Begin implementing a reporting mechanism

server/keychain.js

@@ -0,0 +1,53 @@
+const { Crypto } = require('@peculiar/webcrypto');
+const crypto = new Crypto();
+
+const encoder = new TextEncoder();
+const decoder = new TextDecoder();
+
+module.exports = class Keychain {
+  constructor(secretKeyB64) {
+    if (secretKeyB64) {
+      this.rawSecret = new Uint8Array(Buffer.from(secretKeyB64, 'base64'));
+    } else {
+      throw new Error('key is required');
+    }
+    this.secretKeyPromise = crypto.subtle.importKey(
+      'raw',
+      this.rawSecret,
+      'HKDF',
+      false,
+      ['deriveKey']
+    );
+    this.metaKeyPromise = this.secretKeyPromise.then(function(secretKey) {
+      return crypto.subtle.deriveKey(
+        {
+          name: 'HKDF',
+          salt: new Uint8Array(),
+          info: encoder.encode('metadata'),
+          hash: 'SHA-256'
+        },
+        secretKey,
+        {
+          name: 'AES-GCM',
+          length: 128
+        },
+        false,
+        ['decrypt']
+      );
+    });
+  }
+
+  async decryptMetadata(ciphertext) {
+    const metaKey = await this.metaKeyPromise;
+    const plaintext = await crypto.subtle.decrypt(
+      {
+        name: 'AES-GCM',
+        iv: new Uint8Array(12),
+        tagLength: 128
+      },
+      metaKey,
+      ciphertext
+    );
+    return JSON.parse(decoder.decode(plaintext));
+  }
+};