Base64 Decode/Encode
Base64 is a generic term for a number of similar encoding schemes that encode binary data by treating it numerically and translating it into a base 64 representation. The Base64 term originates from a specific MIME content transfer encoding.
Design
The particular choice of characters to make up the 64 characters required for base varies between implementations. The general rule is to choose a set of 64 characters that is both part of a subset common to most encodings, and also printable. This combination leaves the data unlikely to be modified in transit through systems, such as email, which were traditionally not 8-bit clean. For example, MIME's Base64 implementation uses A-Z, a-z, and 0-9 for the first 62 values. Other variations, usually derived from Base64, share this property but differ in the symbols chosen for the last two values; an example is UTF-7.
Example
A quote snippet from Thomas Hobbes's Leviathan:
"Man is distinguished, not only by his reason, but ..."
represented as an ASCII byte sequence is encoded in MIME's Base64 scheme as follows:
TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCAuLi4=
In the above quote the encoded value of Man is TWFu. Encoded in ASCII, M, a, n are stored as the bytes 77, 97, 110, which are 01001101, 01100001, 01101110 in base 2. These three bytes are joined together in a 24 bit buffer producing 010011010110000101101110. Packs of 6 bits (6 bits have a maximum of 64 different binary values) are converted into 4 numbers (24 = 4 * 6 bits) which are then converted to their corresponding values in Base64.
Text content M a n
ASCII 77 97 110
Bit pattern 0 1 0 0 1 1 0 1 0 1 1 0 0 0 0 1 0 1 1 0 1 1 1 0
Index 19 22 5 46
Base64-encoded T W F u
As this example illustrates, Base64 encoding converts 3 uncoded bytes (in this case, ASCII characters) into 4 encoded ASCII characters.
Source: Wikipedia
