with any invalid components. This also means a restricted set of algorithms
will be used -- the spec limits them to `sha256`, `sha384`, and `sha512`.

Strict mode is recommended if the integrity strings are intended for use in
browsers, or in other situations where strict adherence to the spec is needed.

##### Example

```javascript
ssri.parse('sha512-9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==?foo') // -> Integrity object
```

#### <a name="stringify"></a> `> ssri.stringify(sri, [opts]) -> String`

This function is identical to [`Integrity#toString()`](#integrity-to-string),
except it can be used on _any_ object that [`parse`](#parse) can handle -- that
is, a string, an `Hash`-like, or an `Integrity`-like.

The `opts.sep` option defines the string to use when joining multiple entries
together. To be spec-compliant, this _must_ be whitespace. The default is a
single space (`' '`).

If `opts.strict` is true, the integrity string will be created using strict
parsing rules. See [`ssri.parse`](#parse).

##### Example

```javascript
// Useful for cleaning up input SRI strings:
ssri.stringify('\n\rsha512-foo\n\t\tsha384-bar')
// -> 'sha512-foo sha384-bar'

// Hash-like: only a single entry.
ssri.stringify({
  algorithm: 'sha512',
  digest:'9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==',
  options: ['foo']
})
// ->
// 'sha512-9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==?foo'

// Integrity-like: full multi-entry syntax. Similar to output of `ssri.parse`
ssri.stringify({
  'sha512': [
    {
      algorithm: 'sha512',
      digest:'9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==',
      options: ['foo']
    }
  ]
})
// ->
// 'sha512-9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==?foo'
```

#### <a name="integrity-concat"></a> `> Integrity#concat(otherIntegrity, [opts]) -> Integrity`

Concatenates an `Integrity` object with another IntegrityLike, or an integrity
string.

This is functionally equivalent to concatenating the string format of both
integrity arguments, and calling [`ssri.parse`](#ssri-parse) on the new string.

If `opts.strict` is true, the new `Integrity` will be created using strict
parsing rules. See [`ssri.parse`](#parse).

##### Example

```javascript
// This will combine the integrity checks for two different versions of
// your index.js file so you can use a single integrity string and serve
// either of these to clients, from a single `<script>` tag.
const desktopIntegrity = ssri.fromData(fs.readFileSync('./index.desktop.js'))
const mobileIntegrity = ssri.fromData(fs.readFileSync('./index.mobile.js'))

// Note that browsers (and ssri) will succeed as long as ONE of the entries
// for the *prioritized* algorithm succeeds. That is, in order for this fallback
// to work, both desktop and mobile *must* use the same `algorithm` values.
desktopIntegrity.concat(mobileIntegrity)
```

#### <a name="integrity-to-string"></a> `> Integrity#toString([opts]) -> String`

Returns the string representation of an `Integrity` object. All hash entries
will be concatenated in the string by `opts.sep`, which defaults to `' '`.

If you want to serialize an object that didn't from from an `ssri` function,
use [`ssri.stringify()`](#stringify).

If `opts.strict` is true, the integrity string will be created using strict
parsing rules. See [`ssri.parse`](#parse).

##### Example

```javascript
const integrity = 'sha512-9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==?foo'

ssri.parse(integrity).toString() === integrity
```

#### <a name="integrity-to-json"></a> `> Integrity#toJSON() -> String`

Returns the string representation of an `Integrity` object. All hash entries
will be concatenated in the string by `' '`.

This is a convenience method so you can pass an `Integrity` object directly to `JSON.stringify`.
For more info check out [toJSON() behavior on mdn](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#toJSON%28%29_behavior).

##### Example

```javascript
const integrity = '"sha512-9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A==?foo"'

JSON.stringify(ssri.parse(integrity)) === integrity
```

#### <a name="integrity-match"></a> `> Integrity#match(sri, [opts]) -> Hash | false`

Returns the matching (truthy) hash if `Integrity` matches the argument passed as
`sri`, which can be anything that [`parse`](#parse) will accept. `opts` will be
passed through to `parse` and [`pickAlgorithm()`](#integrity-pick-algorithm).

##### Example

```javascript
const integrity = 'sha512-9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A=='

ssri.parse(integrity).match(integrity)
// Hash {
//   digest: '9KhgCRIx/AmzC8xqYJTZRrnO8OW2Pxyl2DIMZSBOr0oDvtEFyht3xpp71j/r/pAe1DM+JI/A+line3jUBgzQ7A=='
//   algorithm: 'sha512'
// }

ssri.parse(integrity).match('sha1-deadbeef')
// false
```

#### <a name="integrity-pick-algorithm"></a> `> Integrity#pickAlgorithm([opts]) -> String`

Returns the "best" algorithm from those available in the integrity object.

