(function () {
    const encoder = new TextEncoder();
    const decoder = new TextDecoder();
    const form = document.getElementById('unlockForm');
    const passwordInput = document.getElementById('letterPassword');
    const statusEl = document.getElementById('unlockStatus');
    const payloadEl = document.getElementById('encryptedPayload');

    function fromBase64(value) {
        const binary = atob(value);
        const bytes = new Uint8Array(binary.length);
        for (let i = 0; i < binary.length; i += 1) {
            bytes[i] = binary.charCodeAt(i);
        }
        return bytes;
    }

    async function deriveKey(password, salt, iterations) {
        const baseKey = await crypto.subtle.importKey(
            'raw',
            encoder.encode(password),
            'PBKDF2',
            false,
            ['deriveKey']
        );

        return crypto.subtle.deriveKey(
            {
                name: 'PBKDF2',
                salt,
                iterations,
                hash: 'SHA-256'
            },
            baseKey,
            { name: 'AES-GCM', length: 256 },
            false,
            ['decrypt']
        );
    }

    async function unlock(password) {
        const payload = JSON.parse(payloadEl.textContent);
        const salt = fromBase64(payload.salt);
        const iv = fromBase64(payload.iv);
        const ciphertext = fromBase64(payload.ciphertext);
        const key = await deriveKey(password, salt, payload.iterations);
        const plaintext = await crypto.subtle.decrypt(
            { name: 'AES-GCM', iv },
            key,
            ciphertext
        );

        return decoder.decode(plaintext);
    }

    form.addEventListener('submit', async event => {
        event.preventDefault();
        const password = passwordInput.value;
        if (!password) {
            statusEl.textContent = '请输入密码。';
            return;
        }

        statusEl.textContent = '正在解锁...';
        form.querySelector('button').disabled = true;

        try {
            const html = await unlock(password);
            document.open();
            document.write(html);
            document.close();
        } catch (error) {
            statusEl.textContent = '密码不正确，或页面密文已损坏。';
            form.querySelector('button').disabled = false;
            passwordInput.select();
        }
    });
}());