Why do we use Base64?

Your first mistake is thinking that ASCII encoding and Base64 encoding are interchangeable. They are not. They are used for different purposes.

• When you encode text in ASCII, you start with a text string and convert it to a sequence of bytes.
• When you encode data in Base64, you start with a sequence of bytes and convert it to a text string.

To understand why Base64 was necessary in the first place we need a little history of computing.

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Computers communicate in binary - 0s and 1s - but people typically want to communicate with more rich forms data such as text or images. In order to transfer this data between computers it first has to be encoded into 0s and 1s, sent, then decoded again. To take text as an example - there are many different ways to perform this encoding. It would be much simpler if we could all agree on a single encoding, but sadly this is not the case.

Originally a lot of different encodings were created (e.g. Baudot code) which used a different number of bits per character until eventually ASCII became a standard with 7 bits per character. However most computers store binary data in bytes consisting of 8 bits each so ASCII is unsuitable for tranferring this type of data. Some systems would even wipe the most significant bit. Furthermore the difference in line ending encodings across systems mean that the ASCII character 10 and 13 were also sometimes modified.

To solve these problems Base64 encoding was introduced. This allows you to encode arbitrary bytes to bytes which are known to be safe to send without getting corrupted (ASCII alphanumeric characters and a couple of symbols). The disadvantage is that encoding the message using Base64 increases its length - every 3 bytes of data is encoded to 4 ASCII characters.

To send text reliably you can first encode to bytes using a text encoding of your choice (for example UTF-8) and then afterwards Base64 encode the resulting binary data into a text string that is safe to send encoded as ASCII. The receiver will have to reverse this process to recover the original message. This of course requires that the receiver knows which encodings were used, and this information often needs to be sent separately.

Historically it has been used to encode binary data in email messages where the email server might modify line-endings. A more modern example is the use of Base64 encoding to embed image data directly in HTML source code. Here it is necessary to encode the data to avoid characters like '<' and '>' being interpreted as tags.

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Here is a working example:

I wish to send a text message with two lines:

Hello
world!

If I send it as ASCII (or UTF-8) it will look like this:

72 101 108 108 111 10 119 111 114 108 100 33

The byte 10 is corrupted in some systems so we can base 64 encode these bytes as a Base64 string:

SGVsbG8Kd29ybGQh

Which when encoded using ASCII looks like this:

83 71 86 115 98 71 56 75 100 50 57 121 98 71 81 104

All the bytes here are known safe bytes, so there is very little chance that any system will corrupt this message. I can send this instead of my original message and let the receiver reverse the process to recover the original message.

Why not look to the RFC that currently defines Base64?

Base encoding of data is used in many situations to store or transfer
data in environments that, perhaps for legacy reasons, are restricted to US-ASCII [1] data.Base encoding can also be used in new applications that do not have legacy restrictions, simply because it makes it possible to manipulate objects with text editors.

In the past, different applications have had different requirements and thus sometimes implemented base encodings in slightly different ways. Today, protocol specifications sometimes use base encodings in general, and "base64" in particular, without a precise description or reference. Multipurpose Internet Mail Extensions (MIME) [4] is often used as a reference for base64 without considering the consequences for line-wrapping or non-alphabet characters. The purpose of this specification is to establish common alphabet and encoding considerations. This will hopefully reduce ambiguity in other documents, leading to better interoperability.

Base64 was originally devised as a way to allow binary data to be attached to emails as a part of the Multipurpose Internet Mail Extensions.

Encoding binary data in XML

Suppose you want to embed a couple images within an XML document. The images are binary data, while the XML document is text. But XML cannot handle embedded binary data. So how do you do it?

One option is to encode the images in base64, turning the binary data into text that XML can handle.

Instead of:

<images>
  <image name="Sally">{binary gibberish that breaks XML parsers}</image>
  <image name="Bobby">{binary gibberish that breaks XML parsers}</image>
</images>

you do:

<images>
  <image name="Sally" encoding="base64">j23894uaiAJSD3234kljasjkSD...</image>
  <image name="Bobby" encoding="base64">Ja3k23JKasil3452AsdfjlksKsasKD...</image>
</images>

And the XML parser will be able to parse the XML document correctly and extract the image data.