Base64: How it Works

Introduction
Base64 is the de facto standard encoding used for attachments by MIME (multipurpose Internet mail extensions). Almost all small clients use MIME to send files as attachments, which means that a majority of mail clients use Base64 to encode files before they are sent across networks.

Base64 is also used in several other ways. For one, Web servers implementing HTTP-based basic authentication use Base64 to encode the username and password of such servers.

However, Base64 is a very weak encoding standard and can be reverse-engineered in only minutes. This guide will walk you through how.

Encoding
NOTE: You will need a binary converter to use this guide effectively

Base64 encoding is quite simple. When it is used to encode plaintext (as it often is), the following steps occur:

- The ASCII value is converted to its binary equivalent (A becomes 01000001, y becomes 01111001, etc.)
- The 8 bits that make up a byte are split up into groups of 6 bits. For example:
CODE :

- These 6-bit binary values are then converted to their decimal form (using the table below), and then finally converted to their new encoded value. The table is in the format of BIN, DEC, ENCODED.
CODE :

So, using the above table, we can take our previous example and finish it up.

CODE :


word is d29yZA when encoded in Base64. See? Easy.

Decoding
- Convert the character to its 6-bit binary equivalent.
- Working from left to right, convert the 6-bit binary back to 8-bit binary. You may have some extra zeroes from the conversion, as we would in our example above. Trim them off until you get something that makes sense
- Convert the 8-bit binary back to its ASCII equivalents.

That's it!

Using our previous example...
CODE :


It's as easy as that. Have fun sniffing e-mail packets and decoding their attachment contents! (not that I'd recommend this >.>)