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BOLT #12: Flexible Protocol for Lightning Payments

Table of Contents

Limitations of BOLT 11

The BOLT 11 invoice format has proven popular but has several limitations:

  1. The entangling of bech32 encoding makes it awkward to send in other forms (e.g. inside the lightning network itself).
  2. The signature applying to the entire invoice makes it impossible to prove an invoice without revealing its entirety.
  3. Fields cannot generally be extracted for external use: the h field was a boutique extraction of the d field only.
  4. The lack of the 'it's OK to be odd' rule makes backward compatibility harder.
  5. The 'human-readable' idea of separating amounts proved fraught: p was often mishandled, and amounts in pico-bitcoin are harder than the modern satoshi-based counting.
  6. Developers found the bech32 encoding to have an issue with extensions, which means we want to replace or discard it anyway.
  7. The payment_secret designed to prevent probing by other nodes in the path was only useful if the invoice remained private between the payer and payee.
  8. Invoices must be given per user and are actively dangerous if two payment attempts are made for the same user.

Payment Flow Scenarios

Here we use "user" as shorthand for the individual user's lightning node and "merchant" as the shorthand for the node of someone who is selling or has sold something.

There are two basic payment flows supported by BOLT 12:

The general user-pays-merchant flow is:

  1. A merchant publishes an offer, such as on a web page or a QR code.
  2. Every user requests a unique invoice over the lightning network using an invoice_request message, which contains the offer fields.
  3. The merchant replies with the invoice.
  4. The user makes a payment to the merchant as indicated by the invoice.

The merchant-pays-user flow (e.g. ATM or refund):

  1. The merchant publishes an invoice_request which includes an amount it wishes to send to the user.
  2. The user sends an invoice over the lightning network for the amount in the invoice_request, using a (possibly temporary) invoice_node_id.
  3. The merchant confirms the invoice_node_id to ensure it's about to pay the correct person, and makes a payment to the invoice.

Payment Proofs and Payer Proofs

Note that the normal lightning "proof of payment" can only demonstrate that an invoice was paid (by showing the preimage of the payment_hash), not who paid it. The merchant can claim an invoice was paid, and once revealed, anyone can claim they paid the invoice, too.[1]

Providing a key in invoice_request allows the payer to prove that they were the one to request the invoice. In addition, the Merkle construction of the BOLT 12 invoice signature allows the user to reveal invoice fields in case of a dispute selectively.

Encoding

Each of the forms documented here are in TLV format.

The supported ASCII encoding is the human-readable prefix, followed by a 1, followed by a bech32-style data string of the TLVs in order, optionally interspersed with + (for indicating additional data is to come). There is no checksum, unlike bech32m.

Requirements

Writers of a bolt12 string:

  • MUST either use all lowercase or all UPPERCASE.
  • SHOULD use uppercase for QR codes.
  • SHOULD use lower case otherwise.
  • MAY use +, optionally followed by whitespace, to separate large bolt12 strings.

Readers of a bolt12 string:

  • MUST handle strings which are all lowercase, or all uppercase.
  • if it encounters a + followed by zero or more whitespace characters between two bech32 characters:
    • MUST remove the + and whitespace.

Rationale

The use of bech32 is arbitrary but already exists in the bitcoin world. We currently omit the six-character trailing checksum: QR codes have their own checksums anyway, and errors don't result in loss of funds, simply an invalid offer (or inability to parse).

The use of + (which is ignored) allows use over limited text fields like Twitter:

lno1xxxxxxxx+

yyyyyyyyyyyy+

zzzzz

See format-string-test.json.

Signature Calculation

All signatures are created as per BIP-340 and tagged as recommended there. Thus we define H(tag,msg) as SHA256(SHA256(tag) || SHA256(tag) || msg), and SIG(tag,msg,key) as the signature of H(tag,msg) using key.

Each form is signed using one or more signature TLV elements: TLV types 240 through 1000 (inclusive). For these, the tag is "lightning" || messagename || fieldname, and msg is the Merkle-root; "lightning" is the literal 9-byte ASCII string, messagename is the name of the TLV stream being signed (i.e. "invoice_request" or "invoice") and the fieldname is the TLV field containing the signature (e.g. "signature").

The formulation of the Merkle tree is similar to that proposed in BIP-341, with each TLV leaf paired with a nonce leaf to avoid revealing adjacent nodes in proofs.

The Merkle tree's leaves are, in TLV-ascending order for each tlv:

  1. The H("LnLeaf",tlv).
  2. The H("LnNonce"||first-tlv,tlv-type) where first-tlv is the numerically-first TLV entry in the stream, and tlv-type is the "type" field (1-9 bytes) of the current tlv.

