Why does the decode function of the segwit bech32 encoder/decoder take a hrp (human readable part) as input?

Question

Looking at the code referenced in bip-0173, specifically for example here: https://github.com/sipa/bech32/blob/master/ref/javascript/segwit_addr.js

  function decode (hrp, addr) {
      var dec = bech32.decode(addr);
      if (dec === null || dec.hrp !== hrp || dec.data.length < 1 || dec.data[0] > 16) {
        return null;
      }
      ...

I understand that this is an error checking statement, but why check a (user?) inputted hrp vs the decoded hrp from bech32.decode?

Also I'm assuming that the "dec.data[0] > 16" check is to make sure that the byte at index zero doesn't exceed a value of 16, which would be invalid hex. Is this correct?

Answer 1

The HRP is part of the bech32 encoded string, and in the bech32 decoding API, it is returned along with the payload by the decoder, after checking the checksum.

We still want to compare it with the expected HRP in BIP173, which encodes the chain the software is operating on.

Otherwise you could have a testnet node that accepts mainnet BIP173 addresses or the other way around.

Answer 2

I'm assuming that the "dec.data[0] > 16" check is to make sure that the byte at index zero doesn't exceed a value of 16, which would be invalid hex. Is this correct?

No, it's because only versions 0 - 16 are specified. Other versions might behave entirely differently. (As an aside, if it were a comparison for the sake of clamping to the range of a single hex character the test would be >15 not >16).

Answer 3

From BIP173 > Specification > Segwit address format > Decoding:

Software interpreting a segwit address:

• MUST verify that the human-readable part is "bc" for mainnet and "tb" for testnet.
• MUST verify that the first decoded data value (the witness version) is between 0 and 16, inclusive