string c_memo = "Inhoud van de memo die versleuteld wordt.";
using (SqlCommand cmd = new SqlCommand("update CLI set c_memo=@c_memo where c_id=..."))
{
    cmd.Parameters.AddWithValue("@c_memo", MercatorCryptorEngine.CryptorEngine.MercatorEncryptRsaAes(c_memo)).SqlDbType = System.Data.SqlDbType.VarChar;
    Api.SqlExec(MercatorUi.Globals.RepData, cmd);
}