[C#/復号]Base64ではない文字/パッディング文字が含まれている？？？

###実現したいこととトラブル
C#で復号プログラムを作ろうとしています。
Unhandled Exception:以下のメッセージが調べても理解できず、お力をお貸しいただけませんでしょうか
BASE64形式のテキストに変換する必要があるのでしょうか？

###エラーメッセージ

Unhandled Exception:

System.FormatException: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
at System.Convert.FromBase64CharPtr (System.Char* inputPtr, System.Int32 inputLength) [0x00068] in <12b418a7818c4ca0893feeaaf67f1e7f>:0
at System.Convert.FromBase64String (System.String s) [0x00028] in <12b418a7818c4ca0893feeaaf67f1e7f>:0
at Sample.Sample.DecS (System.String text, System.String p) [0x0002c] in /workspace/Main.cs:25
at Sample.Sample.Main () [0x00012] in /workspace/Main.cs:11
[ERROR] FATAL UNHANDLED EXCEPTION: System.FormatException: The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
at System.Convert.FromBase64CharPtr (System.Char* inputPtr, System.Int32 inputLength) [0x00068] in <12b418a7818c4ca0893feeaaf67f1e7f>:0
at System.Convert.FromBase64String (System.String s) [0x00028] in <12b418a7818c4ca0893feeaaf67f1e7f>:0
at Sample.Sample.DecS (System.String text, System.String p) [0x0002c] in /workspace/Main.cs:25
at Sample.Sample.Main () [0x00012] in /workspace/Main.cs:11

###コード

C#
1using System;
2
3namespace Sample
4{
5  class Sample
6  {
7    static void Main()
8    {
9
10      Sample sample = new Sample();
11      Console.WriteLine(sample.Decode("...","aaa"));
12
13
14    }
15
16    private string Decode(string text, string p)
17    {
18        System.Security.Cryptography.RijndaelManaged _rijndael = new System.Security.Cryptography.RijndaelManaged();
19
20        byte[] _key, _iv;
21        GenerateKeyFromPassWord(p, _rijndael.KeySize, out _key, _rijndael.BlockSize, out _iv);
22        _rijndael.Key = _key;
23        _rijndael.IV = _iv;
24
25        byte[] _strByte = System.Convert.FromBase64String(text);
26
27        System.Security.Cryptography.ICryptoTransform _decryptor = _rijndael.CreateDecryptor();
28
29        byte[] _decByte = _decryptor.TransformFinalBlock(_strByte, 0, _strByte.Length);
30        _decryptor.Dispose();
31
32        return System.Text.Encoding.UTF8.GetString(_decByte);
33    }
34
35    public static void GKFP(string pass, int keySize, out byte[] key, int blockSize, out byte[] iv)
36    {
37        byte[] _salt = System.Text.Encoding.UTF8.GetBytes("・・・");
38
39        System.Security.Cryptography.Rfc2898DeriveBytes _deviceBytes = new System.Security.Cryptography.Rfc2898DeriveBytes(password, _salt);
40       
41        key = _deviceBytes.GetBytes(keySize);
42        iv = _deviceBytes.GetBytes(blockSize);
43    }
44  }
45}