If `opts.pickAlgorithm` is provided, it will be passed two algorithms as
arguments. ssri will prioritize whichever of the two algorithms is returned by
this function. Note that the function may be called multiple times, and it
**must** return one of the two algorithms provided. By default, ssri will make
a best-effort to pick the strongest/most reliable of the given algorithms. It
may intentionally deprioritize algorithms with known vulnerabilities.

##### Example

```javascript
ssri.parse('sha1-WEakDigEST sha512-yzd8ELD1piyANiWnmdnpCL5F52f10UfUdEkHywVZeqTt0ymgrxR63Qz0GB7TKPoeeZQmWCaz7T1').pickAlgorithm() // sha512
```

#### <a name="integrity-hex-digest"></a> `> Integrity#hexDigest() -> String`

`Integrity` is assumed to be either a single-hash `Integrity` instance, or a
`Hash` instance. Returns its `digest`, converted to a hex representation of the
base64 data.

##### Example

```javascript
ssri.parse('sha1-deadbeef').hexDigest() // '75e69d6de79f'
```

#### <a name="from-hex"></a> `> ssri.fromHex(hexDigest, algorithm, [opts]) -> Integrity`

Creates an `Integrity` object with a single entry, based on a hex-formatted
hash. This is a utility function to help convert existing shasums to the
Integrity format, and is roughly equivalent to something like:

```javascript
algorithm + '-' + Buffer.from(hexDigest, 'hex').toString('base64')
```

`opts.options` may optionally be passed in: it must be an array of option
strings that will be added to all generated integrity hashes generated by
`fromData`. This is a loosely-specified feature of SRIs, and currently has no
specified semantics besides being `?`-separated. Use at your own risk, and
probably avoid if your integrity strings are meant to be used with browsers.

If `opts.strict` is true, the integrity object will be created using strict
parsing rules. See [`ssri.parse`](#parse).

If `opts.single` is true, a single `Hash` object will be returned.

##### Example

```javascript
ssri.fromHex('75e69d6de79f', 'sha1').toString() // 'sha1-deadbeef'
```

#### <a name="from-data"></a> `> ssri.fromData(data, [opts]) -> Integrity`

Creates an `Integrity` object from either string or `Buffer` data, calculating
all the requested hashes and adding any specified options to the object.

`opts.algorithms` determines which algorithms to generate hashes for. All
results will be included in a single `Integrity` object. The default value for
`opts.algorithms` is `['sha512']`. All algorithm strings must be hashes listed
in `crypto.getHashes()` for the host Node.js platform.

`opts.options` may optionally be passed in: it must be an array of option
strings that will be added to all generated integrity hashes generated by
`fromData`. This is a loosely-specified feature of SRIs, and currently has no
specified semantics besides being `?`-separated. Use at your own risk, and
probably avoid if your integrity strings are meant to be used with browsers.

If `opts.strict` is true, the integrity object will be created using strict
parsing rules. See [`ssri.parse`](#parse).

##### Example

```javascript
const integrityObj = ssri.fromData('foobarbaz', {
  algorithms: ['sha256', 'sha384', 'sha512']
})
integrity.toString('\n')
// ->
// sha256-l981iLWj8kurw4UbNy8Lpxqdzd7UOxS50Glhv8FwfZ0=
// sha384-irnCxQ0CfQhYGlVAUdwTPC9bF3+YWLxlaDGM4xbYminxpbXEq+D+2GCEBTxcjES9
// sha512-yzd8ELD1piyANiWnmdnpCL5F52f10UfUdEkHywVZeqTt0ymgrxR63Qz0GB7TKPoeeZQmWCaz7T1+9vBnypkYWg==
```

#### <a name="from-stream"></a> `> ssri.fromStream(stream, [opts]) -> Promise<Integrity>`

Returns a Promise of an Integrity object calculated by reading data from
a given `stream`.

It accepts both `opts.algorithms` and `opts.options`, which are documented as
part of [`ssri.fromData`](#from-data).

Additionally, `opts.Promise` may be passed in to inject a Promise library of
choice. By default, ssri will use Node's built-in Promises.

If `opts.strict` is true, the integrity object will be created using strict
parsing rules. See [`ssri.parse`](#parse).

##### Example

```javascript
ssri.fromStream(fs.createReadStream('index.js'), {
  algorithms: ['sha1', 'sha512']
}).then(integrity => {
  return ssri.checkStream(fs.createReadStream('index.js'), integrity)
}) // succeeds
```

#### <a name="create"></a> `> ssri.create([opts]) -> <Hash>`

Returns a Hash object with `update(<Buffer or string>[,enc])` and `digest()` methods.


The Hash object provides the same methods as [crypto class Hash](https://nodejs.org/dist/latest-v6.x/docs/api/crypto.html#crypto_class_hash).
`digest()` accepts no arguments and returns an Integrity object calculated by reading data from
calls to update.

It accepts both `opts.algorithms` and `opts.options`, which are documented as
part of [`ssri.fromData`](#from-data).

If `opts.strict` is true, the integrity object will be created using strict
parsing rules. See [`ssri.parse`](#parse).

