Base64 Encoding

Sometimes we need to share binary data with others and one way to do this is in text form. But as we saw in the Character Encoding section, there is a careful mapping between characters and numerical values.

Let's say we want to share a single byte. If that byte has a value between 32 and 126, it would be possible to send the ASCII representation of the value over plaintext. For example, a byte of value 0x41 could be represented as A.

But what happens to the byte values outside the printable range? To accommodate for scenarios such as these a binary to text encoding scheme known as Base64 was invented. Every 6 bits of binary data are represented by an 8 bit character. Since 2^6 is 64, we need only part of the typical ASCII character set.

This ratio of 8 bit characters to 6 bit encoding means that, for every 3 bytes of data, we need 4 bytes of characters to encode it.

What happens in the case where the data is not a simple multiple of 6 bits? The values are padded with zeros and the = character appears.

Example 1: The hex value 0xc0ffee is a 24-bit number. To encode this we will need 24/6 * 8 = 32 bits, or 4 characters. db

0xc0ffee is therefore encoded as wP/u in Base64.

Example 2: Encoding 0xdeadbeef.

We have four bytes to encode here, or 32-bits, which does not divide evenly by 6. Instead, some padding will be introduced which leads to the characteristic '=' sign at the end of many Base64 encoded strings.

We see 0xdeadbeef encoded as 3q2+7w== after padding.