transient.chat/static/assets/js/crypto.js

/**
 * Transient.chat - Encryption First Engine
 * Zero-Knowledge End-to-End Encryption (E2EE)
 * All chat payloads (text & voice audio) are encrypted client-side using AES-256-GCM.
 * Server memory stores strictly ephemeral encrypted ciphertext and IVs.
 */
(function() {
    // Helper: string to ArrayBuffer (UTF-8)
    function strToBuf(str) {
        return new TextEncoder().encode(str);
    }

    // Helper: ArrayBuffer to string (UTF-8)
    function bufToStr(buf) {
        return new TextDecoder().decode(buf);
    }

    // Helper: ArrayBuffer to hex string
    function bufToHex(buf) {
        return Array.from(new Uint8Array(buf))
            .map(b => b.toString(16).padStart(2, '0'))
            .join('');
    }

    // Helper: hex string to ArrayBuffer
    function hexToBuf(hex) {
        if (!hex) return new Uint8Array(0).buffer;
        const bytes = new Uint8Array(hex.length / 2);
        for (let i = 0; i < bytes.length; i++) {
            bytes[i] = parseInt(hex.substr(i * 2, 2), 16);
        }
        return bytes.buffer;
    }

    /**
     * Derives a 256-bit AES-GCM key from the room password and room name.
     * Uses PBKDF2 with SHA-256 and 100,000 iterations with a room-bound salt.
     */
    window.deriveRoomKey = async function(password, roomName) {
        const passwordBuffer = strToBuf(password);
        const saltBuffer = strToBuf("transient.chat.room." + roomName);

        const baseKey = await window.crypto.subtle.importKey(
            "raw",
            passwordBuffer,
            "PBKDF2",
            false,
            ["deriveKey"]
        );

        return await window.crypto.subtle.deriveKey(
            {
                name: "PBKDF2",
                salt: saltBuffer,
                iterations: 100000,
                hash: "SHA-256"
            },
            baseKey,
            { name: "AES-GCM", length: 256 },
            false,
            ["encrypt", "decrypt"]
        );
    };

    /**
     * Encrypts a message payload object (text + optional audio) using the derived room key.
     * Returns hex-encoded ciphertext and 96-bit (12-byte) initialization vector (IV).
     */
    window.encryptRoomPayload = async function(cryptoKey, payloadObj) {
        const jsonStr = JSON.stringify(payloadObj);
        const iv = window.crypto.getRandomValues(new Uint8Array(12));

        const encryptedBuffer = await window.crypto.subtle.encrypt(
            { name: "AES-GCM", iv: iv },
            cryptoKey,
            strToBuf(jsonStr)
        );

        return {
            ciphertext: bufToHex(encryptedBuffer),
            iv: bufToHex(iv)
        };
    };

    /**
     * Decrypts ciphertext and IV hex strings back to the original payload object.
     */
    window.decryptRoomPayload = async function(cryptoKey, ciphertextHex, ivHex) {
        const decryptedBuffer = await window.crypto.subtle.decrypt(
            { name: "AES-GCM", iv: hexToBuf(ivHex) },
            cryptoKey,
            hexToBuf(ciphertextHex)
        );

        return JSON.parse(bufToStr(decryptedBuffer));
    };
})();