import { ed25519 } from '@noble/curves/ed25519.js';

/**
 * Deterministically derive an Ed25519 public key from a 32-byte seed.
 *
 * In the @noble/curves convention the "private key" *is* the seed —
 * `sign(seed, msg)` works directly, and `getPublicKey(seed)` recovers
 * the matching public key. V4.9's encrypted-blob primitive uses this
 * to mint a per-slot signing keypair from an HKDF output rooted at the
 * user's master key, so the same credentials always reproduce the same
 * keypair.
 */
export function ed25519PublicKeyFromSeed(seed: Uint8Array): Uint8Array {
  if (seed.length !== 32) {
    throw new Error(`Ed25519 seed must be 32 bytes, got ${seed.length}`);
  }
  return ed25519.getPublicKey(seed);
}
