keysend: enforce BOLT11 description length limit #8535
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noblepayne
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Sep 5, 2025
plugins/keysend.c
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| json_add_string(req->js, "description", desc); | ||
| /* Don't exceed max possible desc length! */ | ||
| if (strlen(desc) > 1023) | ||
| if (strlen(desc) >= BOLT11_FIELD_BYTE_LIMIT) |
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Should this be > or >=? lightningd/invoice.c uses >, however the BOLT11 spec says:
Note that the maximum length of a Tagged Field's data is constricted by the maximum value of data_length. This is 1023 x 5 bits, or 639 bytes.
In common/bolt11.c BOLT11_FIELD_BYTE_LIMIT has a value of 640.
Far from an expert here and appreciate any insight. Happy to update as needed.
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Hmm, we should fix invoice.c too. Let me write a test.
The keysend plugin previously used `> 1023` as the cutoff for description length when inserting an invoice. This was inconsistent with invoice.c, which enforces the BOLT11 description field limit defined in `common/bolt11.h`. This patch switches to using `BOLT11_FIELD_BYTE_LIMIT` directly. As a result, keysend no longer fails on descriptions between 641–1023 bytes, which previously caused unexpected failures. A new regression test (`test_keysend_description_size_limit`) exercises boundary cases just below, at, and above the limit. Changelog-Fixed: Protocol: `keysend` with descriptions of length 640-1023 bytes fixed. Signed-off-by: Wes Payne <[email protected]>
They are invalid! This is because our BOLT11_FIELD_BYTE_LIMIT is not the limit, it's one greater than the limit. Reported-by: https://github.com/noblepayne Signed-off-by: Rusty Russell <[email protected]> Changelog-Fixed: JSON-RPC: `invoice` no longer accepts 640-byte descriptions (it would produce malformed invoices).
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OK, I fixed the invoice 640-byte desc error you found! And I modified the Changelog-None to a proper changelog line for your fix. Thanks! |
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The keysend plugin uses
> 1023as the cutoff for description length when inserting an invoice. This was inconsistent withlightnind/invoice.c, which enforces the BOLT11 description field limit defined incommon/bolt11.h.This patch switches to using
BOLT11_FIELD_BYTE_LIMITdirectly. As a result, keysend no longer fails on descriptions between 641–1023 bytes, which previously caused unexpected failures.A new regression test (
test_keysend_description_size_limit) exercises boundary cases just below, at, and above the limit.Changelog-None
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