Encrypting Passwords In C# code behind

private const int prefix = 376;
    private const string postfix = "P$Y";
 
    public string encrypt(string _message)
    {
        string afterEncode;
        byte[] b = ConvertToByte(_message);
        int bl = b.Length;
 
        afterEncode = Convert.ToBase64String(ConvertToByte(_message), 0, _message.Length);
 
        //get length of encoded string and mask it by adding the prefix
        int codeLengthint = afterEncode.Length + prefix;
 
        //turn it back into a string, append the encoded word, and add a postfix to mask it
        string codeLength = codeLengthint.ToString();
        afterEncode = codeLength + afterEncode + postfix;
 
        //encode again for good measure
        afterEncode = Convert.ToBase64String(ConvertToByte(afterEncode));
 
        //remove problematic characters and add a string that is unlikely to exist by chance
        string testString = "";
        string outString = "";
        for (int i = 0; i < afterEncode.Length; i++)
        {
            testString = afterEncode[i].ToString();
            if (testString == "?")
            {
                testString = "QUESTION";
            }
            else if (testString == "&")
            {
                testString = "AMPERSAND";
            }
            else if (testString == "=")
            {
                testString = "EQUALSTO";
            }
            outString += testString;
        }
        afterEncode = outString;
 
        return afterEncode;
    }
 
    public string decrypt(string _message)
    {
        try
        {
            string afterEncode = _message;
            string testString = "";
            int codeLengthint = 0;
 
            //replace key words with the characters they represent
            while (afterEncode.IndexOf("QUESTION") != -1)
            {
                testString = afterEncode.Substring(0, afterEncode.IndexOf("QUESTION"));
                afterEncode = testString + "?" + afterEncode.Substring(afterEncode.IndexOf("QUESTION") + 8);
            }
            while (afterEncode.IndexOf("AMPERSAND") != -1)
            {
                testString = afterEncode.Substring(0, afterEncode.IndexOf("AMPERSAND"));
                afterEncode = testString + "&" + afterEncode.Substring(afterEncode.IndexOf("AMPERSAND") + 9);
            }
            while (afterEncode.IndexOf("EQUALSTO") != -1)
            {
                testString = afterEncode.Substring(0, afterEncode.IndexOf("EQUALSTO"));
                afterEncode = testString + "=" + afterEncode.Substring(afterEncode.IndexOf("EQUALSTO") + 8);
            }
 
            afterEncode = ConvertToString(Convert.FromBase64String(afterEncode));
 
            //get length of encoded character
            codeLengthint = Convert.ToInt32(afterEncode.Substring(0, 3)) - prefix;
 
            //retrieve encoded word and decrypt it
            afterEncode = afterEncode.Substring(3, codeLengthint);
            string afterDecode = ConvertToString(Convert.FromBase64String(afterEncode));
 
            return afterDecode;
        }
        catch (Exception ex)
        {
            string sMsg = string.Format("SE2LegacyEncoding.Decode error - message = [{0}]", _message);
            throw new Exception(sMsg, ex);
        }
    }
 
    private byte[] ConvertToByte(string _string)
    {
        byte[] result = new byte[_string.Length];
 
        // Convert each of the characters in the string to byte
        for (int i = 0; i < _string.Length; i++) { result[i] = (byte)_string[i]; }
 
        return result;
    }
 
    private string ConvertToString(byte[] _bytes)
    {
        char[] result = new char[_bytes.Length];
 
        // Convert each of the characters in the string to byte
        for (int i = 0; i < _bytes.Length; i++) { result[i] = (char)_bytes[i]; }
 
        return new string(result);
    }
   

                                              

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:
40:
41:
42:
43:
44:
45:
46:
47:
48:
49:
50:
51:
52:
53:
54:
55:
56:
57:
58:
59:
60:
61:
62:
63:
64:
65:
66:
67:
68:
69:
70:
71:
72:
73:
74:
75:
76:
77:
78:
79:
80:
81:
82:
83:
84:
85:
86:
87:
88:
89:
90:
91:
92:
93:
94:
95:
96:
97:
98:
99:
100:
101:
102:
103:
104:
105:
106:
107:
108:
109: