Base64 Encode Decode Algorithm

I wrote this code about five years ago. I used it to encode query strings in web urls so that some sensitive information(IDs, keys) are not presented to the user directly. Then the query string is decoded to retrieved parameters. Since the output of Convert.ToBase64String in .Net framework is not suitable for URLs, I reimplemented the algorithm. You can find more description about Base64 in WikiPedia.

The Code

/// <summary>
/// Base64 Encode Decode Methods
/// </summary>
public class Base64 {
/// <summary>
/// Alphabet used for encoded string representation.
/// It should contain exactly 64 characters.
/// This can be changed to other text, but must be the same in encode/decode.
/// </summary>
public static string Alphabet
= "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz[]";
public static char Pad = '!';

public static string EncodeText(string text){
return EncodeText(text,Encoding.UTF8);
}

public static string DecodeText(string code){
return DecodeText(code,Encoding.UTF8);
}

public static string EncodeText(string text,Encoding encoding){
return Encode(encoding.GetBytes(text));
}

public static string DecodeText(string code,Encoding encoding){
return encoding.GetString(Decode(code));
}

public static string Encode(byte[] _data){
int n = _data.Length;
if(n==0)return"";
int r = n % 3;
int p = 3 - r;
if(p==3)p=0;
int k = n + p;
byte[] data = new byte[k];
_data.CopyTo(data,0);
int m = (k/3)*4;
int[] code = new int[m];
/* 6 2 4 4 2 6
* data|###### ##|#### ####|## ######|
* code|######|## ####|#### ##|######|
*
* code[0]=data[0]>>2;
* code[1]=data[0]<<4&X | data[1]>>4; X = 0b00110000 = 48;
* code[2]=data[1]<<2&Y | data[2]>>6; Y = 0b00111100 = 60;
* code[3]=data[2]&Z; Z = 0b00111111 = 63;
*/
for(int i=0;i<k/3;i++){
code[i*4+0] = data[i*3+0]>>2;
code[i*4+1] = data[i*3+0]<<4 & 48 | data[i*3+1]>>4;
code[i*4+2] = data[i*3+1]<<2 & 60 | data[i*3+2]>>6;
code[i*4+3] = data[i*3+2] & 63;
}
StringBuilder codestring = new StringBuilder(new string(Pad,m));
for(int i=0;i<m-p;i++){
codestring[i] = Alphabet[code[i]];
}
return codestring.ToString();
}

public static byte[] Decode(string _code){
int n = _code.Length;
byte[] code = new byte
;
for(int i=0;i<n;i++){
code[i] = LetterIndex(_code[i]);
}
int p = _code.IndexOf(Pad);
if(p==-1){
p = 0;
}else{
p = n - p;
}
int m = (n/4)*3;
byte[] data = new byte[m];
/* 6 2 4 4 2 6
* data|###### ##|#### ####|## ######|
* code|######|## ####|#### ##|######|
*
* data[0]=code[0]<<2 | code[1]>>4;
* data[1]=code[1]<<4 | code[2]>>2;
* data[2]=code[2]<<6 | code[3];
*/
for(int i=0;i<n/4;i++){
data[i*3+0]=(byte)(code[i*4+0]<<2 | code[i*4+1]>>4);
data[i*3+1]=(byte)(code[i*4+1]<<4 | code[i*4+2]>>2);
data[i*3+2]=(byte)(code[i*4+2]<<6 | code[i*4+3]);
}
byte[] _data = new byte[m-p];
for(int i=0;i<_data.Length;i++){
_data[i] = data[i];
}
return _data;
}

static byte LetterIndex(char letter){
int index = Alphabet.IndexOf(letter);
if(index == -1){
index = 0;
}
return (byte)index;
}
}