feat: Add Integrity#merge method

This is going to be used in Pacote to upgrade the Fetcher.integrity
value with the data that comes out of cacache, but in such a way that we
don't accidentally suppress integrity errors.

PR-URL: #4
Credit: @isaacs
Close: #4
Reviewed-by: @isaacs

README.md

@@ -19,6 +19,7 @@ Integrity](https://w3c.github.io/webappsec/specs/subresourceintegrity/) hashes.
     * [`parse`](#parse)
     * [`stringify`](#stringify)
     * [`Integrity#concat`](#integrity-concat)
+    * [`Integrity#merge`](#integrity-merge)
     * [`Integrity#toString`](#integrity-to-string)
     * [`Integrity#toJSON`](#integrity-to-json)
     * [`Integrity#match`](#integrity-match)
@@ -184,6 +185,45 @@ const mobileIntegrity = ssri.fromData(fs.readFileSync('./index.mobile.js'))
 desktopIntegrity.concat(mobileIntegrity)
 ```
 
+#### <a name="integrity-merge"></a> `> Integrity#merge(otherIntegrity, [opts])`
+
+Safely merges another IntegrityLike or integrity string into an `Integrity`
+object.
+
+If the other integrity value has any algorithms in common with the current
+object, then the hash digests must match, or an error is thrown.
+
+Any new hashes will be added to the current object's set.
+
+This is useful when an integrity value may be upgraded with a stronger
+algorithm, you wish to prevent accidentally supressing integrity errors by
+overwriting the expected integrity value.
+
+##### Example
+
+```javascript
+const data = fs.readFileSync('data.txt')
+
+// integrity.txt contains 'sha1-X1UT+IIv2+UUWvM7ZNjZcNz5XG4='
+// because we were young, and didn't realize sha1 would not last
+const expectedIntegrity = ssri.parse(fs.readFileSync('integrity.txt', 'utf8'))
+const match = ssri.checkData(data, expectedIntegrity, {
+  algorithms: ['sha512', 'sha1']
+})
+if (!match) {
+  throw new Error('data corrupted or something!')
+}
+
+// get a stronger algo!
+if (match && match.algorithm !== 'sha512') {
+  const updatedIntegrity = ssri.fromData(data, { algorithms: ['sha512'] })
+  expectedIntegrity.merge(updatedIntegrity)
+  fs.writeFileSync('integrity.txt', expectedIntegrity.toString())
+  // file now contains
+  // 'sha1-X1UT+IIv2+UUWvM7ZNjZcNz5XG4= sha512-yzd8ELD1piyANiWnmdnpCL5F52f10UfUdEkHywVZeqTt0ymgrxR63Qz0GB7TKPoeeZQmWCaz7T1+9vBnypkYWg=='
+}
+```
+
 #### <a name="integrity-to-string"></a> `> Integrity#toString([opts]) -> String`
 
 Returns the string representation of an `Integrity` object. All hash entries

index.js

@@ -47,9 +47,9 @@ class IntegrityStream extends MiniPass {
   [_getOptions] () {
     const opts = this.opts
     // For verification
-    this.sri = opts.integrity && parse(opts.integrity, opts)
+    this.sri = opts.integrity ? parse(opts.integrity, opts) : null
     this.expectedSize = opts.size
-    this.goodSri = this.sri ? Object.keys(this.sri).length || null : null
+    this.goodSri = this.sri ? !!Object.keys(this.sri).length : false
     this.algorithm = this.goodSri ? this.sri.pickAlgorithm(opts) : null
     this.digests = this.goodSri ? this.sri[this.algorithm] : null
     this.optString = getOptString(opts.options)
@@ -198,6 +198,24 @@ class Integrity {
     return parse(this, { single: true }).hexDigest()
   }
 
+  // add additional hashes to an integrity value, but prevent
+  // *changing* an existing integrity hash.
+  merge (integrity, opts) {
+    opts = SsriOpts(opts)
+    const other = parse(integrity, opts)
+    for (const algo in other) {
+      if (this[algo]) {
+        if (!this[algo].find(hash =>
+          other[algo].find(otherhash =>
+            hash.digest === otherhash.digest))) {
+          throw new Error('hashes do not match, cannot update integrity')
+        }
+      } else {
+        this[algo] = other[algo]
+      }
+    }
+  }
+
   match (integrity, opts) {
     opts = SsriOpts(opts)
     const other = parse(integrity, opts)

test/update.js

@@ -0,0 +1,13 @@
+const ssri = require('../')
+const t = require('tap')
+
+const i = ssri.parse('sha1-foo')
+const o = ssri.parse('sha512-bar')
+i.merge(o)
+t.equal(i.toString(), 'sha1-foo sha512-bar', 'added second algo')
+t.throws(() => i.merge(ssri.parse('sha1-baz')), {
+  message: 'hashes do not match, cannot update integrity'
+})
+i.merge(o)
+i.merge(ssri.parse('sha1-foo'))
+t.equal(i.toString(), 'sha1-foo sha512-bar', 'did not duplicate')