The Merkle tree inner nodes are H("LnBranch", lesser-SHA256||greater-SHA256); this ordering means proofs are more compact since left/right is inherently determined.

If there is not exactly a power of 2 leaves, then the tree depth will be uneven, with the deepest tree on the lowest-order leaves.

e.g. consider the encoding of an invoice signature with TLVs TLV0, TLV1, and TLV2 (of types 0, 1, and 2 respectively):

L1=H("LnLeaf",TLV0)
L1nonce=H("LnNonce"||TLV0,0)
L2=H("LnLeaf",TLV1)
L2nonce=H("LnNonce"||TLV0,1)
L3=H("LnLeaf",TLV2)
L3nonce=H("LnNonce"||TLV0,2)

Assume L1 < L1nonce, L2 > L2nonce and L3 > L3nonce.

   L1    L1nonce                      L2   L2nonce                L3   L3nonce
     \   /                             \   /                       \   /
      v v                               v v                         v v
L1A=H("LnBranch",L1||L1nonce) L2A=H("LnBranch",L2nonce||L2)  L3A=H("LnBranch",L3nonce||L3)
                 
Assume L1A < L2A:

       L1A   L2A                                 L3A=H("LnBranch",L3nonce||L3)
         \   /                                    |
          v v                                     v
  L1A2A=H("LnBranch",L1A||L2A)                   L3A=H("LnBranch",L3nonce||L3)
  
Assume L1A2A > L3A:

  L1A2A=H("LnBranch",L1A||L2A)          L3A
                          \            /
                           v          v
                Root=H("LnBranch",L3A||L1A2A)

Signature = SIG("lightninginvoicesignature", Root, nodekey)

Offers

Offers are a precursor to an invoice_request: readers will request an invoice (or multiple) based on the offer. An offer can be much longer-lived than a particular invoice, so it has some different characteristics; in particular the amount can be in a non-lightning currency. It's also designed for compactness to fit inside a QR code easily.

Note that the non-signature TLV elements get mirrored into invoice_request and invoice messages, so they each have specific and distinct TLV ranges.

The human-readable prefix for offers is lno.

TLV Fields for Offers

  1. tlv_stream: offer
  2. types:
    1. type: 2 (offer_chains)
    2. data:
      • [...*chain_hash:chains]
    3. type: 4 (offer_metadata)
    4. data:
      • [...*byte:data]
    5. type: 6 (offer_currency)
    6. data:
      • [...*utf8:iso4217]
    7. type: 8 (offer_amount)
    8. data:
      • [tu64:amount]
    9. type: 10 (offer_description)
    10. data:
      • [...*utf8:description]
    11. type: 12 (offer_features)
    12. data:
      • [...*byte:features]
    13. type: 14 (offer_absolute_expiry)
    14. data:
      • [tu64:seconds_from_epoch]
    15. type: 16 (offer_paths)
    16. data:
      • [...*blinded_path:paths]
    17. type: 18 (offer_issuer)
    18. data:
      • [...*utf8:issuer]
    19. type: 20 (offer_quantity_max)
    20. data:
      • [tu64:max]
    21. type: 22 (offer_issuer_id)
    22. data:
      • [point:id]

Requirements For Offers

A writer of an offer:

  • MUST NOT set any TLV fields outside the inclusive ranges: 1 to 79 and 1000000000 to 1999999999.
  • if the chain for the invoice is not solely bitcoin:
    • MUST specify offer_chains the offer is valid for.
  • otherwise:
    • SHOULD omit offer_chains, implying that bitcoin is only chain.
  • if a specific minimum offer_amount is required for successful payment:
    • MUST set offer_amount to the amount expected (per item).
    • if the currency for offer_amount is that of all entries in chains:
      • MUST specify offer_amount in multiples of the minimum lightning-payable unit (e.g. milli-satoshis for bitcoin).
    • otherwise:
      • MUST specify offer_currency iso4217 as an ISO 4217 three-letter code.
      • MUST specify offer_amount in the currency unit adjusted by the ISO 4217 exponent (e.g. USD cents).
    • MUST set offer_description to a complete description of the purpose of the payment.
  • otherwise:
    • MUST NOT set offer_amount
    • MUST NOT set offer_currency
    • MAY set offer_description
  • MAY set offer_metadata for its own use.
  • if it supports bolt12 offer features:
    • MUST set offer_features.features to the bitmap of bolt12 features.
  • if the offer expires:
    • MUST set offer_absolute_expiry seconds_from_epoch to the number of seconds after midnight 1 January 1970, UTC that invoice_request should not be attempted.
  • if it is connected only by private channels:
    • MUST include offer_paths containing one or more paths to the node from publicly reachable nodes.
  • otherwise:
    • MAY include offer_paths.
  • if it includes offer_paths:
    • MAY set offer_issuer_id.
  • otherwise:
    • MUST set offer_issuer_id to the node's public key to request the invoice from.
  • if it sets offer_issuer:
    • SHOULD set it to identify the issuer of the invoice clearly.
    • if it includes a domain name:
      • SHOULD begin it with either user@domain or domain
      • MAY follow with a space and more text
  • if it can supply more than one item for a single invoice:
    • if the maximum quantity is known:
      • MUST set that maximum in offer_quantity_max.
      • MUST NOT set offer_quantity_max to 0.
    • otherwise:
      • MUST set offer_quantity_max to 0.
  • otherwise:
    • MUST NOT set offer_quantity_max.

