我的C＃加密器拒绝打开3 GB以上的文件

Question

因此，我正在为一个学校项目工作，该项目允许我加密大文件，但是对于某些（可能是32位相关的）问题，它无法打开大于3 GB的文件。 你能发现我的错误吗？

我已经用long代替了整数

        private void Form1_Load(object sender, EventArgs e)
        {
            rbEncrypt.Checked = true;

            // init abc and table
            abc = new byte[256];
            for (long i = 0; i < 256; i++)
                abc[i] = Convert.ToByte(i);

            table = new byte[256, 256];
            for(long i = 0; i < 256; i++)
                for(long j = 0; j< 256; j++)
                {
                    table[i, j] = abc[(i + j) % 256];
                }

        }

        private void BtStart_Click(object sender, EventArgs e)
        {
            // Checking input values
            if(!File.Exists(tbPath.Text))
            {
                MessageBox.Show("File does not exist");
                return;
            }
            if(string.IsNullOrEmpty(tbPassword.Text))
            {
                MessageBox.Show("Pass is empty");
                return;
            }
            // get file content and key for encrypting
            try
            {
                byte[] filecontent = File.ReadAllBytes(tbPath.Text);
                byte[] passwordTmp = Encoding.ASCII.GetBytes(tbPassword.Text);
                byte[] keys = new byte[filecontent.Length];
                for (long i = 0; i < filecontent.Length; i++)
                    keys[i] = passwordTmp[i % passwordTmp.Length];

                //Encrypt
                byte[] result = new byte[filecontent.Length];
                if (rbEncrypt.Checked)
                {
                    for (long i = 0; i < filecontent.Length; i++)
                    {
                        long value = filecontent[i];
                        long key = keys[i];
                        long valueIndex = -1, keyIndex = -1;
                        for (long j = 0; j < 256; j++)
                            if (abc[j] == value)
                            {
                                valueIndex = j;
                                break;
                            }
                        for (long j = 0; j < 256; j++)
                            if (abc[j] == key)
                            {
                                keyIndex = j;
                                break;
                            }
                        result[i] = table[keyIndex, valueIndex];

                    }
                }
                //Decrypt
                else
                { 
                    for (long i = 0; i < filecontent.Length; i++)
                    {
                        long value = filecontent[i];
                        long key = keys[i];
                        long valueIndex = -1, keyIndex = -1;
                        for (long j = 0; j < 256; j++)
                            if (abc[j] == key)
                            {
                                keyIndex = j;
                                break;
                            }
                        for (long j = 0; j < 256; j++)
                            if (table[keyIndex, j] == value)
                            {
                                valueIndex = j;
                                break;
                            }
                        result[i] = abc[valueIndex];
                        }
                    }

感谢您的帮助。我很想弄清楚这一点，但我找不到任何东西，并且一直在为HOURS而苦苦挣扎。