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Cover Story: Understanding Base64 Encoding

What it is, when to use it, and how to write custom Base64 encoding

If you work in a .NET environment you have probably come across Base64 encoded data. For example, Base64 encoding is used in ASP.NET for a Web application's ViewState value, as shown in Figure 1. Base64 encoding is also used to transmit binary data over e-mail. However, if you are like most of my colleagues (and me until recently) you do not have a thorough understanding of precisely what Base64 encoding is and when Base64 encoding should be used. In the this article I will explain exactly what Base64 encoding is, show you how to use the two primary .NET Framework methods that support Base64 encoding and decoding, and present a lightweight, custom C# implementation of Base64 encoding and decoding methods. This article assumes you are a .NET developer, tester, or manager and have intermediate level C# coding skill. After reading the article you'll have a solid grasp of Base64 encoding as well as the ability to write your own custom encoding methods. I think you'll find the ability to use Base64 encoded data is a valuable addition to your skill set.

The best way to show you where I'm headed in this article is with a screenshot. If you examine Figure 2 you'll see that I start with the arbitrary string "Hello" and use it to generate some binary data. After displaying the starting binary data in hexadecimal form, I convert the binary data to a Base64 encoded string using a method from the .NET Framework, and also using my custom encoding method. Notice the encoding results are the same. In the next part of the screenshot in Figure 2 I decode the Base64 encoded strings back to their original byte arrays, using both the built-in .NET Framework method and my custom implementation, and display in hexadecimal form. The complete program, which produced the screenshot in Figure 2, is presented in Listing 3.

What Is Base64 Encoding?
Exactly what is Base64 encoding? Base64 encoding is a scheme that encodes arbitrary binary data as a string composed from a set of 64 characters. The exact character set can be any 64 distinct ASCII characters, but by far the most common set is "A" through "Z," "a" through "z," "0" through "9," "+," and "/." For example, using this character set the 40-bit data:

01001000 01100101 01101100 01101100 01101111

can be Base64 encoded as the string:

SGVsbG8=

The trailing "=" character is a padding character as I will explain shortly. After seeing Base64 encoding for the first time, most engineers have an immediate question: Why would anyone want to use Base64 encoding? Base64 encoding is useful when you want to transmit binary data over a communication channel that is designed to transmit character data. For example, consider e-mail. The e-mail protocol SMTP was originally designed to send and receive only simple text data. However, suppose you want to transmit binary data such as a JPEG image. If you can encode the image as a Base64 string, then you can send the image just like any other message. Another common example is sending an ASP.NET Web application's ViewState value (which is a binary value representing the overall state of the application) over HTTP (which is an inherently text-based transport protocol). But why go to the trouble of Base64 encoding when "ordinary" encoding already exists? By ordinary encoding I mean regular hexadecimal encoding. For example, the 40-bit binary data above can be represented as a hexadecimal string: 48 65 6C 6C 6F (where the spaces are included just for readability). The answer is that Base64 encoding is more efficient than hexadecimal encoding in the sense that Base64 encoding requires fewer characters to represent the same data. Notice that the hexadecimal encoding of the 40-bit data above requires 10 characters while Base64 encoding requires only 8 characters - a 20 percent reduction.

Because Base64 encoding only uses 64 characters, any of the characters can be represented with just 6 bits because 26 = 64. Or, put another way, using 6 bits you can represent data in the range 000000b to 111111b, which is 0d to 63d. This is the key to Base64 encoding efficiency. The best way to explain how Base64 encoding works is with a picture as shown in Figure 3.

Suppose the first three bytes of input to be encoded are 48h, 65h, and 6Ch. In Figure 3 these values are shown in their binary representation: 01001000, 01100101, and 01101100. The first six bits of input, 010010, have value 18d, which in turn maps to character [18] in the Base64 character set, which is "S." The second six bits of input - the last two bits of the first byte of input and the first four bits of the second byte of input - equal 6d, which maps to character "G," and so on. Notice that three bytes of input map neatly to four character of output. Because of this it is convenient to implement Base64 encoding in "blocks" that represent a group of three bytes of input, or four characters of output.

