1
Visit the channel
Subscribe if you haven't. It supports the project and there's nothing to pay.
Open the channel
2
Come back and get your key
Trip confirmed — you can create a key.
3
Or paste one you already have
Keys are tied to this phone's device ID.
What this actually checks
Only that you went to the channel and came back. YouTube gives no way for any app to confirm a subscription.
Better yet: do it in the app
The app can do the whole thing itself. Tap "go to channel" there, come back, and it mints your key.
Key Format
NNNN
- 4-digit nonce (changes every time)
HHHH
- first 4 chars of SHA-256 hash
HHHH
- last 4 chars of SHA-256 hash
L
- checksum character (A-Z)