feat(specs): add NXP-2 - Derivation Path Standards #11
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Define comprehensive key derivation standards for Nexum: - BIP-44 paths for account signing keys - EIP-1581 paths for encryption and non-wallet keys - Delegation key derivation from account addresses - Domain-separated key derivation patterns Closes nxm-rs/nexum#86 Related to nxm-rs/pm#3 (EPIC 0.0)
0xferrous
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Dec 8, 2025
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| @dataclass | ||
| class ExtendedKey: | ||
| private_key: int | ||
| chain_code: bytes | ||
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| def try_derive(parent: ExtendedKey, index: int) -> ExtendedKey | None: | ||
| """Attempt to derive a child key. Returns None if derivation is invalid.""" | ||
| data = serialize(parent, index) | ||
| i = hmac.new(parent.chain_code, data, hashlib.sha512).digest() | ||
| i_l, i_r = i[:32], i[32:] | ||
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| # Check validity per BIP-32 | ||
| parsed_key = int.from_bytes(i_l, byteorder='big') | ||
| if parsed_key >= SECP256K1_ORDER: | ||
| return None # Invalid: I_L >= curve order | ||
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| child_key = (parsed_key + parent.private_key) % SECP256K1_ORDER | ||
| if child_key == 0: | ||
| return None # Invalid: resulting key is zero | ||
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| return ExtendedKey(private_key=child_key, chain_code=i_r) | ||
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| def derive_with_retry(parent: ExtendedKey, start_index: int) -> tuple[ExtendedKey, int]: | ||
| """Derive child key, incrementing index on failure until valid.""" | ||
| index = start_index | ||
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| while True: | ||
| result = try_derive(parent, index) | ||
| if result is not None: | ||
| return (result, index) | ||
| index += 1 # Increment and retry | ||
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| # Safety check (should never be reached in practice) | ||
| if index > 0xFFFFFFFF: | ||
| raise ValueError("Exhausted index space") |
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we dont need to show bip32 implementation in this nxp
| ``` | ||
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| Where: | ||
| - `44'` — BIP-44 purpose (hardened) |
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i think its more interesting to set this 44 to a DELEGATING_PURPOSE instead that is defined in some other nxp for common delegation purposes.
| |------|------------|-------| | ||
| | `m/44'/60'/*/0/*` | No | Standard accounts stay on-card | | ||
| | `m/44'/60'/*/1/*` | No | Delegation keys stay on-card | | ||
| | `m/43'/60'/1581'/*` | Yes | EIP-1581 encryption keys only | |
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so maybe you want to explicitly DISALLOW identity index to be 1581 in nxp-1?
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Not an issue as 1581 for an identity index is fine as the identity index will be under the 44' purpose code, whereas for EIP-1581 derived keys, these fall under the 43' purpose code.
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Summary
Context
NXP-2 builds on NXP-1's identity model to define the exact derivation paths used for all key types. It distinguishes between signing keys (which remain on secure elements) and encryption/delegation keys (which must be exported for software operations).
Closes nxm-rs/nexum#86
Related to nxm-rs/pm#3 (EPIC 0.0)
Test plan