##### Example

```javascript
const integrity = ssri.create().update('foobarbaz').digest()
integrity.toString()
// ->
// sha512-yzd8ELD1piyANiWnmdnpCL5F52f10UfUdEkHywVZeqTt0ymgrxR63Qz0GB7TKPoeeZQmWCaz7T1+9vBnypkYWg==
```

#### <a name="check-data"></a> `> ssri.checkData(data, sri, [opts]) -> Hash|false`

Verifies `data` integrity against an `sri` argument. `data` may be either a
`String` or a `Buffer`, and `sri` can be any subresource integrity
representation that [`ssri.parse`](#parse) can handle.

If verification succeeds, `checkData` will return the name of the algorithm that
was used for verification (a truthy value). Otherwise, it will return `false`.

If `opts.pickAlgorithm` is provided, it will be used by
[`Integrity#pickAlgorithm`](#integrity-pick-algorithm) when deciding which of
the available digests to match against.

If `opts.error` is true, and verification fails, `checkData` will throw either
an `EBADSIZE` or an `EINTEGRITY` error, instead of just returning false.

##### Example

```javascript
const data = fs.readFileSync('index.js')
ssri.checkData(data, ssri.fromData(data)) // -> 'sha512'
ssri.checkData(data, 'sha256-l981iLWj8kurw4UbNy8Lpxqdzd7UOxS50Glhv8FwfZ0')
ssri.checkData(data, 'sha1-BaDDigEST') // -> false
ssri.checkData(data, 'sha1-BaDDigEST', {error: true}) // -> Error! EINTEGRITY
```

#### <a name="check-stream"></a> `> ssri.checkStream(stream, sri, [opts]) -> Promise<Hash>`

Verifies the contents of `stream` against an `sri` argument. `stream` will be
consumed in its entirety by this process. `sri` can be any subresource integrity
representation that [`ssri.parse`](#parse) can handle.

`checkStream` will return a Promise that either resolves to the
`Hash` that succeeded verification, or, if the verification fails
or an error happens with `stream`, the Promise will be rejected.

If the Promise is rejected because verification failed, the returned error will
have `err.code` as `EINTEGRITY`.

If `opts.size` is given, it will be matched against the stream size. An error
with `err.code` `EBADSIZE` will be returned by a rejection if the expected size
and actual size fail to match.

If `opts.pickAlgorithm` is provided, it will be used by
[`Integrity#pickAlgorithm`](#integrity-pick-algorithm) when deciding which of
the available digests to match against.

##### Example

```javascript
const integrity = ssri.fromData(fs.readFileSync('index.js'))

ssri.checkStream(
  fs.createReadStream('index.js'),
  integrity
)
// ->
// Promise<{
//   algorithm: 'sha512',
//   digest: 'sha512-yzd8ELD1piyANiWnmdnpCL5F52f10UfUdEkHywVZeqTt0ymgrxR63Qz0GB7TKPoeeZQmWCaz7T1'
// }>

ssri.checkStream(
  fs.createReadStream('index.js'),
  'sha256-l981iLWj8kurw4UbNy8Lpxqdzd7UOxS50Glhv8FwfZ0'
) // -> Promise<Hash>

ssri.checkStream(
  fs.createReadStream('index.js'),
  'sha1-BaDDigEST'
) // -> Promise<Error<{code: 'EINTEGRITY'}>>
```

#### <a name="integrity-stream"></a> `> integrityStream([opts]) -> IntegrityStream`

Returns a `Transform` stream that data can be piped through in order to generate
and optionally check data integrity for piped data. When the stream completes
successfully, it emits `size` and `integrity` events, containing the total
number of bytes processed and a calculated `Integrity` instance based on stream
data, respectively.

If `opts.algorithms` is passed in, the listed algorithms will be calculated when
generating the final `Integrity` instance. The default is `['sha512']`.

If `opts.single` is passed in, a single `Hash` instance will be returned.

If `opts.integrity` is passed in, it should be an `integrity` value understood
by [`parse`](#parse) that the stream will check the data against. If
verification succeeds, the integrity stream will emit a `verified` event whose
value is a single `Hash` object that is the one that succeeded verification. If
verification fails, the stream will error with an `EINTEGRITY` error code.

If `opts.size` is given, it will be matched against the stream size. An error
with `err.code` `EBADSIZE` will be emitted by the stream if the expected size
and actual size fail to match.

If `opts.pickAlgorithm` is provided, it will be passed two algorithms as
arguments. ssri will prioritize whichever of the two algorithms is returned by
this function. Note that the function may be called multiple times, and it
**must** return one of the two algorithms provided. By default, ssri will make
a best-effort to pick the strongest/most reliable of the given algorithms. It
may intentionally deprioritize algorithms with known vulnerabilities.

##### Example

```javascript
const integrity = ssri.fromData(fs.readFileSync('index.js'))
fs.createReadStream('index.js')
.pipe(ssri.integrityStream({integrity}))
```