# Crypto

The `crypto` module provides cryptographic functionality that includes a set of
wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify functions.

Use `require('crypto')` to access this module.

## Class: Hash

The `Hash` class is a utility for creating hash digests of data. It can be
used in one of two ways:

- As a [stream][] that is both readable and writable, where data is written
  to produce a computed hash digest on the readable side, or
- Using the [`hash.update()`][] and [`hash.digest()`][] methods to produce the
  computed hash.

The [`crypto.createHash()`][] method is used to create `Hash` instances. `Hash`
objects are not to be created directly using the `new` keyword.

Example: Using `Hash` objects as streams:

```js
const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.on('readable', () => {
  const data = hash.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
  }
});

hash.write('some data to hash');
hash.end();
```

Example: Using `Hash` and piped streams:

```js
const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');

const input = fs.createReadStream('test.js');
input.pipe(hash).pipe(process.stdout);
```

Example: Using the [`hash.update()`][] and [`hash.digest()`][] methods:

```js
const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
```

### hash.digest([encoding])

- `encoding` {string}

Calculates the digest of all of the data passed to be hashed (using the
[`hash.update()`][] method). The `encoding` can be `'hex'`, `'latin1'` or
`'base64'`. If `encoding` is provided a string will be returned; otherwise
a [`Buffer`][] is returned.

The `Hash` object can not be used again after `hash.digest()` method has been
called. Multiple calls will cause an error to be thrown.

### hash.update(data[, inputEncoding])

- `data` {string | Buffer | TypedArray | DataView}
- `inputEncoding` {string}

Updates the hash content with the given `data`, the encoding of which
is given in `inputEncoding` and can be `'utf8'`, `'ascii'` or
`'latin1'`. If `encoding` is not provided, and the `data` is a string, an
encoding of `'utf8'` is enforced. If `data` is a [`Buffer`][], `TypedArray`, or
`DataView`, then `inputEncoding` is ignored.

This can be called many times with new data as it is streamed.

## `crypto` module methods and properties

### crypto.createHash(algorithm[, options])

- `algorithm` {string}
- `options` {Object} [`stream.transform` options][]

Creates and returns a `Hash` object that can be used to generate hash digests
using the given `algorithm`. Optional `options` argument controls stream
behavior.

The `algorithm` is dependent on the available algorithms supported by the
version of OpenSSL on the platform. Examples are `'sha256'`, `'sha512'`, etc.
On recent releases of OpenSSL, `openssl list-message-digest-algorithms` will
display the available digest algorithms.

Example: generating the sha256 sum of a file

```js
const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});
```

### crypto.getHashes()

Returns an array of the names of the supported hash algorithms,
such as `RSA-SHA256`.

Example:

```js
const hashes = crypto.getHashes();
console.log(hashes);
```

### crypto.randomBytes(size[, callback])

* size {Number}
* callback {Function}
  * err {Error}
  * buf {Buffer}

Generates cryptographically strong pseudo-random data. The size argument is a number indicating the number of bytes 
to generate.

If a callback function is provided, the bytes are generated asynchronously and the callback function is invoked with two 
arguments: `err` and `buf`. If an error occurs, err will be an Error object; otherwise it is null. The buf argument is a 
Buffer containing the generated bytes.