A reader of an offer:

  • if the offer contains any TLV fields outside the inclusive ranges: 1 to 79 and 1000000000 to 1999999999:
    • MUST NOT respond to the offer.
  • if offer_features contains unknown odd bits that are non-zero:
    • MUST ignore the bit.
  • if offer_features contains unknown even bits that are non-zero:
    • MUST NOT respond to the offer.
    • SHOULD indicate the unknown bit to the user.
  • if offer_chains is not set:
    • if the node does not accept bitcoin invoices:
      • MUST NOT respond to the offer
  • otherwise: (offer_chains is set):
    • if the node does not accept invoices for any of the chains:
      • MUST NOT respond to the offer
  • if offer_amount is set and offer_description is not set:
    • MUST NOT respond to the offer.
  • if offer_currency is set and offer_amount is not set:
    • MUST NOT respond to the offer.
  • if neither offer_issuer_id nor offer_paths are set:
    • MUST NOT respond to the offer.
  • if num_hops is 0 in any blinded_path in offer_paths:
    • MUST NOT respond to the offer.
  • if it uses offer_amount to provide the user with a cost estimate:
    • MUST take into account the currency units for offer_amount:
      • offer_currency field if set
      • otherwise, the minimum lightning-payable unit (e.g. milli-satoshis for bitcoin).
    • MUST warn the user if the received invoice_amount differs significantly from that estimate.
  • if the current time is after offer_absolute_expiry:
    • MUST NOT respond to the offer.
  • if it chooses to send an invoice request, it sends an onion message:
    • if offer_paths is set:
      • MUST send the onion message via any path in offer_paths to the final onion_msg_hop.blinded_node_id in that path
    • otherwise:
      • MUST send the onion message to offer_issuer_id
    • MAY send more than one invoice request onion message at once.

Rationale

The entire offer is reflected in the invoice_request, both for completeness (so all information will be returned in the invoice), and so that the offer node can be stateless. This makes offer_metadata particularly useful, since it can contain an authentication cookie to validate the other fields.

Because offer fields are copied into the invoice request (and then the invoice), they require distinct ranges. The range 1-79 is the normal range, with another billion for self-assigning experimental ranges.

A signature is unnecessary, and makes for a longer string (potentially limiting QR code use on low-end cameras); if the offer has an error, no invoice will be given since the request includes all the non-signature fields.

The offer_issuer_id can be omitted for brevity, if offer_paths is set, as each of the final blinded_node_id in the paths can serve as a valid public key for the destination.

Because offer_amount can be in a different currency (using the offer_currency field) it is merely a guide: the issuer will convert it into a number of millisatoshis for invoice_amount at the time they generate an invoice, or the invoice request can specify the exact amount in invreq_amount, but the issuer may then reject it if it disagrees.

offer_quantity_max is allowed to be 1, which seems useless, but useful in a system which bases it on available stock. It would be painful to have to special-case the "only one left" offer generation.

Offers can be used to simply send money without expecting anything in return (tips, kudos, donations, etc), which means the description field is optional (the offer_issuer field is very useful for this case!); if you are charging for something specific, the description is vital for the user to know what it was they paid for.

Invoice Requests

Invoice Requests are a request for an invoice; the human-readable prefix for invoice requests is lnr.

There are two similar-looking uses for invoice requests, which are almost identical from a workflow perspective, but are quite different from a user's point of view.

