Fix currency UTF-8 test vector to actually test invalid UTF-8 sequences

[erickcestari]

The previous test vector for "Malformed: invalid currency UTF-8" was incorrectly testing invalid length encoding (02 length with 2804 value) rather than testing invalid UTF-8 byte sequences in the currency field itself.

This commit replaces it with a test vector that:

• Uses correct length encoding (03 length with proper 3-byte value)
• Contains an actual invalid UTF-8 sequence in the currency data
• Properly validates that parsers reject malformed UTF-8 in currency fields

[t-bast]

@thomash-acinq can you review this and check what eclair does?

[thomash-acinq]

The current test vector uses the following TLV stream:

06 02 8041  // Type: 6 (offer_currency), Length: 2, Value: two bytes that form an invalid utf8 string
0a 05 414c494345  // Type: 10 (offer_description), Length: 5, Value: "ALICE"
16 21 020202020202020202020202020202020202020202020202020202020202020202  // Type: 22 (offer_issuer_id), Length: 33, Value : some public key

It does what it claims to do (test that the offer_currency is validated as a utf8 string) and I don't see any need to change it.

Your proposed change is

06 03 4d2aa4  // Type: 6 (offer_currency), Length: 3, Value: three bytes that form an invalid utf8 string
08 02 2710  // Type: 8 (offer_amount)
0a 0c 5465737420766563746f7273 // Type: 10 (offer_description)
16 21 02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619  // Type: 22 (offer_issuer_id)

which does not seem to be an improvement.

[erickcestari]

The current test vector uses the following TLV stream:

06 02 8041  // Type: 6 (offer_currency), Length: 2, Value: two bytes that form an invalid utf8 string
0a 05 414c494345  // Type: 10 (offer_description), Length: 5, Value: "ALICE"
16 21 020202020202020202020202020202020202020202020202020202020202020202  // Type: 22 (offer_issuer_id), Length: 33, Value : some public key

It does what it claims to do (test that the offer_currency is validated as a utf8 string) and I don't see any need to change it.

Your proposed change is

06 03 4d2aa4  // Type: 6 (offer_currency), Length: 3, Value: three bytes that form an invalid utf8 string
08 02 2710  // Type: 8 (offer_amount)
0a 0c 5465737420766563746f7273 // Type: 10 (offer_description)
16 21 02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619  // Type: 22 (offer_issuer_id)

which does not seem to be an improvement.

Some applications (rust-lightning) validate currency codes by first checking for exactly 3 bytes before parsing the UTF-8 content. Since this test vector isn't 3 bytes long, those applications would reject it based on length rather than detecting the invalid UTF-8 encoding. Using a 3-byte invalid UTF-8 sequence would better test the UTF-8 validation logic specifically. I'll also remove the other fields to keep this test vector focused solely on offer_currency.

[thomash-acinq]

OK, I understand the problem now. But if we really want to check that this field is a valid ISO 4217 code we should also add test vectors with symbols (US$), lowercase letters (usd) or even codes that look good but are not part of the standard (ABC).
Eclair is not affected as it doesn't support currency conversion so it will reject any offer that sets offer_currency.