Add content verification

Signed-off-by: Jacob Kiers <jacob@jacobkiers.net>

content/scripts/verifier.js

@@ -0,0 +1,21 @@
+class Verifier {
+    /***
+     * @param content Content
+     */
+    async verify(content) {
+        if (!window.isSecureContext) return;
+        if (typeof TextEncoder === "undefined") return;
+        if (typeof crypto === "undefined") return;
+        const encoder = new TextEncoder();
+        const digestBuffer = await crypto.subtle.digest(content.index.hashAlgorithm, encoder.encode(content.content));
+
+        const hashArray = Array.from(new Uint8Array(digestBuffer));                     // convert buffer to byte array
+        const digest = hashArray.map(b => b.toString(16).padStart(2, '0')).join(''); // convert bytes to hex string
+
+        if (content.index.hash === digest) {
+            document.getElementById("verification").innerHTML =
+                "The hash of the content is verified to correspond with the hash" +
+                " in the metadata. You are now reading exactly what was intended.";
+        }
+    }
+}