15_payment_requests.asciidoc

Lightning Payment Requests

In this chapter we will look at Lightning payment requests, or as they are more commonly known, Lightning invoices.

Invoices in the Lightning Protocol Suite

Payment requests, aka invoices, are part of the payment layer and are shown in the upper left of Payment requests in the Lightning protocol suite.

Figure 1. Payment requests in the Lightning protocol suite

Introduction

As we’ve learned throughout the book, minimally two pieces of data are required to complete a Lightning payment: a payment hash and a destination. As SHA-256 is used in the Lightning Network to implement HTLCs, this information requires 32 bytes to communicate. Destinations, on the other hand, are simply the secp256k1 public key of the node that wishes to receive a payment. The purpose of a payment request in the context of the Lightning Network is to communicate these two pieces of information from sender to receiver. The QR-code-friendly format for communicating the information required to complete a payment from receiver to sender is described in BOLT #11: Invoice Protocol for Lightning Payments. In practice, more than just the payment hash and destination are communicated in a payment request to make the encoding more fully featured.

Lightning Payment Requests Versus Bitcoin Addresses

A commonly asked question when people first encounter a Lightning Payment request is: why can’t a normal static address format be used instead?

To answer this question, you must first internalize how Lightning differs from base layer Bitcoin as a payment method. Compared to a Bitcoin address which may be used to make a potentially unbounded number of payments (though reusing a Bitcoin address may degrade one’s privacy), a Lightning payment request should only ever be used once. This is due to the fact that sending a payment to a Bitcoin address essentially uses a public key cryptosystem to "encode" the payment in a manner that only the true "owner" of that Bitcoin address can redeem it.

In contrast, to complete a Lightning payment, the recipient must reveal a "secret" to the entire payment route, including the sender. This can be interpreted as usage of a kind of domain-specific symmetric cryptography because the payment preimage is for practical purposes a nonce (number only used once). If the sender attempts to make another payment using that identical payment hash, then they risk losing funds because the payment may not actually be delivered to the destination. It’s safe to assume that after a preimage has been revealed, all nodes in the path will keep it around forever, then rather than forward the HTLC to collect a routing fee if the payment is completed, they can simply settle the payment at that instance and gain the entire payment amount in return. As a result, it’s unsafe to ever use a payment request more than once.

New variants of the original Lightning payment request exist that allow the sender to reuse them as many times as they want. These variants flip the normal payment flow as the sender transmits a preimage within the encrypted onion payload to the receiver, who is the only one that is able to decrypt it and settle the payment. Alternatively, assuming a mechanism that allows a sender to typically request a new payment request from the receiver, then an interactive protocol can be used to allow a degree of payment request reuse.

BOLT #11: Lightning Payment Request Serialization and Interpretation

In this section, we’ll describe the mechanism used to encode the set of information required to complete a payment on the Lightning Network. As mentioned earlier, the payment hash and destination is the minimum amount of information required to complete a payment. However, in practice, more information such as timelock information, payment request expiration, and possibly an on-chain fallback address are also communicated. The full specification document is BOLT #11: Invoice Protocol for Lightning Payments.

Payment Request Encoding in Practice

First, let’s examine what a real payment request looks like in practice. The following is a valid payment request that could have been used to complete a payment on the mainnet Lightning Network at the time it was created:

lnbc2500u1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdq5xysx
xatsyp3k7enxv4jsxqzpuaztrnwngzn3kdzw5hydlzf03qdgm2hdq27cqv3agm2awhz5se903vruatf
hq77w3ls4evs3ch9zw97j25emudupq63nyw24cg27h2rspfj9srp

The Human-Readable Prefix

Looking at the string, we can tease out a portion that we can parse with our eyes, while the rest of it just looks like a random set of strings. The part that is somewhat parsable by a human is referred to as the human-readable prefix. It allows a human to quickly extract some relevant information from a payment request at a glance. In this case, we can see that this payment is for the mainnet instance of the Lightning Network (lnbc), and is requesting 2,500 uBTC (microbitcoin), or 25,000,000 satoshis. The latter potion is referred to as the data portion and uses an extensible format to encode the information required to complete a payment.

Each version of instance of the Lightning Network (mainnet, testnet, etc.) has its own human-readable prefix (see BOLT #11 network prefixes). This allows client software and also humans to quickly determine if a payment request can be satisfied by their node or not.

