[bitcoindev] Changes to BIP-360 - Pay to Quantum Resistant Hash (P2QRH)

[Ethan Heilman <eth3rs@gmail.com>]

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We made the following changes to BIP-360 (Pay to Quantum Resistant Hash) PR
<https://github.com/bitcoin/bips/pull/1670>:


   -

   P2QRH (Pay to Quantum Resistant Hash) is now taproot (P2TR) but with the
   quantum vulnerable key-spend path removed.
   -

   PQ signatures have been moved to a future BIP (coming soon).
   -

   The plan for PQ signatures is to redefine OP_SUCCESSx opcodes:
   OP_CHECKMLSIG


Below we go into these changes one by one, see BIP-360 PR for full
details (BIP-360
mediawiki render as of 7/7/2025
<https://github.com/bitcoin/bips/blob/a375b65e2df340915a74bff5401a8bc83472b1a3/bip-0360.mediawiki>
).

P2QRH is now script-spend only P2TR (taproot), i.e. no quantum vulnerable
key-spend. P2QRH outputs commit directly to the tapleaf merkle root
computed by taproot.

The scriptPubKey for a P2QRH output is:

OP_PUSHNUM_3 OP_PUSHBYTES_32 <tapleaf merkle root>

Advantages of this approach


   1.

   We can reuse taproot code, but just skip taptweak steps.
   2.

   Everyone who understands P2TR, already understands P2QRH.
   3.

   By supporting tapscript and tapleaf, it supports everything that
   supports tapscript.
   4.

   P2QRH protects tapscript outputs against long-exposure attacks. This is
   a big win because long-exposure attacks will be practical before
   short-exposure attacks. Note: protecting against short-exposure attacks
   requires PQ signatures.
   5.

   P2QRH gives us similar functionality as the much discussed option of
   disabling key-spends in P2TR on Q-Day (when quantum attacks become
   practical), but with the added benefit that the ecosystem can upgrade well
   before Q-Day. This removes the risks of attempting a consensus change
   during an emergency or acting too late.


We moved PQ signatures specification out of BIP-360 so that P2QRH can be
debated independently of the debate over PQ signature algorithms. This
allows us to move forward on P2QRH without forcing a commitment to any
particular algorithm.

BIP-360 includes a purely informational plan for adding PQ signature
algorithms to tapscript. This plan to add tapscript PQ signature
verification opcodes for ML-DSA (CRYSTALS-Dilithium) and SLH-DSA (SPHINCS+)
via OP_SUCCESSx. This allows separate activation of PQ signature algorithms
if desired and provides a pattern for adding new signature algorithms in
the future. No new tapleaf version needed. The full specification will be
given in a new BIP.

See BIP-360 for details.

Thanks,
Ethan and Hunter

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