Module nut28 

Pay-to-Blinded-Key (P2BK) Implementation

This module implements NUT-28: Pay-to-Blinded-Key, a privacy enhancement for P2PK (NUT-11) that allows “silent payments” - tokens can be locked to a public key without exposing which public key they’re locked to, even to the mint.

Key Concepts

• Ephemeral Keys: Sender generates a fresh ephemeral keypair (e, E) for each transaction
• ECDH: Both sides derive the same shared secret via Elliptic Curve Diffie-Hellman
• Blinding: Public keys are blinded before being sent to the mint
• Key Recovery: Receiver uses ECDH to recover the original blinding factor and derive signing key

Feature Highlights

• Privacy-preserving P2PK operations
• Compatible with existing mints (no mint-side changes needed)
• BIP-340 compatibility for x-only pubkeys
• Canonical slot mapping for multi-key proofs

Implementation Details

• Uses SHA-256 for key derivation with domain separation
• Supports rejection sampling for out-of-range blinding factors
• Properly handles SEC1 and BIP-340 key formats

See the NUT-28 specification for full details: https://github.com/cashubtc/nuts/blob/main/28.md

Enums

Error

NUT-28 Error

Functions

blind_public_key

Blind a public key with a random scalar r

derive_signing_key_bip340

Derive BIP-340 compatible signing key from private key and blinding scalar

ecdh_kdf

Perform ECDH and get blinding factor r