One is a response to an offer; this contains the offer_issuer_id or offer_paths and all other offer details, and is generally received over an onion message: if it's valid and refers to a known offer, the response is generally to reply with an invoice using the reply_path field of the onion message.

The second case is publishing an invoice request without an offer, such as via QR code. It contains neither offer_issuer_id nor offer_paths, setting the invreq_payer_id (and possibly invreq_paths) instead, as it in the one paying: the other offer fields are filled by the creator of the invoice_request, forming a kind of offer-to-send-money.

Note: the invreq_metadata is numbered 0 (not in the 80-159 range for other invreq fields) making it the "numerically-first TLV entry" for Signature Calculation. This ensures that merkle leaves are unguessable, allowing a future compact representation to hide fields while still allowing signature validation.

TLV Fields for invoice_request

  1. tlv_stream: invoice_request
  2. types:
    1. type: 0 (invreq_metadata)
    2. data:
      • [...*byte:blob]
    3. type: 2 (offer_chains)
    4. data:
      • [...*chain_hash:chains]
    5. type: 4 (offer_metadata)
    6. data:
      • [...*byte:data]
    7. type: 6 (offer_currency)
    8. data:
      • [...*utf8:iso4217]
    9. type: 8 (offer_amount)
    10. data:
      • [tu64:amount]
    11. type: 10 (offer_description)
    12. data:
      • [...*utf8:description]
    13. type: 12 (offer_features)
    14. data:
      • [...*byte:features]
    15. type: 14 (offer_absolute_expiry)
    16. data:
      • [tu64:seconds_from_epoch]
    17. type: 16 (offer_paths)
    18. data:
      • [...*blinded_path:paths]
    19. type: 18 (offer_issuer)
    20. data:
      • [...*utf8:issuer]
    21. type: 20 (offer_quantity_max)
    22. data:
      • [tu64:max]
    23. type: 22 (offer_issuer_id)
    24. data:
      • [point:id]
    25. type: 80 (invreq_chain)
    26. data:
      • [chain_hash:chain]
    27. type: 82 (invreq_amount)
    28. data:
      • [tu64:msat]
    29. type: 84 (invreq_features)
    30. data:
      • [...*byte:features]
    31. type: 86 (invreq_quantity)
    32. data:
      • [tu64:quantity]
    33. type: 88 (invreq_payer_id)
    34. data:
      • [point:key]
    35. type: 89 (invreq_payer_note)
    36. data:
      • [...*utf8:note]
    37. type: 90 (invreq_paths)
    38. data:
      • [...*blinded_path:paths]
    39. type: 240 (signature)
    40. data:
      • [bip340sig:sig]

Requirements for Invoice Requests

The writer:

  • if it is responding to an offer:
    • MUST copy all fields from the offer (including unknown fields).
    • if offer_chains is set:
      • MUST set invreq_chain to one of offer_chains unless that chain is bitcoin, in which case it SHOULD omit invreq_chain.
    • otherwise:
      • if it sets invreq_chain it MUST set it to bitcoin.
    • MUST set signature.sig as detailed in Signature Calculation using the invreq_payer_id.
    • if offer_amount is not present:
      • MUST specify invreq_amount.
    • otherwise:
      • MAY omit invreq_amount.
      • if it sets invreq_amount:
        • MUST specify invreq_amount.msat as greater or equal to amount expected by offer_amount (and, if present, offer_currency and invreq_quantity).
    • MUST set invreq_payer_id to a transient public key.
    • MUST remember the secret key corresponding to invreq_payer_id.
    • if offer_quantity_max is present:
      • MUST set invreq_quantity to greater than zero.
      • if offer_quantity_max is non-zero:
        • MUST set invreq_quantity less than or equal to offer_quantity_max.
    • otherwise:
      • MUST NOT set invreq_quantity
  • otherwise (not responding to an offer):
    • MUST set offer_description to a complete description of the purpose of the payment.
    • MUST set (or not set) offer_absolute_expiry and offer_issuer as it would for an offer.
    • MUST set invreq_payer_id (as it would set offer_issuer_id for an offer).
    • MUST set invreq_paths as it would set (or not set) offer_paths for an offer.
    • MUST NOT include signature, offer_metadata, offer_chains, offer_amount, offer_currency, offer_features, offer_quantity_max, offer_paths or offer_issuer_id
    • if the chain for the invoice is not solely bitcoin:
      • MUST specify invreq_chain the offer is valid for.
    • MUST set invreq_amount.
  • MUST NOT set any non-signature TLV fields outside the inclusive ranges: 0 to 159 and 1000000000 to 2999999999
  • MUST set invreq_metadata to an unpredictable series of bytes.
  • if it sets invreq_amount:
    • MUST set msat in multiples of the minimum lightning-payable unit (e.g. milli-satoshis for bitcoin) for invreq_chain (or for bitcoin, if there is no invreq_chain).
  • if it supports bolt12 invoice request features:
    • MUST set invreq_features.features to the bitmap of features.

