[bug]: BOLT 11 Compliance: Invoice description field not validated as UTF-8

[erickcestari]

Background

After doing some differential fuzzing between rust-lightining and LND using bitcoinfuzz I noticed that LND currently accepts Lightning invoices with invalid UTF-8 in the description field (d), violating BOLT 11 specification requirements.

BOLT 11 Requirements:

• d field "Short description of purpose of payment (UTF-8)"
• Writer "MUST set d to a valid UTF-8 string"

Current Behavior: The parseDescription function converts bech32 data to string without UTF-8 validation, allowing invalid byte sequences.

Expected Behavior: Reject invoices with non-UTF-8 description fields per spec.

Impact:

• Spec non-compliance
• Potential interoperability issues with other Lightning implementations
• Display problems in wallets/UIs

Proposed Fix: Add utf8.Valid() check in parseDescription before string conversion.

if !utf8.Valid(base256Data) {
    return nil, fmt.Errorf("description is not valid UTF-8")
}

Example invoice with invalid description UTF-8 bytes:

lnbc100n1pllllllpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdz823jhxapqv3jhxcmjd9c8g6t0dcs924zx95uzqamfw35zq6twweskc6tyyp38jar9wvs0llsnp4q0n326hr8v9zprg8gsvezcch06gfaqqhde2aj730yg0durunfhv669qrsgquuwsa0vmqyngdnj3j2sz0hc29eaq0sypz2lpr8v2aa0csjufcw9j3hzp6sfzzuu6jz7gzs22sw0msxaq7uc0jzp7usmg2dsa6cxahrqpjdwkzz