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Deterministic pay to public key generation #331

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2ce332e
pay to public key standard derivation
lescuer97 Jan 19, 2026
75fa183
fix: wording
lescuer97 Jan 21, 2026
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12 changes: 12 additions & 0 deletions 13.md
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Expand Up @@ -39,6 +39,18 @@ The HMAC-SHA256 KDF is built as the following:
2. `hmac_digest = HMAC_SHA256(seed, message)`, where `HMAC_SHA256` is the [hash-based message authentication code](https://en.wikipedia.org/wiki/HMAC) using SHA-256 as the hashing algorithm.
3. `secret = hmac_digest` and `blinding_factor = hmac_digest % N`.

#### P2PK Derivation

Wallet are able to generate private keys is a deterministic way to have proofs locked to them.

The following BIP32 derivation path for derivation of the key: `m/129372'/10'/0'/0'/{counter}`:
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@prusnak prusnak Jan 22, 2026 •
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Can you explain why the last element is not hardened?

Is the xpub of m/129372'/10'/0'/0' ever shared anywhere?

If it is not shared, then instead of hardening it, I propose we change the scheme to Cashu_KDF_HMAC_SHA256 (same as used in Keyset v2, since BIP32 used in Keyset v1 is deprecated).

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@lescuer97 lescuer97 Jan 26, 2026 •
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We are following what Bitcoin does when it comes to key derivation for locking.

there is no reason right now for sharing an XPUB but can't guarantee that in the future.

The reason we want to use bip32 is because Bip32 is specifically made for this case.The Cashu_KDF_HMAC_SHA256 scheme is used because of aggregation to avoid certain issues when generating the keyset id.

This keys are never aggregated and are use individually.

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@prusnak prusnak Jan 26, 2026

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The reason we want to use bip32 is because Bip32 is specifically made for this case.

No, it's not. BIP32 usecase is when you need to share a pubkey for a certain key subtree. Unless you need this requirement, going with Cashu_KDF_HMAC_SHA256 is simply faster and easier.

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@Egge21M Egge21M Jan 26, 2026

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@prusnak if we get the option to query the mint for quotes connected to a pubkey via NUT-20, public key derivation would become more meaningful than it currently is. I can not think of a usecase right now, but with this, extended keys would be able to essentially create a watch-only wallet of a wallets quotes, without spending from it.

So either we think about proper usecases for this for 2 weeks and go with HMAC if we can't find any, or we take the performance L, go with BIP32 and hope someone finds a usecase some day in the future

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@prusnak prusnak Jan 26, 2026

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if we want to have a watch-only wallet eventually which should show p2pk tokens too, this seems like a good use-case. ACK


- 129372': Registered SLIP-0044 coin type for Cashu.
- 10': Purpose for generating private keys for usage in P2PK.
- {counter}: Incrementing counter encoded as an unsigned 64-bit integer in big-endian format.
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@a1denvalu3 a1denvalu3 Jan 19, 2026

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is this using BIP32? Should be more clear.

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@lescuer97 lescuer97 Jan 20, 2026

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yes, I'll make it clearer


This will allow wallets to swap proof that are still locked to a public key during a restore process.
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@robwoodgate robwoodgate Jan 22, 2026 •
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Suggested change
This will allow wallets to swap proof that are still locked to a public key during a restore process.
Wallets can deterministically generate private keys for P2PK using the following BIP32 derivation path:
`m/129372'/10'/0'/0'/{counter}`
Where:
- Purpose' = `129372'` (UTF-8 for 🥜)
- Coin type' = `10'` - reserved for generating private keys.
- `{counter}` is an Incrementing counter, encoded as an unsigned 64-bit integer in big-endian format.
This allows wallets to swap Proofs still locked to a corresponding public key during a restore process.
In line with BIP-32, if the resulting private key is out of range (`> N`), it should be discarded.


### Code Examples

#### Versioned Secret Derivation
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22 changes: 22 additions & 0 deletions tests/13-tests.md
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Expand Up @@ -91,3 +91,25 @@ The corresponding blinding factors `r` are:
"r_4": "5550337312d223ba62e3f75cfe2ab70477b046d98e3e71804eade3956c7b98cf"
}
```

## P2PK Derivation

Using [NUT-13](13.md) derivation procedure for P2PK, we derive values starting from the following BIP39 mnemonic:

```json
{
"mnemonic": "half depart obvious quality work element tank gorilla view sugar picture humble"
}
```

The public keys derived for the first five counters from `counter=0` to `counter=4` are:

```json
{
"m/129372'/10'/0'/0'/0": "03381fbf0996b81d49c35bae17a70d71db9a9e802b1af5c2516fc90381f4741e06",
"m/129372'/10'/0'/0'/1": "039bbb7a9cd234da13a113cdd8e037a25c66bbf3a77139d652786a1d7e9d73e600",
"m/129372'/10'/0'/0'/2": "02ffd52ed54761750d75b67342544cc8da8a0994f84c46d546e0ab574dd3651a29",
"m/129372'/10'/0'/0'/3": "02751ab780960ff177c2300e440fddc0850238a78782a1cab7b0ae03c41978d92d",
"m/129372'/10'/0'/0'/4": "0391a9ba1c3caf39ca0536d44419a6ceeda922ee61aa651a72a60171499c02b423"
}
```