The reader:

  • MUST reject the invoice request if invreq_payer_id or invreq_metadata are not present.
  • MUST reject the invoice request if any non-signature TLV fields are outside the inclusive ranges: 0 to 159 and 1000000000 to 2999999999
  • if invreq_features contains unknown odd bits that are non-zero:
    • MUST ignore the bit.
  • if invreq_features contains unknown even bits that are non-zero:
    • MUST reject the invoice request.
  • MUST reject the invoice request if signature is not correct as detailed in Signature Calculation using the invreq_payer_id.
  • if num_hops is 0 in any blinded_path in invreq_paths:
    • MUST reject the invoice request.
  • if offer_issuer_id is present, and invreq_metadata is identical to a previous invoice_request:
    • MAY simply reply with the previous invoice.
  • otherwise:
    • MUST NOT reply with a previous invoice.
  • if offer_issuer_id or offer_paths are present (response to an offer):
    • MUST reject the invoice request if the offer fields do not exactly match a valid, unexpired offer.
    • if offer_paths is present:
      • MUST ignore the invoice_request if it did not arrive via one of those paths.
    • otherwise:
      • MUST ignore any invoice_request if it arrived via a blinded path.
    • if offer_quantity_max is present:
      • MUST reject the invoice request if there is no invreq_quantity field.
      • if offer_quantity_max is non-zero:
        • MUST reject the invoice request if invreq_quantity is zero, OR greater than offer_quantity_max.
    • otherwise:
      • MUST reject the invoice request if there is an invreq_quantity field.
    • if offer_amount is present:
      • MUST calculate the expected amount using the offer_amount:
        • if offer_currency is not the invreq_chain currency, convert to the invreq_chain currency.
        • if invreq_quantity is present, multiply by invreq_quantity.quantity.
      • if invreq_amount is present:
        • MUST reject the invoice request if invreq_amount.msat is less than the expected amount.
        • MAY reject the invoice request if invreq_amount.msat greatly exceeds the expected amount.
    • otherwise (no offer_amount):
      • MUST reject the invoice request if it does not contain invreq_amount.
    • SHOULD send an invoice in response using the onionmsg_tlv reply_path.
  • otherwise (no offer_issuer_id or offer_paths, not a response to our offer):
    • MUST reject the invoice request if any of the following are present:
      • offer_chains, offer_features or offer_quantity_max.
    • MUST reject the invoice request if invreq_amount is not present.
    • MAY use offer_amount (or offer_currency) for informational display to user.
    • if it sends an invoice in response:
      • MUST use invreq_paths if present, otherwise MUST use invreq_payer_id as the node id to send to.
  • if invreq_chain is not present:
    • MUST reject the invoice request if bitcoin is not a supported chain.
  • otherwise:
    • MUST reject the invoice request if invreq_chain.chain is not a supported chain.

Rationale

invreq_metadata might typically contain information about the derivation of the invreq_payer_id. This should not leak any information (such as using a simple BIP-32 derivation path); a valid system might be for a node to maintain a base payer key and encode a 128-bit tweak here. The payer_id would be derived by tweaking the base key with SHA256(payer_base_pubkey || tweak). It's also the first entry (if present), ensuring an unpredictable nonce for hashing.

invreq_payer_note allows you to compliment, taunt, or otherwise engrave graffiti into the invoice for all to see.

Users can give a tip (or obscure the amount sent) by specifying an invreq_amount in their invoice request, even though the offer specifies an offer_amount. The recipient will only accept this if the invoice request amount exceeds the amount it's expecting (i.e. its offer_amount after any currency conversion, multiplied by invreq_quantity, if any).

Non-offer-response invoice requests are currently required to explicitly state the invreq_amount in the chain currency, so offer_amount and offer_currency are redundant (but may be informative for the payer to know how the sender claims invreq_amount was derived).

The requirement to use offer_paths if present, ensures a node does not reveal it is the source of an offer if it is asked directly. Similarly, the requirement that the correct path is used for the offer ensures that cannot be made to reveal that it is the same node that created some other offer.

Invoices

Invoices are a payment request, and when the payment is made, the payment preimage can be combined with the invoice to form a cryptographic receipt.

