BIP352: align reference address decoding with silent payment rules

[ginavalent]

BIP 352 specifies that silent payment addresses use Bech32m encoding and defines forward-compatible address decoding rules for versions 0 through 30.

The BIP352 reference implementation currently uses the generic SegWit decoder without checking the encoding type, which allows a Bech32 checksum for a silent payment address. It also rejects versions above 16 and passes the complete payload to the spend key parser instead of discarding extension data for future versions.

Update the BIP352-specific decoder to:

• require Bech32m encoding;
• accept versions 0 through 30 and reject version 31;
• require exactly 66 decoded bytes for version 0;
• accept at least 66 decoded bytes for versions 1 through 30 and consume only the first 66 bytes.

[murchandamus]

Thanks. @theStack, this seems to fix a discrepancy between the specification and the reference implementation. Do you want it?

[theStack]

Concept ACK

[fametrano]

ACK c39bae2

The decoder now does what the BIP says: bech32m only, exactly 66 bytes for v0, the first 66 bytes for v1 through v30, fail on v31. At c39bae2 the five new tests pass and reference.py send_and_receive_test_vectors.json passes all 28 cases; against master's decoder four of the five new tests fail. I also ran 33 addresses, the ten from the test vectors and 23 constructed edge cases, through the BIP's rules over btclib's bech32m decoder: accept/reject and the decoded keys agree with this PR on all 33, while master differs on nine, among them accepting a v0 address with a bech32 checksum.