BOLT 11 - Why is the description field limited to 639 bytes?

Question

Why is the description field in a Lightning Network invoice limited to 639 bytes?

https://github.com/lightningnetwork/lightning-rfc/blob/master/11-payment-encoding.md#tagged-fields

https://github.com/lightningnetwork/lnd/blob/95502da7e87d7f1ae96e9d69fcdb2f1b038d6497/channeldb/invoices.go#L85 (LND limits it to 1024)

I would like to send data over the lightning network using the description field for the anonymizing aspects of onion routing in conjunction with the economic incentive for honest nodes.

Answer 1

Why is the description field limited to 639 bytes?

Every field in lightning invoice is of the form:

1. type (5 bits)
2. data_length (10 bits, big-endian)
3. data (data_length x 5 bits)

The data_length indicates the size of the data that is to be followed. Since data_length itself has a size of 10 bits, it can represent a number up to 1023 (2¹⁰ -1). Thus the size of data can be at most 1023*5 bits or 639.375 bytes. This puts the upper limit of 639 bytes on the description field.