The recipient sends an invoice in response to an invoice_request using the onion_message invoice field.

  1. tlv_stream: invoice

  2. types:

    1. type: 0 (invreq_metadata)
    2. data:
      • [...*byte:blob]
    3. type: 2 (offer_chains)
    4. data:
      • [...*chain_hash:chains]
    5. type: 4 (offer_metadata)
    6. data:
      • [...*byte:data]
    7. type: 6 (offer_currency)
    8. data:
      • [...*utf8:iso4217]
    9. type: 8 (offer_amount)
    10. data:
      • [tu64:amount]
    11. type: 10 (offer_description)
    12. data:
      • [...*utf8:description]
    13. type: 12 (offer_features)
    14. data:
      • [...*byte:features]
    15. type: 14 (offer_absolute_expiry)
    16. data:
      • [tu64:seconds_from_epoch]
    17. type: 16 (offer_paths)
    18. data:
      • [...*blinded_path:paths]
    19. type: 18 (offer_issuer)
    20. data:
      • [...*utf8:issuer]
    21. type: 20 (offer_quantity_max)
    22. data:
      • [tu64:max]
    23. type: 22 (offer_issuer_id)
    24. data:
      • [point:id]
    25. type: 80 (invreq_chain)
    26. data:
      • [chain_hash:chain]
    27. type: 82 (invreq_amount)
    28. data:
      • [tu64:msat]
    29. type: 84 (invreq_features)
    30. data:
      • [...*byte:features]
    31. type: 86 (invreq_quantity)
    32. data:
      • [tu64:quantity]
    33. type: 88 (invreq_payer_id)
    34. data:
      • [point:key]
    35. type: 89 (invreq_payer_note)
    36. data:
      • [...*utf8:note]
    37. type: 90 (invreq_paths)
    38. data:
      • [...*blinded_path:paths]
    39. type: 160 (invoice_paths)
    40. data:
      • [...*blinded_path:paths]
    41. type: 162 (invoice_blindedpay)
    42. data:
      • [...*blinded_payinfo:payinfo]
    43. type: 164 (invoice_created_at)
    44. data:
      • [tu64:timestamp]
    45. type: 166 (invoice_relative_expiry)
    46. data:
      • [tu32:seconds_from_creation]
    47. type: 168 (invoice_payment_hash)
    48. data:
      • [sha256:payment_hash]
    49. type: 170 (invoice_amount)
    50. data:
      • [tu64:msat]
    51. type: 172 (invoice_fallbacks)
    52. data:
      • [...*fallback_address:fallbacks]
    53. type: 174 (invoice_features)
    54. data:
      • [...*byte:features]
    55. type: 176 (invoice_node_id)
    56. data:
      • [point:node_id]
    57. type: 240 (signature)
    58. data:
      • [bip340sig:sig]

  3. subtype: blinded_payinfo

  4. data:

    • [u32:fee_base_msat]
    • [u32:fee_proportional_millionths]
    • [u16:cltv_expiry_delta]
    • [u64:htlc_minimum_msat]
    • [u64:htlc_maximum_msat]
    • [u16:flen]
    • [flen*byte:features]

  5. subtype: fallback_address

  6. data:

    • [byte:version]
    • [u16:len]
    • [len*byte:address]

Invoice Features

Bits Description Name
16 Multi-part-payment support MPP/compulsory
17 Multi-part-payment support MPP/optional

The 'MPP support' invoice feature indicates that the payer MUST (16) or MAY (17) use multiple part payments to pay the invoice.

Some implementations may not support MPP (e.g. for small payments), or may (due to capacity limits on a single channel) require it.

Requirements

A writer of an invoice:

