# x25519.select

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## Purpose

`x25519.select.ts` is the build-time primitive selector for X25519 in `@suaveplan/crypto-curve`. Generated by `bun scripts/gen-crypto-selects.ts`, it re-exports one backend — `x25519.noble.ts` or `x25519.native.ts` — based on the `x25519` entry in `crypto.config.ts`.

Callers import through `x25519.ts`, which delegates here. The selector isolates callers from backend changes: switching from Noble to a native implementation requires only regenerating this file. Currently the Noble backend is active because the native stub throws `PrimitiveNotImplemented`.

## Features

- Single re-export of `getPublicKey`, `getSharedSecret`, and `randomPrivateKey`.
- Generated — never hand-edited.
- Controlled by `crypto.config.ts`: set `x25519` to `'noble'` or `'native'`.
- Enables dead-code elimination of the inactive backend.

## Usage: Basic Example

```ts
import * as x25519 from "@suaveplan/crypto-curve/x25519";

const sk = x25519.randomPrivateKey();
const pk = x25519.getPublicKey(sk);
console.log(pk.length); // 32
```

## Usage: Advanced Example

```ts
import * as x25519 from "@suaveplan/crypto-curve/x25519";

// Simple ECDH exchange.
function exchange(localSk: Uint8Array, remotePk: Uint8Array): Uint8Array {
    return x25519.getSharedSecret(localSk, remotePk);
}
```

## API Reference

Re-exports from the active implementation:

- `getPublicKey(privateKey: Uint8Array): Uint8Array` — 32-byte public key.
- `getSharedSecret(privateKey: Uint8Array, peerPublicKey: Uint8Array): Uint8Array` — 32-byte raw shared secret.
- `randomPrivateKey(): Uint8Array` — 32 random bytes.

## Implementation Notes

This file is a generated one-liner: `export * from "./impl/x25519.noble.js"`. The generator resolves the configured backend and writes the appropriate path. The file is idempotent and safe to regenerate at any time. Do not add logic here — it will be overwritten.

### Selection Logic

At build time `crypto.config.ts` picks either the Noble or the native engine for every primitive; the selector exported here reads the generated `crypto-selects` map and routes calls to the chosen impl. When the selected engine is not yet implemented, the `PrimitiveNotImplemented` sentinel from `@suaveplan/crypto-utils` is thrown with the primitive-id baked in so logs identify the exact missing slot.