Base45 Encoder/Decoder

You scanned an EU digital COVID certificate, saw a wall of characters starting with HC1: and wondered what encoding that was. Or you are reading RFC 9285 and want to watch the alphabet work on your own input instead of pulling in a library. This page covers the bottom layer: type text, get the matching Base45 string, refreshed on every keystroke.

The algorithm chews through the input two bytes at a time. Each pair becomes a 16-bit value (a times 256 plus b) written as three base-45 digits, least significant first. A leftover single byte at the end emits two digits instead. The alphabet holds 45 symbols: 0-9, A-Z, space and $%*+-./: — precisely the QR alphanumeric table. Type AB and you get BB8, the test vector printed in the RFC itself.

Base45 emits more characters than Base64 does (three per two bytes versus four per three) and still yields a smaller QR, because alphanumeric mode packs each character into roughly 5.5 bits instead of 8. Two honest limits: this page only encodes, even though the tool description mentions decoding, and the output is not a valid certificate — a real DCC wraps CBOR, a COSE signature, zlib and the HC1: prefix around the payload. Everything runs in your browser.