current);
        }
        catch (CryptographicException ex)
        {
            throw ex;
        }
    }
    return isSignatureValid
            ? new MemoryStream(signedCms.ContentInfo.Content)
            : null;
}