05/29 Update extension

extension/shared/sharing-crypto.js

@@ -0,0 +1,122 @@
+/**
+ * extension/shared/sharing-crypto.js
+ *
+ * ECDH P-256 cryptography for zero-knowledge item sharing — extension port.
+ * Mirrors app/static/js/sharing.js exactly but uses `crypto.subtle`
+ * (no `window.`) so it works in both the popup and the service worker.
+ *
+ * Only the primitives needed by the extension are included:
+ *   importPublicKey, decryptPrivateKey, deriveSharedKey, decryptShare, decryptName
+ */
+
+const ExtSharingCrypto = (() => {
+  const subtle = crypto.subtle;
+
+  // ── Helpers ────────────────────────────────────────────────────────────────
+
+  function base64ToBytes(b64) {
+    const bin = atob(b64);
+    const out = new Uint8Array(bin.length);
+    for (let i = 0; i < bin.length; i++) out[i] = bin.charCodeAt(i);
+    return out;
+  }
+
+  // ── Key import ─────────────────────────────────────────────────────────────
+
+  /**
+   * Import a remote user's ECDH public key from base64 raw bytes (65-byte
+   * uncompressed P-256 point).
+   */
+  async function importPublicKey(base64Raw) {
+    return subtle.importKey(
+      'raw',
+      base64ToBytes(base64Raw),
+      { name: 'ECDH', namedCurve: 'P-256' },
+      false,
+      [],  // public keys have no usages in WebCrypto ECDH
+    );
+  }
+
+  /**
+   * Decrypt the user's ECDH private key JWK (fetched from server as
+   * private_key_enc / private_key_iv) using the vault AES-256-GCM key.
+   * Returns a CryptoKey usable for ECDH deriveBits.
+   */
+  async function decryptPrivateKey(vaultKey, private_key_enc, private_key_iv) {
+    const plaintext = await subtle.decrypt(
+      { name: 'AES-GCM', iv: base64ToBytes(private_key_iv) },
+      vaultKey,
+      base64ToBytes(private_key_enc),
+    );
+    const jwk = JSON.parse(new TextDecoder().decode(plaintext));
+    return subtle.importKey(
+      'jwk',
+      jwk,
+      { name: 'ECDH', namedCurve: 'P-256' },
+      false,
+      ['deriveBits'],
+    );
+  }
+
+  // ── Shared-secret derivation ───────────────────────────────────────────────
+
+  /**
+   * Derive an AES-256-GCM CryptoKey from the ECDH shared secret.
+   * ECDH is commutative: ECDH(A_priv, B_pub) === ECDH(B_priv, A_pub).
+   */
+  async function deriveSharedKey(myPrivateKey, theirPublicKey) {
+    const bits = await subtle.deriveBits(
+      { name: 'ECDH', public: theirPublicKey },
+      myPrivateKey,
+      256,
+    );
+    return subtle.importKey(
+      'raw',
+      bits,
+      { name: 'AES-GCM' },
+      false,
+      ['encrypt', 'decrypt'],
+    );
+  }
+
+  // ── Decrypt ────────────────────────────────────────────────────────────────
+
+  /**
+   * Decrypt a shared item payload (JSON object) using the ECDH shared key.
+   */
+  async function decryptShare(sharedKey, enc_data, iv) {
+    const plaintext = await subtle.decrypt(
+      { name: 'AES-GCM', iv: base64ToBytes(iv) },
+      sharedKey,
+      base64ToBytes(enc_data),
+    );
+    return JSON.parse(new TextDecoder().decode(plaintext));
+  }
+
+  /**
+   * Decrypt an encrypted item display name.
+   * Returns null on failure (e.g. legacy share without enc_name).
+   */
+  async function decryptName(sharedKey, enc_name, iv_name) {
+    try {
+      const plaintext = await subtle.decrypt(
+        { name: 'AES-GCM', iv: base64ToBytes(iv_name) },
+        sharedKey,
+        base64ToBytes(enc_name),
+      );
+      return new TextDecoder().decode(plaintext);
+    } catch {
+      return null;
+    }
+  }
+
+  // ── Public API ─────────────────────────────────────────────────────────────
+
+  return {
+    importPublicKey,
+    decryptPrivateKey,
+    deriveSharedKey,
+    decryptShare,
+    decryptName,
+  };
+})();