feat(fromHex): ssri.fromHex to make it easier to generate them from h…

…ex valus

README.md

@@ -23,6 +23,7 @@ Integrity](https://w3c.github.io/webappsec/specs/subresourceintegrity/) hashes.
     * [`Integrity#pickAlgorithm`](#integrity-pick-algorithm)
     * [`IntegrityMetadata#hexDigest`](#integrity-metadata-hex-digest)
   * Integrity Generation
+    * [`fromHex`](#from-hex)
     * [`fromData`](#from-data)
     * [`fromStream`](#from-stream)
   * Integrity Verification
@@ -227,6 +228,33 @@ to a hex representation of the base64 data.
 ssri.parse('sha1-deadbeef', {single: true}).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 metadata 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 `IntegrityMetadata` 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

index.js

@@ -31,11 +31,7 @@ class IntegrityMetadata {
     this.options = rawOpts ? rawOpts.slice(1).split('?') : []
   }
   hexDigest () {
-    return this.digest && (
-      Buffer.from
-      ? Buffer.from(this.digest, 'base64')
-      : new Buffer(this.digest, 'base64')
-    ).toString('hex')
+    return this.digest && bufFrom(this.digest, 'base64').toString('hex')
   }
   toString (opts) {
     if (opts && opts.strict) {
@@ -136,6 +132,18 @@ function stringify (obj, opts) {
   }
 }
 
+module.exports.fromHex = fromHex
+function fromHex (hexDigest, algorithm, opts) {
+  const optString = (opts && opts.options && opts.options.length)
+  ? `?${opts.options.join('?')}`
+  : ''
+  return parse(
+    `${algorithm}-${
+      bufFrom(hexDigest, 'hex').toString('base64')
+    }${optString}`, opts
+  )
+}
+
 module.exports.fromData = fromData
 function fromData (data, opts) {
   opts = opts || {}
@@ -254,3 +262,7 @@ function getPrioritizedHash (algo1, algo2) {
   ? algo1
   : algo2
 }
+
+function bufFrom (data, enc) {
+  return Buffer.from ? Buffer.from(data, enc) : new Buffer(data, enc)
+}

test/from.js

@@ -16,6 +16,20 @@ function fileStream () {
   return fs.createReadStream(__filename)
 }
 
+test('fromHex', t => {
+  t.equal(
+    ssri.fromHex('deadbeef', 'sha1').toString(),
+    `sha1-3q2+7w==`,
+    'created an Integrity object from a given hex + sha'
+  )
+  t.equal(
+    ssri.fromHex('deadbeef', 'sha512', {options: ['a', 'b', 'c']}).toString(),
+    `sha512-3q2+7w==?a?b?c`,
+    'options added to entry'
+  )
+  t.done()
+})
+
 test('fromData', t => {
   t.equal(
     ssri.fromData(TEST_DATA).toString(),