Crypto in C#/4.0 using PKCS#10 and SHA256

ninodago

I'm facing a new situation involving a program, written in .NET Framework 4.0/C#, which has to encode/decode and sign messages to be sent/received in a particular kind of WAN. This is the current scenario, which is working fine. We have a .p12 file (which contains the sender certificate) and a .cer file (which contains the receiver certificate), which are both installed in the pc. The scenario is working using SHA1 and PKCS#7 at 1024 bit, so the following piece of code works:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography.Pkcs;
using System.Security.Cryptography;
using System.Windows;
using System.Windows.Controls;
using System.Collections;
private static string senderCert = "sendername"; //the name registered in .p12 file
private static string receiverCert = "receivername"; //the name registered in .cer file
/// ///verify that the receiving message is signed
///and returns the data without sign
///
/// dataflow with sign
///
public static byte[] Verify(this byte[] signedData)
{
X509Certificate2 certPub = GetReceiverCert();
if (certPub == null) return null;

    ContentInfo decodeContentInfo = new ContentInfo(signedData);
    SignedCms decodeCMS = new SignedCms(decodeContentInfo, false);

    try
    {
        //decode the message, if it isn't signed, raise an exception
        decodeCMS.Decode(signedData);
        SignerInfo signerInfo = decodeCMS.SignerInfos\[0\];

        X509Certificate2Collection certCollection = new 
                    X509Certificate2Collection(certPub);                
        return decodeCMS.ContentInfo.Content;
    }
    catch (CryptographicException err)
    {
        Logger.Log(err);
        return null;
    }
}

/// /// Returns the certificate used to sign the sending messages
/// 
/// 
private static X509Certificate2 GetSenderCert()
{
    //Open the personal certificates folder
    X509Store storeMy = new X509Store(StoreName.My, StoreLocation.CurrentUser);
    storeMy.Open(OpenFlags.ReadOnly);

    //find the proper certificate
    X509Certificate2Collection certColl = storeMy.Certificates.Find 
    (X509FindType.