OP_PAIRCOMMIT as a candidate for addition to LNhance

Protocol Design
5

But the question was really not about this, but rather if it makes any sense to do a mini-hashing of the sizes instead of static padding, for making the preimage more mutable when you redistribute bytes between the stack elements?

The “original” variant:

const HashWriter HASHER_PAIRCOMMIT{TaggedHash("PairCommit")};

uint256 PairCommitHash(const std::vector<unsigned char>& x1, const std::vector<unsigned char>& x2)
{
    // PAD is 0x00000001 in little endian serializaton
    const uint32_t PAD = 0x01000000;

    HashWriter ss{HASHER_PAIRCOMMIT};
    ss << x1
       << x2
       << uint32_t(x1.size()) << PAD
       << uint32_t(x2.size()) << PAD;

    return ss.GetSHA256();
}