src/js.rs

// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use crate::Error;

extern crate std;
use std::thread_local;

use js_sys::Uint8Array;
use wasm_bindgen::{prelude::*, JsCast};

// Maximum is 65536 bytes see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
const BROWSER_CRYPTO_BUFFER_SIZE: usize = 256;

enum RngSource {
    Node(NodeCrypto),
    Browser(BrowserCrypto, Uint8Array),
}

// JsValues are always per-thread, so we initialize RngSource for each thread.
//   See: https://github.com/rustwasm/wasm-bindgen/pull/955
thread_local!(
    static RNG_SOURCE: Result<RngSource, Error> = getrandom_init();
);

pub(crate) fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
    RNG_SOURCE.with(|result| {
        let source = result.as_ref().map_err(|&e| e)?;

        match source {
            RngSource::Node(n) => {
                if n.random_fill_sync(dest).is_err() {
                    return Err(Error::NODE_RANDOM_FILL_SYNC);
                }
            }
            RngSource::Browser(crypto, buf) => {
                // getRandomValues does not work with all types of WASM memory,
                // so we initially write to browser memory to avoid exceptions.
                for chunk in dest.chunks_mut(BROWSER_CRYPTO_BUFFER_SIZE) {
                    // The chunk can be smaller than buf's length, so we call to
                    // JS to create a smaller view of buf without allocation.
                    let sub_buf = buf.subarray(0, chunk.len() as u32);

                    if crypto.get_random_values(&sub_buf).is_err() {
                        return Err(Error::WEB_GET_RANDOM_VALUES);
                    }
                    sub_buf.copy_to(chunk);
                }
            }
        };
        Ok(())
    })
}

fn getrandom_init() -> Result<RngSource, Error> {
    let global: Global = js_sys::global().unchecked_into();
    if is_node(&global) {
        let crypto = require("crypto").map_err(|_| Error::NODE_CRYPTO)?;
        return Ok(RngSource::Node(crypto));
    }

    // Assume we are in some Web environment (browser or web worker). We get
    // `self.crypto` (called `msCrypto` on IE), so we can call
    // `crypto.getRandomValues`. If `crypto` isn't defined, we assume that
    // we are in an older web browser and the OS RNG isn't available.
    let crypto = match (global.crypto(), global.ms_crypto()) {
        (c, _) if c.is_object() => c,
        (_, c) if c.is_object() => c,
        _ => return Err(Error::WEB_CRYPTO),
    };

    let buf = Uint8Array::new_with_length(BROWSER_CRYPTO_BUFFER_SIZE as u32);
    Ok(RngSource::Browser(crypto, buf))
}

// Taken from https://www.npmjs.com/package/browser-or-node
fn is_node(global: &Global) -> bool {
    let process = global.process();
    if process.is_object() {
        let versions = process.versions();
        if versions.is_object() {
            return versions.node().is_string();
        }
    }
    false
}

#[wasm_bindgen]
extern "C" {
    type Global; // Return type of js_sys::global()

    // Web Crypto API (https://www.w3.org/TR/WebCryptoAPI/)
    #[wasm_bindgen(method, getter, js_name = "msCrypto")]
    fn ms_crypto(this: &Global) -> BrowserCrypto;
    #[wasm_bindgen(method, getter)]
    fn crypto(this: &Global) -> BrowserCrypto;
    type BrowserCrypto;
    #[wasm_bindgen(method, js_name = getRandomValues, catch)]
    fn get_random_values(this: &BrowserCrypto, buf: &Uint8Array) -> Result<(), JsValue>;

    // Node JS crypto module (https://nodejs.org/api/crypto.html)
    #[wasm_bindgen(catch, js_name = "module.require")]
    fn require(s: &str) -> Result<NodeCrypto, JsValue>;
    type NodeCrypto;
    #[wasm_bindgen(method, js_name = randomFillSync, catch)]
    fn random_fill_sync(this: &NodeCrypto, buf: &mut [u8]) -> Result<(), JsValue>;

    // Node JS process Object (https://nodejs.org/api/process.html)
    #[wasm_bindgen(method, getter)]
    fn process(this: &Global) -> Process;
    type Process;
    #[wasm_bindgen(method, getter)]
    fn versions(this: &Process) -> Versions;
    type Versions;
    #[wasm_bindgen(method, getter)]
    fn node(this: &Versions) -> JsValue;
}