Why PQHD?

PQHD is Tidecoin’s post-quantum hierarchical deterministic wallet system. It exists because Bitcoin’s BIP32 wallet model depends on elliptic-curve behavior that does not transfer cleanly to post-quantum signature schemes.

This page is the plain-language explanation. The protocol specification is Protocol / PQHD.

The BIP32 Problem

BIP32 lets a wallet derive child public keys from an extended public key (xpub). That is useful for watch-only wallets and address generation, but it depends on elliptic-curve arithmetic.

In a quantum setting, exposing public keys is more dangerous because Shor’s algorithm can recover the corresponding private key from a public key once a sufficiently large fault-tolerant quantum computer exists. An xpub model built around exposed public derivation is the wrong foundation for a post-quantum wallet.

There is also a practical issue: NIST post-quantum signature schemes do not provide the same homomorphic public child derivation that BIP32 uses.

What PQHD Does Instead

PQHD uses hardened-only derivation. That means every child key is derived from secret material, not from public derivation data.

The path shape is:


m / purpose' / coin_type' / scheme' / account' / change' / index'

All elements are hardened. The scheme' element selects the post-quantum signature scheme, such as Falcon-512 or ML-DSA-65.

Why Hardened-Only Matters

Hardened-only derivation gives up one BIP32 convenience: you cannot publish an xpub and let an untrusted server derive all future child public keys.

That tradeoff is intentional. In a post-quantum wallet, avoiding public derivation is cleaner than trying to recreate the xpub model with primitives that were not designed for it.

Watch-only and offline workflows still exist, but they use descriptors, addresses, UTXO state, and PSBT metadata rather than an xpub-style public derivation root.

Multi-Seed and Multi-Scheme

Tidecoin Core can track multiple PQHD seeds in one wallet and set default receive/change scheme policy. That supports:

separating account domains;
importing recovery material without mixing it into normal receive paths;
testing activation-gated schemes on testnet or regtest;
moving gradually between scheme policies when consensus rules allow it.

The important rule is that wallet policy never overrides consensus activation. If a scheme is not active on mainnet, a wallet should not use it for production mainnet receives.

Backup Boundary

A descriptor can describe a PQHD path, but it does not contain the master seed. To recover funds, you need the wallet backup, PQHD seed material, or the wallet-specific recovery artifact.

Think of PQHD like this:

Artifact	Role
Master seed	Secret material that can derive spending keys
Seed ID	Non-secret identifier for matching descriptors and origins
Descriptor	Script and path metadata
PSBT origin metadata	Handoff data that helps an offline signer locate the right derivation

Practical Takeaway

PQHD is less convenient than xpub-based public derivation, but it matches the post-quantum threat model. It keeps deterministic wallet recovery while avoiding public child derivation assumptions that do not belong in a lattice-signature wallet.