Working with and studying lnd a bit recently, I'm finding the topic of macaroons increasingly interesting. Here's an easily digestible overview of how lnd uses them:
Here's a simple crypto quiz question coming out of that: how is an HMAC different from a digital signature with the HMAC secret as private key? (note: 'simple'; there's nothing advanced here, and it's not a trick question!)
To give a motivating example: you could create a macaroon that only allows the owner to spend up to 0.1btc and not more; and you could delegate that to a less secure server, or something.
Currently they don't have something as complicated as that, but they do have a macaroon specifically for generating invoices only (and not diddling with the wallet otherwise).
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