Table 1. BOLT #11 network prefixes

Network	BOLT #11 prefix
mainnet	lnbc
testnet	lntb
simnet/regtest	lnbcrt

The first portion of the human-readable prefix is a compact expression of the amount of the payment request. The compact amount is encoded in two parts. First, an integer is used as the base amount. This is then followed by a multiplier that allows us to specify distinct order of magnitude increases offset by the base amount. If we return to our initial example, then we can take the 2500u portion and decrease it by a factor of 1,000 to instead use 2500m or (2,500 mBTC). As a rule of thumb, to ascertain the amount of an invoice at a glance, take the base factor and multiply it by the multiplier.

A full list of the currently defined multipliers is given in BOLT #11 amount multipliers.

Table 2. BOLT #11 amount multipliers

Multiplier	Bitcoin unit	Multiplication factor
m	milli	0.001
u	micro	0.000001
n	nano	0.000000001
p	pico	0.000000000001

bech32 and the Data Segment

If the "unreadable" portion looks familiar, then that’s because it uses the very same encoding scheme as SegWit compatible Bitcoin addresses use today, namely bech32. Describing the bech32 encoding scheme is outside the scope of this chapter. In brief, it’s a sophisticated way to encode short strings that has very good error correction as well as detection properties.

The data portion can be separated into three sections:

• The timestamp

• Zero or more tagged key-value pairs

• The signature of the entire invoice

The timestamp is expressed in seconds since the year 1970, or the Unix Epoch. This timestamp allows the sender to gauge how old the invoice is, and as we’ll see later, allows the receiver to force an invoice to only be valid for a period of time if they wish.

Similar to the TLV format we learned about in [tlv], the BOLT #11 invoice format uses a series of extensible key-value pairs to encode information needed to satisfy a payment. Because key-value pairs are used, it’s easy to add new values in the future if a new payment type or additional requirement/functionality is introduced.

Finally, a signature is included that covers the entire invoice signed by the destination of the payment. This signature allows the sender to verify that the payment request was indeed created by the destination of the payment. Unlike Bitcoin payment requests which aren’t signed, this allows us to ensure that a particular entity signed the payment request. The signature itself is encoded using a recovery ID, which allows a more compact signature to be used that allows public key extraction. When verifying the signature, the recovery ID extracts the public key, then verifies that against the public key included in the invoice.

Tagged invoice fields

The tagged invoice fields are encoded in the main body of the invoice. These fields represent different key-value pairs that express either additional information that may help complete the payment or information which is required to complete the payment. Because a slight variant of bech32 is utilized, each of these fields are actually in the "base 5" domain.

A given tag field is comprised of three components:

• The type of the field (5 bits)

• The length of the data of the field (10 bits)

• The data itself, which is length * 5 bytes in size

A full list of all the currently defined tagged fields is given in BOLT #11 tagged invoice fields.

Table 3. BOLT #11 tagged invoice fields

Field tag	Data length	Usage
p	52	The SHA-256 payment hash.
s	52	A 256-bit secret that increases the end-to-end privacy of a payment by mitigating probing by intermediate nodes.
d	Variable	The description, a short UTF-8 string of the purpose of the payment.
n	53	The public key of the destination node.
h	52	A hash that represents a description of the payment itself. This can be used to commit to a description that’s over 639 bytes in length.
x	Variable	The expiry time, in seconds, of the payment. The default is 1 hour (3,600) if not specified.
c	Variable	The min_cltv_expiry to use for the final hop in the route. The default is 9 if not specified.
f	Variable	A fallback on-chain address to be used to complete the payment if the payment cannot be completed over the Lightning Network.
r	Variable	One or more entries that allow a receiver to give the sender additional ephemeral edges to complete the payment.
9	Variable	A set of 5-bit values that contain the feature bits that are required in order to complete the payment.

The elements contained in the field r are commonly referred to as routing hints. They allow the receiver to communicate an extra set of edges that may help the sender complete their payment. The hints are usually used when the receiver has some/all private channels, and they wish to guide the sender into this "unmapped" portion of the channel graph. A routing hint encodes effectively the same information that a normal channel_update message does. The update is itself packed into a single value with the following fields:

• The pubkey of the outgoing node in the edge (264 bits)

• The short_channel_id of the "virtual" edge (64 bits)

• The base fee (fee_base_msat) of the edge (32 bits)

• The proportional fee (fee_proportional_millionths) (32 bits)

• The CLTV expiry delta (cltv_expiry_delta) (16 bits)

The final portion of the data segment is the set of feature bits that communicate to the sender the functionality needed to complete a payment. As an example, if a new payment type is added in the future that isn’t backward compatible with the original payment type, then the receiver can set a required feature bit to communicate that the payer needs to understand that feature to complete the payment.

Conclusion

As we have seen, invoices are a lot more than just a request for an amount. They contain critical information about how to make the payment, such as routing hints, the destination node’s public key, ephemeral keys to increase security, and much more.