  • MUST set invoice_created_at to the number of seconds since Midnight 1 January 1970, UTC when the invoice was created.
  • MUST set invoice_amount to the minimum amount it will accept, in units of the minimal lightning-payable unit (e.g. milli-satoshis for bitcoin) for invreq_chain.
  • if the invoice is in response to an invoice_request:
    • MUST copy all non-signature fields from the invoice request (including unknown fields).
    • if invreq_amount is present:
      • MUST set invoice_amount to invreq_amount
    • otherwise:
      • MUST set invoice_amount to the expected amount.
  • MUST set invoice_payment_hash to the SHA256 hash of the payment_preimage that will be given in return for payment.
  • if offer_issuer_id is present:
    • MUST set invoice_node_id to the offer_issuer_id
  • otherwise, if offer_paths is present:
    • MUST set invoice_node_id to the final blinded_node_id on the path it received the invoice request
  • MUST specify exactly one signature TLV element: signature.
  • if it requires multiple parts to pay the invoice:
    • MUST set invoice_features.features bit MPP/compulsory
  • or if it allows multiple parts to pay the invoice:
    • MUST set invoice_features.features bit MPP/optional
  • if the expiry for accepting payment is not 7200 seconds after invoice_created_at:
    • MUST set invoice_relative_expiry.seconds_from_creation to the number of seconds after invoice_created_at that payment of this invoice should not be attempted.
  • if it accepts onchain payments:
    • MAY specify invoice_fallbacks
    • SHOULD specify invoice_fallbacks in order of most-preferred to least-preferred if it has a preference.
    • for the bitcoin chain, it MUST set each fallback_address with version as a valid witness version and address as a valid witness program
  • MUST include invoice_paths containing one or more paths to the node.
    • MUST specify invoice_paths in order of most-preferred to least-preferred if it has a preference.
    • MUST include invoice_blindedpay with exactly one blinded_payinfo for each blinded_path in paths, in order.
    • MUST set features in each blinded_payinfo to match encrypted_data_tlv.allowed_features (or empty, if no allowed_features).
    • SHOULD ignore any payment which does not use one of the paths.

A reader of an invoice:

  • MUST reject the invoice if invoice_amount is not present.
  • MUST reject the invoice if invoice_created_at is not present.
  • MUST reject the invoice if invoice_payment_hash is not present.
  • MUST reject the invoice if invoice_node_id is not present.
  • if invreq_chain is not present:
    • MUST reject the invoice if bitcoin is not a supported chain.
  • otherwise:
    • MUST reject the invoice if invreq_chain.chain is not a supported chain.
  • if invoice_features contains unknown odd bits that are non-zero:
    • MUST ignore the bit.
  • if invoice_features contains unknown even bits that are non-zero:
    • MUST reject the invoice.
  • if invoice_relative_expiry is present:
    • MUST reject the invoice if the current time since 1970-01-01 UTC is greater than invoice_created_at plus seconds_from_creation.
  • otherwise:
    • MUST reject the invoice if the current time since 1970-01-01 UTC is greater than invoice_created_at plus 7200.
  • MUST reject the invoice if invoice_paths is not present or is empty.
  • MUST reject the invoice if num_hops is 0 in any blinded_path in invoice_paths.
  • MUST reject the invoice if invoice_blindedpay is not present.
  • MUST reject the invoice if invoice_blindedpay does not contain exactly one blinded_payinfo per invoice_paths.blinded_path.
  • For each invoice_blindedpay.payinfo:
    • MUST NOT use the corresponding invoice_paths.path if payinfo.features has any unknown even bits set.
    • MUST reject the invoice if this leaves no usable paths.
  • if the invoice is a response to an invoice_request:
    • MUST reject the invoice if all fields in ranges 0 to 159 and 1000000000 to 2999999999 (inclusive) do not exactly match the invoice request.
    • if offer_issuer_id is present (invoice_request for an offer):
      • MUST reject the invoice if invoice_node_id is not equal to offer_issuer_id
    • otherwise, if offer_paths is present (invoice_request for an offer without id):
      • MUST reject the invoice if invoice_node_id is not equal to the final blinded_node_id it sent the invoice request to.
    • otherwise (invoice_request without an offer):
      • MAY reject the invoice if it cannot confirm that invoice_node_id is correct, out-of-band.
  • MUST reject the invoice if signature is not a valid signature using invoice_node_id as described in Signature Calculation.
  • SHOULD prefer to use earlier invoice_paths over later ones if it has no other reason for preference.
  • if invoice_features contains the MPP/compulsory bit:
    • MUST pay the invoice via multiple separate blinded paths.
  • otherwise, if invoice_features contains the MPP/optional bit:
    • MAY pay the invoice via multiple separate payments.
  • otherwise:
    • MUST NOT use multiple parts to pay the invoice.
  • if invreq_amount is present:
    • MUST reject the invoice if invoice_amount is not equal to invreq_amount
  • otherwise:
    • SHOULD confirm authorization if invoice_amount.msat is not within the amount range authorized.
  • for the bitcoin chain, if the invoice specifies invoice_fallbacks:
    • MUST ignore any fallback_address for which version is greater than 16.
    • MUST ignore any fallback_address for which address is less than 2 or greater than 40 bytes.
    • MUST ignore any fallback_address for which address does not meet known requirements for the given version
  • if invreq_paths is present:
    • MUST reject the invoice if it did not arrive via one of those paths.
  • otherwise, neither offer_issuer_id nor offer_paths are present (not derived from an offer):
    • MUST reject the invoice if it arrived via a blinded path.
  • otherwise (derived from an offer):
    • MUST reject the invoice if it did not arrive via invoice request onionmsg_tlv reply_path.

