RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
RSAParameters RSAKeyInfo = new RSAParameters();
RSAKeyInfo.Modulus = m_public.Modulus;
RSAKeyInfo.Exponent = m_public.Exponent;

X509Store certStore = new X509Store(StoreLocation.CurrentUser);
certStore.Open(OpenFlags.ReadOnly);

var signature = File.ReadAllBytes("Test.ecp"); 
var data = File.ReadAllBytes("Test.txt");

var x509 = certStore.Certificates[4];  // Нужный мне сертификат 4ый
         
// Вот тут огромные сомнения... не знаю точно, как нужно !!
var rsa = x509.PublicKey.Key as RSACryptoServiceProvider;     
string sha1Oid = CryptoConfig.MapNameToOID("SHA1");
string md5Oid = CryptoConfig.MapNameToOID("MD5");            

    try
    {
        bool sha1Valid = rsa.VerifyData(data, md5Oid, signature);    // false
    }
    catch (Exception )
    {}

    try
    {
        bool md5Valid = rsa.VerifyHash(data, md5Oid, signature);   // false
    }
    catch (Exception)       
    { }

bool sha1Valid = rsa.VerifyData(data, sha1Oid, signature);    // false

bool sha1Valid = rsa.VerifyData(data, md5Oid, signature);    // false

var x509 = certStore.Certificates[4];

 RSACryptoServiceProvider csp = (RSACryptoServiceProvider)x509.PrivateKey;            
 // Hash the data
 SHA1Managed sha1 = new SHA1Managed();
 //UnicodeEncoding encoding = new UnicodeEncoding();
 //byte[] data1 = encoding.GetBytes(text);
 byte[] hash = sha1.ComputeHash(data);

 var signedHash = csp.SignHash(hash, CryptoConfig.MapNameToOID("SHA1"));

    // Get public key
    RSACryptoServiceProvider csp = (RSACryptoServiceProvider)x509.PublicKey.Key;

    // Hash the text, the text is the expected command by the client application.
    // Remember hased data cannot be unhash. It is irreversable
    SHA1Managed sha1 = new SHA1Managed();
    byte[] hash = sha1.ComputeHash(data);

    // Verify the signature with the hash
    valid =  csp.VerifyData(hash, CryptoConfig.MapNameToOID("SHA1"), signature);   // false

    using (var fs = new FileStream("3.FLD", FileMode.Open, FileAccess.Read))
    {
        signData = provider.SignData(fs, new SHA1CryptoServiceProvider());
        //File.WriteAllBytes("SignedData.bin", signData);
    }

    File.WriteAllBytes("realSign.result", signData);