Base64 Encoder/Decoder

What is Base64?

Base64 is a binary-to-text encoding scheme that represents binary data using a set of 64 ASCII characters. It was designed to ensure that binary data can be safely transmitted over text-based systems that may not handle raw bytes correctly — such as email (SMTP), JSON, XML, and URL parameters.

The name “Base64” refers to the 64-character alphabet used for encoding. Unlike hexadecimal (Base16), which uses 16 characters, Base64 is more compact — it represents 6 bits per character instead of 4.

How Base64 Encoding Works

The encoding process operates on groups of 3 bytes (24 bits) at a time:

1. Take 3 bytes of input (24 bits total)
2. Split into 4 groups of 6 bits each
3. Map each 6-bit value to a character from the Base64 alphabet
4. If the input length isn’t a multiple of 3, pad with = characters

The Base64 Alphabet

Index Range	Characters	Count
0-25	A to Z	26
26-51	a to z	26
52-61	0 to 9	10
62	+	1
63	/	1
Padding	=	—

Base64URL Variant

For URLs and filenames, the standard + and / characters are problematic. The Base64URL variant (RFC 4648 §5) replaces them:

• + becomes -
• / becomes _
• Padding = is often omitted

This variant is used in JWTs, OAuth tokens, and anywhere tokens appear in URLs.

Padding Explained

Base64 works on 3-byte groups. When the input doesn’t divide evenly by 3:

Input Bytes	Base64 Output	Padding
3 bytes	4 characters	None
2 bytes	3 characters + =	1 pad
1 byte	2 characters + ==	2 pads

Common Use Cases

• Email attachments (MIME): Binary files are Base64-encoded to travel safely through SMTP, which only supports 7-bit ASCII
• Data URIs: Embed images directly in HTML or CSS as data:image/png;base64,iVBORw0KGgo...
• HTTP Basic Auth: The Authorization: Basic header carries username:password as Base64
• JSON payloads: Binary data (files, images, certificates) can be embedded in JSON strings via Base64
• JWT tokens: Each segment (header, payload, signature) is Base64URL-encoded