fix: reject CBOR data with extraneous back-to-back encoded data

The streaming form of CBOR (3.1) may use back-to-back top-level objects without
an explicit container and borc will decode this without failure.
`decodeFirst()` will only return the first of these but additional data may
exist but be hidden.

Ref: dignifiedquire/borc#47 (comment)
Ref: ipld/specs#268

src/util.js

@@ -153,9 +153,14 @@ exports.deserialize = (data) => {
     throw new Error('Data is too large to deserialize with current decoder')
   }
 
-  const deserialized = decoder.decodeFirst(data)
+  // borc will decode back-to-back objects into an implicit top-level array, we
+  // strictly want to only see a single explicit top-level object
+  const all = decoder.decodeAll(data)
+  if (all.length !== 1) {
+    throw new Error('Extraneous CBOR data found beyond initial top-level object')
+  }
 
-  return deserialized
+  return all[0]
 }
 
 /**

test/util.spec.js

@@ -122,4 +122,12 @@ describe('util', () => {
     expect(dagCBOR.util.deserialize(s1)).to.be.eql({ data: bytes })
     expect(dagCBOR.util.deserialize(s2)).to.be.eql({ data: bytes })
   })
+
+  it('reject extraneous, but valid CBOR data after initial top-level object', () => {
+    expect(() =>
+      // two top-level CBOR objects, the original and a single uint=0, valid if using
+      // CBOR in streaming mode, not valid here
+      dagCBOR.util.deserialize(Buffer.concat([serializedObj, Buffer.alloc(1)]))
+    ).to.throw(Error, 'Extraneous CBOR data found beyond initial top-level object')
+  })
 })