Lightning payment requests

You have pasted many invoices by now. A BOLT11 invoice is a bech32 string with three parts:

lnbcrt500u 1 pj9x…(data)…  (signature)
└── hrp ─┘ ↑ └──── tagged fields ────┘
       separator

The human-readable part (hrp)

• ln: Lightning.
• bc mainnet, tb testnet, tbs signet, bcrt regtest.
• An optional amount with a multiplier: m milli (10⁻³ BTC), u micro (10⁻⁶), n nano (10⁻⁹), p pico (10⁻¹²).

So lnbcrt500u means 500 × 10⁻⁶ BTC = 0.0005 BTC = 50,000 sats. The separator is the last 1 in the string; bech32 data never contains a 1.

Tagged fields

After a timestamp, the data holds tagged fields: p the payment hash (the hash of the preimage), d a description, x expiry, c the final time lock, s a payment secret, r routing hints for private channels, and 9 feature bits. The invoice ends with Dina's signature, which also reveals her public key.

Decode the amount

⚡Read the amount inside an invoice

Python

NodeAlice · LNDBob · c-lightningChan · c-lightningDina · LNDNo node

from jifunze_lab import lnd MULTIPLIER = {"m": 10**-3, "u": 10**-6, "n": 10**-9, "p": 10**-12} def amount_sats(invoice: str) -> int: """Read the amount from the human-readable part, e.g. lnbcrt500u1... -> 50000 sats.""" hrp = invoice[: invoice.rfind("1")] # TODO: strip the 'lnbc' / 'lnbcrt' prefix, split the number from the multiplier letter, # and convert bitcoin to sats (1 BTC = 100,000,000 sats). return 0 invoice = lnd("dina").addinvoice(50000, "kitabu")["payment_request"] print(invoice[:30], "...") print("my decoder:", amount_sats(invoice), "sats") print("lnd says: ", lnd("dina").decode(invoice)["num_satoshis"], "sats")

▶ RunCheckReset lessonRestore starter code

These exercises need a laptop with the lab running. You can read the lesson on your phone.

The check tries your amount_sats on several kinds of invoices: micro, milli and nano amounts on mainnet and regtest.

Beyond BOLT11: offers

BOLT 11 invoices are single-use and carry everything in one string. BOLT 12 offers are reusable: a shop prints one offer, and each payer's wallet fetches a fresh invoice over Lightning itself. That allows refunds, recurring payments and more privacy for the receiver.