# p521.select

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## Purpose

`p521.select.ts` is the build-time primitive selector for NIST P-521 in
`@suaveplan/crypto-curve`. Generated by `bun scripts/gen-crypto-selects.ts`,
it re-exports one concrete backend — `p521.noble.ts` or `p521.native.ts` —
according to the `p521` field in `crypto.config.ts`.

Callers import through the stable `p521.ts` entry point. This selector
level insulates all call sites from backend changes: regenerating this
file is the only step required to switch implementations. Currently the
Noble backend is active because the native stub throws
`PrimitiveNotImplemented`.

## Features

- Single re-export of `getPublicKey`, `sign`, `verify`, and `randomPrivateKey`.
- Generated — never hand-edited.
- Controlled by `crypto.config.ts`: set `p521` to `'noble'` or `'native'`.
- Enables dead-code elimination of the inactive backend.
- Maintains a stable API across backend switches.

## Usage: Basic Example

```ts
import * as p521 from "@suaveplan/crypto-curve/p521";

const sk = p521.randomPrivateKey();
const pk = p521.getPublicKey(sk);
console.log(pk.length); // 67
```

## Usage: Advanced Example

```ts
import * as p521 from "@suaveplan/crypto-curve/p521";
import { sha512 } from "@suaveplan/crypto-hash";

function roundTrip(message: Uint8Array): boolean {
    const sk = p521.randomPrivateKey();
    const pk = p521.getPublicKey(sk);
    const digest = sha512(message);
    const sig = p521.sign(digest, sk);
    return p521.verify(sig, digest, pk);
}
```

## API Reference

Re-exports from the active implementation:

- `getPublicKey(privateKey: Uint8Array): Uint8Array` — 67-byte compressed
  P-521 public key.
- `sign(digest: Uint8Array, privateKey: Uint8Array): Uint8Array` — 132-byte
  compact ECDSA signature.
- `verify(signature: Uint8Array, digest: Uint8Array, publicKey: Uint8Array): boolean`
- `randomPrivateKey(): Uint8Array` — rejection-sampled 66-byte key.

## Implementation Notes

The selector file contains a single `export * from` line and is marked
`/* GENERATED — DO NOT EDIT */`. P-521 keys carry an additional invariant
that callers need to be aware of: the high byte of every scalar / coordinate
must have the top 7 bits zero (the curve has a 521-bit order, byte-aligned
to 66 bytes). The noble backend enforces this in `isValidPrivateKey`; the
rejection loop in `randomPrivateKey` retries until the constraint holds.