/**
 * 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,
  };
})();