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Require minimally-encoded features in BOLT 11 invoices #1245

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TheBlueMatt merged 2 commits into from
May 1, 2025
Merged

Require minimally-encoded features in BOLT 11 invoices #1245

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Require minimally-encoded fields in BOLT 11 invoices
TheBlueMatt Apr 3, 2025
4042520
Add note that non-largest multipliers have been seen in BOLT 11
TheBlueMatt Apr 7, 2025
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15 changes: 12 additions & 3 deletions 11-payment-encoding.md
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Original file line number Diff line number Diff line change
Expand Up @@ -98,6 +98,9 @@ whatever is being offered in return.
The `p` multiplier would allow to specify sub-millisatoshi amounts, which cannot be transferred on the network, since HTLCs are denominated in millisatoshis.
Requiring a trailing `0` decimal ensures that the `amount` represents an integer number of millisatoshis.

Note that non-largest multipliers have been encountered in the wild, and as
such invoice parsers should handle them.

# Data Part

The data part of a Lightning invoice consists of multiple sections:
Expand Down Expand Up @@ -176,11 +179,11 @@ A writer:
- SHOULD use a complete description of the purpose of the payment.
- MAY include one `x` field.
- if `x` is included:
- SHOULD use the minimum `data_length` possible.
- MUST use the minimum `data_length` possible, i.e. no leading 0 field-elements.
- SHOULD include one `c` field (`min_final_cltv_expiry_delta`).
- MUST set `c` to the minimum `cltv_expiry` it will accept for the last
HTLC in the route.
- SHOULD use the minimum `data_length` possible.
- MUST use the minimum `data_length` possible, i.e. no leading 0 field-elements.
- MAY include one `n` field. (Otherwise performing signature recovery is required)
- MUST set `n` to the public key used to create the `signature`.
- MAY include one or more `f` fields.
Expand All @@ -197,7 +200,8 @@ A writer:
specified in [BOLT #7](07-routing-gossip.md#the-channel_update-message).
- MAY include more than one `r` field to provide multiple routing options.
- if `9` contains non-zero bits:
- SHOULD use the minimum `data_length` possible.
- MUST use the minimum `data_length` possible to encode the non-zero bits
with no 0 field-elements at the start.
- otherwise:
- MUST omit the `9` field altogether.
- MUST pad field data to a multiple of 5 bits, using 0s.
Expand All @@ -222,6 +226,11 @@ A reader:
- MUST use an expiry delta of at least 18 when making the payment
- if an `m` field is provided:
- MUST use that as [`payment_metadata`](04-onion-routing.md#tlv_payload-payload-format)
- if a `c`, `x`, or `9` field is provided which has a non-minimal `data_length`
(i.e. begins with 0 field elements):
- SHOULD treat the invoice as invalid.


### Rationale

The type-and-length format allows future extensions to be backward
Expand Down