# `math/os2ip`

## Purpose

Octet-String-to-Integer Primitive (RFC 8017 §4.2). Decodes a big-endian byte string
into a non-negative `bigint`. Used by every RSA scheme to convert a modulus-sized
byte payload back into the integer the modular operations expect.

Thin re-export of `@suaveplan/crypto-utils`'s canonical implementation so the
entire project agrees on encoding semantics.

## Features

- Always returns `bigint`, never `number`, so precision is preserved for any
  byte length.
- Pure decoder; never throws.

## Usage: Basic Example

```ts
import { os2ip } from "@suaveplan/crypto-rsa/math";

const value = os2ip(new Uint8Array([0, 0, 0, 255])); // 255n
```

## Usage: Advanced Example

```ts
import { i2osp, modPow, os2ip } from "@suaveplan/crypto-rsa/math";

function privateOpRoundTrip(
    sigBytes: Uint8Array,
    e: bigint,
    n: bigint,
    modulusByteLength: number
): Uint8Array {
    const sigInt = os2ip(sigBytes);
    const recovered = modPow(sigInt, e, n);
    return i2osp(recovered, modulusByteLength);
}
```

## API Reference

### `os2ip(bytes: Uint8Array): bigint`

Returns the big-endian-decoded `bigint`. Empty input returns `0n`.

## Implementation Notes

The canonical implementation lives in `@suaveplan/crypto-utils/bytes/os2ip/os2ip.ts`.
This re-export gives RSA consumers a path-local import mirroring RFC 8017 §4.2.