public static byte[] DoIt(string thumbprint, string msg)
{
    Encoding unicode = Encoding.UTF8;
    byte[] msgBytes = unicode.GetBytes(msg);
    X509Certificate2 signerCert = GetSignerCert(thumbprint);
    byte[] encodedSignature = SignMsg(msgBytes, signerCert);
    VerifyMsg(msgBytes, encodedSignature);
    return encodedSignature;
}

private static X509Certificate2 GetSignerCert(string thumbprint)
{
    var storeMy = new X509Store(StoreName.My, StoreLocation.CurrentUser);
    storeMy.Open(OpenFlags.ReadOnly);
    X509Certificate2Collection certColl = storeMy.Certificates.Find(X509FindType.FindByThumbprint, thumbprint, false);
    storeMy.Close();
    return certColl[0];
}

private static byte[] SignMsg(byte[] msg, X509Certificate2 signerCert)
{
    var contentInfo = new ContentInfo(msg);
    var signedCms = new SignedCms(contentInfo, true);
    var cmsSigner = new CmsSigner(signerCert);
    signedCms.ComputeSignature(cmsSigner);
    return signedCms.Encode();
}

private static void VerifyMsg(byte[] msg, byte[] encodedSignature)
{
    var contentInfo = new ContentInfo(msg);
    var signedCms = new SignedCms(contentInfo, true);
    signedCms.Decode(encodedSignature);
    signedCms.CheckSignature(true);
}

public static string Encode(byte[] bytes)
{
    string data = Convert.ToBase64String(bytes);
    return data;
}

public static string PrepareSafeUrl(string data)
{
    data = data.TrimEnd('=');
    data = data.Replace("+", "%2b");
    data = data.Replace("/", "%2f");
    data = data.Replace("=", "%3d");
    return data;
}

byte[] signatureArray = DetachedSignatureCompact.DoIt(thumbprint, apiCode);
string signature = TrueEncodeHelper.Encode(signatureArray);
signature = TrueEncodeHelper.PrepareSafeUrl(signature);