add spec and README

README.md

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+# rsp6-decoder
+
+This is a set of tools to decode UK National Rail electronic ticket barcodes, specifically RSP6 format,
+as explained in the blog post ["Reversing UK mobile rail tickets"](https://eta.st/2023/01/31/rail-tickets.html).
+
+If you're interested in a working demo, check out the hosted version at [eta.st/tickets/](https://eta.st/tickets).
+
+## What's in this repo?
+
+- This repo is primarily a Rust crate that can decode these tickets, given a scanned barcode string (starting with `06...`).
+  Look at the `decode_ticket` function for the main entry point.
+- A website that embeds the crate for demo purposes (powering the hosted version) is inside `rsp6-webshite/`.
+  - This will require compiling the main crate for WASM using `wasm-pack build --features wasm`.
+  - If you get errors decoding at runtime, you might need to replace references to `wbindgen_realloc` in
+    `pkg/rsp6_decoder_bg.js` with `undefined`. It's unclear why this happens.
+- `spec.pdf` has a janky table showing what all the fields in a decoded ticket look like.
+  - `spec.ods` is the janky spreadsheet that was used to make this.
+- `keys.json` has a copy of all the ticket public keys, as of 2023-01.
+  - `decode.py` was a hacky Python script to make these from some binary plist I found at some point. You can ignore it.
+
+## License
+
+This is all MIT. Please go forth and use it to make your own ticket stuff!