Codec lesson · Base64URL

Base64URL: Base64 for filenames and URLs

Base64URL is the RFC 4648 URL- and filename-safe alphabet. It replaces + and / with - and _, while Encode Keyboard emits the optional padding-free form.

Standard · RFC 4648 Round trip: Exact for canonical Base64URL whose decoded bytes are valid UTF-8 text, including an initial U+FEFF when present.

Learning objective

Distinguish Base64URL from standard Base64 and validate a canonical UTF-8 text value without confusing encoding with encryption.

Encode Keyboard converts UTF-8 bytes with the RFC 4648 Base64URL alphabet and removes trailing = padding. Decode accepts canonical unpadded text or exactly the required padding, then requires valid UTF-8.

How Encode Keyboard handles it

Start with UTF-8 bytes

Text first becomes the same byte sequence used by the other exact data formats.

Use the URL-safe alphabet

The indices that standard Base64 writes as + and / are written as - and _.

Validate before returning text

Decode checks the alphabet, impossible lengths, allowed padding, canonical re-encoding, and UTF-8 output.

Worked example

InputHello?

OutputSGVsbG8_

The final _ is the URL-safe symbol where standard Base64 would use /. Encode Keyboard omits = padding.

Good uses

Reading URL-safe tokens that are documented as Base64URL
Learning the difference between Base64 alphabets
Moving non-sensitive UTF-8 text through filename-safe fields

Validation and common errors

Use only A–Z, a–z, 0–9, - and _ before optional trailing =
Do not mix + or / from standard Base64
Padding, when present, must be exactly the amount required by the final block

Practice in your browser

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