Base64

Purpose

It solved the problem of early mail systems that could carry only 7-bit ASCII text and could not transport binary attachments directly, while keeping control characters from being mangled by gateways.

Mechanism

The input byte stream is processed in 3-byte (24-bit) groups split into four 6-bit units, each mapped to one character of the Base64 alphabet. A final short group is padded with = to 4 characters. The encoded length is about 4/3 of the input, and the output length is always a multiple of 4.

Use cases

• Embedding binary data in JSON, XML, and URLs (e.g., small images in Data URLs)
• MIME attachment transport in email
• Encoding username:password in HTTP Basic authentication
• Encoding the header and payload segments of a JWT

Examples

• Input

  Hello

  Output

  SGVsbG8=

  5 bytes to 8 characters with one trailing = pad

Common errors

Using standard Base64 directly in URLs, where + and / get percent-encoded and decoding fails

Use Base64URL instead (- and _ replace + and /, and = padding is dropped)