Rationale

Because the messaging layer is unreliable, it's quite possible to receive multiple requests for the same offer. As it's the caller's responsibility to make invreq_metadata both unpredictable and unique, the writer doesn't have to check all the fields are duplicates before simply returning a previous invoice. Note that such caching is optional, and should be carefully limited when e.g. currency conversion is involved, or if the invoice has expired.

The invoice duplicates fields rather than committing to the previous invreq. This flattened format simplifies storage at some space cost, as the payer need only remember the invoice for any refunds or proof.

The reader of the invoice cannot trust the invoice correctly reflects the invreq fields, hence the requirements to check that they are correct, although allowance is made for simply sending an unrequested invoice directly.

Note that the recipient of the invoice can determine the expected amount from either the offer it received, or the invreq it sent, so often already has authorization for the expected amount.

The default invoice_relative_expiry of 7200 seconds, which is generally a sufficient time for payment, even if new channels need to be opened.

Blinded paths provide an equivalent to payment_secret and payment_metadata used in BOLT 11. Even if invoice_node_id or invreq_payer_id is public, we force the use of blinding paths to keep these features. If the recipient does not care about the added privacy offered by blinded paths, they can create a path of length 1 with only themselves.

Rather than provide detailed per-hop-payinfo for each hop in a blinded path, we aggregate the fees and CLTV deltas. This avoids trivially revealing any distinguishing non-uniformity which may distinguish the path.

In the case of an invoice where there was no offer (just an invoice request), the payer needs to ensure that the invoice is from the intended payment recipient. This is the basis for the suggestion to confirm the invoice_node_id for this case.

Raw invoices (not based on an invoice_request) are generally not supported, though an implementation is allowed to support them, and we may define the behavior in future. The redundant requirement to check invreq_chain explicitly is a nod to this: if the invoice is a response to an invoice request, that field must have existed due to the invoice request requirements, and we also require it to be mirrored here.

Invoice Errors

Informative errors can be returned in an onion message invoice_error field (via the onion reply_path) for either invoice_request or invoice.

TLV Fields for invoice_error

  1. tlv_stream: invoice_error
  2. types:
    1. type: 1 (erroneous_field)
    2. data:
      • [tu64:tlv_fieldnum]
    3. type: 3 (suggested_value)
    4. data:
      • [...*byte:value]
    5. type: 5 (error)
    6. data:
      • [...*utf8:msg]

Requirements

A writer of an invoice_error:

  • MUST set error to an explanatory string.
  • MAY set erroneous_field to a specific field number in the invoice or invoice_request which had a problem.
  • if it sets erroneous_field:
    • MAY set suggested_value.
    • if it sets suggested_value:
      • MUST set suggested_value to a valid field for that tlv_fieldnum.
  • otherwise:
    • MUST NOT set suggested_value.

A reader of an invoice_error: FIXME!

Rationale

Usually an error message is sufficient for diagnostics, however future enhancements may make automated handling useful.

In particular, we could allow non-offer-response invoice_requests to omit invreq_amount in future and use offer fields to indicate alternate currencies. ("I will send you 10c!"). Then the sender of the invoice would have to guess how many msat that was, and could use the invoice_error to indicate if the recipient disagreed with the conversion so the sender can send a new invoice.

FIXME: Possible future extensions:

  1. The offer can require delivery info in the invoice_request.
  2. An offer can be updated: the response to an invoice_request is another offer, perhaps with a signature from the original offer_issuer_id
  3. Any empty TLV fields can mean the value is supposed to be known by other means (i.e. transport-specific), but is still hashed for sig.
  4. We could upgrade to allow multiple offers in one invreq and invoice, to make a shopping list.
  5. All-zero offer_id == gratuitous payment.
  6. Streaming invoices?
  7. Re-add recurrence.
  8. Re-add invreq_refund_for to support proofs.
  9. Re-add invoice_replace for requesting replacement of a (stuck-payment) invoice with a new one.
  10. Allow non-offer invoice_request with alternate currencies?
  11. Add offer_quantity_unit to indicate stepping for quantity (e.g. 100 grams).

[1] https://www.youtube.com/watch?v=4SYc_flMnMQ