NET Support for Base64 Encoding
The .NET Framework supports Base64 encoding with two methods. The Convert.ToBase64String() method accepts a byte array as an input argument and returns a Base64 encoded string (using the usual 64-character set described in the previous section). The Convert.FromBase64String() accepts a string argument (which is assumed to be a valid Base64 encoded string) and returns the corresponding byte array. Using these two methods is very easy. Notice both methods belong to the Convert class and are static so you do not need to instantiate an object to use the methods. The Convert class is part of the System namespace. Consider this code snippet:

byte[] bytes = new byte[] { 0x5F, 0xC9, 0xBF, 0x17 };
string base64 = Convert.ToBase64String(bytes);
Console.WriteLine(base64);

This code would produce "X8m/Fw==" as output. Notice that the input has size 4 bytes. The first three bytes (3 * 8 = 24 bits) of input are used to produce the first four Base64 characters (4 * 6 = 24 bits) of output. The last input byte produces the rest of the output, and the output is padded with "=" characters to bring the output size up to an even multiple of 4. The technique to decode is similar. The statements

string encoded = "X8m/Fw==";
byte[] result = Convert.FromBase64String(encoded);
Console.WriteLine(BitConverter.ToString(result));

produce "5F-C9-BF-17" as output. The Framework Base64 methods are simple and straightforward. However, if you are encoding, transmitting, and decoding large amounts of binary data, it is up to you write auxiliary code, which buffers the process by breaking the input data into manageable-sized chunks.

A Lightweight Custom Base64 Encoder
The .NET Framework's ToBase64-String() and FromBase64String() methods will meet most of your Base64 encoding needs. However what if you are developing a system and need a slightly different encoding scheme? For instance, you may want to use a different character set than the normal{"A"-"Z," "a"-"z," "0"-9," "+," "/"} set. In this section I'll present a lightweight, custom Base64 encoder written in C# that you can use as a starting point for your own custom encoder. If you search the Internet you'll find quite a few Base64 encoding examples. The one I present here is a hybrid of several I found combined with one I wrote recently, and is designed for maximum clarity rather than for efficiency. The custom encoder is presented in Listing 1.

Because I want my customer encoder to mimic the interface of the Framework encoder, I begin by creating an overall structure of:

public class MyConverter
{
    public static string ToBase64String(byte[] value)
   {
      // implementation goes here
}
} // class MyConverter

Of course there are many other design alternatives, but making the custom encoder signature the same as the Framework's encoder signature makes sense. I begin my encoder implementation by declaring an array of the 64 characters I want to use for my encoding:

char[] base64Chars = new char[]
{ 'A','B','C','D','E','F','G','H','I','J','K','L','M',
'N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
'a','b','c','d','e','f','g','h','i','j','k','l','m',
'n','o','p','q','r','s','t','u','v','w','x','y','z',
'0','1','2','3','4','5','6','7','8','9','+','/' };

This array acts as a lookup table to map a decimal value in the range 0 - 63 to a Base64 character. For example, 0 maps to "A," 1 maps to "B," and 26 maps to "a." I use the normal character set but you can use different characters, or change the order for a custom encoding scheme. Next I compute two values that will control the encoding algorithm:

int numBlocks;
int padBytes;
if ((value.Length % 3) == 0)
{
     numBlocks = value.Length / 3;
     padBytes = 0;
}
else
{
     numBlocks = 1 + (value.Length / 3);
     padBytes = 3 - (value.Length % 3);
}

More Stories By James McCaffrey

Dr. James McCaffrey works for Volt Information Sciences, Inc., where he manages technical training for software engineers working at Microsoft's Redmond, WA campus. He has worked on several Microsoft products, including Internet Explorer and MSN Search. James can be reached at [email protected] or [email protected]

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