Base45 Encoder & Decoder

Base45 is a binary-to-text encoding defined in RFC 9285. It packs two bytes into three characters from a 45-symbol alphabet, which is efficient to store in QR codes.

Most notably in the EU Digital COVID Certificate and other QR-based systems, where the 45-character alphabet maps well onto QR code alphanumeric mode.

Text is encoded as UTF-8 first, so accented letters, emoji and CJK characters become their byte sequences and decode back exactly.

Decoding checks that every character is in the alphabet and that each group is a valid length and value. Invalid input shows an error instead of broken output.

Because length in characters is the wrong measure. Base45 turns every 2 bytes into 3 characters, a fixed 1.5×, where Base64 is about 1.36×. But Base45's alphabet is exactly QR's alphanumeric set, which packs two characters into 11 bits — 5.5 bits each — while Base64 needs lowercase and so forces byte mode at 8 bits per character. For 100 bytes that is 825 bits of QR payload against 1,088, about 24% less, which is often a whole size class of symbol.

Not without quoting it. The alphabet's last nine symbols are space, $, %, *, +, -, . / and : — so a Base45 string routinely contains a literal space, a percent sign that URL decoding will read as an escape, and a plus that form decoding will turn into a space. It is also uppercase-only, so anything that lowercases the string in transit destroys it. Base45 is designed for the inside of a QR symbol, not for a query parameter; percent-encode it or use Base64url if it has to travel in a URL.