anagram-games/www/scramblestring.js

'use strict';

/* Utilities for scrambling strings to obscure puzzle answers. Not
 * intended to be secure. */

// From https://stackoverflow.com/a/34310051
function toHexString(buffer) {
    let byteArray = new Uint8Array(buffer);
    return Array.from(byteArray, function(byte) {
        return ('0' + (byte & 0xFF).toString(16)).slice(-2);
    }).join('');
}
function fromHexString(str) {
    let buffer = new ArrayBuffer(str.length / 2);
    let byteArray = new Uint8Array(buffer);
    let i = 0;
    for (let idx = 0; idx < str.length; idx += 2) {
        let hex = str.substring(idx, idx+2);
        byteArray[i++] = parseInt(hex, 16);
    }
    return buffer;
}

// "Encrypt" a string and include the key just as a way of scrambling it
//  to make it hard to read for a human.
async function getDefaultKeyAndIv() {
    let key = await window.crypto.subtle.importKey('raw', new ArrayBuffer(32),
        { name: "AES-GCM" }, false, ['encrypt', 'decrypt']);
    let iv = new Uint8Array(12);
    return [key, iv];
}
async function encryptStringNoKey(plaintext) {
    let encoded = new TextEncoder().encode(plaintext);
    let [key, iv] = await getDefaultKeyAndIv();
    let ciphertext = await window.crypto.subtle.encrypt(
        {
            name: "AES-GCM",
            iv: iv
        },
        key,
        encoded
    );
    return toHexString(ciphertext);
}
async function decryptStringNoKey(ciphertextStr) {
    let [key, iv] = await getDefaultKeyAndIv();
    let ciphertext = fromHexString(ciphertextStr);

    let decrypted = await window.crypto.subtle.decrypt(
        {
            name: "AES-GCM",
            iv: iv
        },
        key,
        ciphertext
    );
    let plaintext = new TextDecoder().decode(decrypted);
    return plaintext;
}
async function encryptString(plaintext) {
    let encoded = new TextEncoder().encode(plaintext);
    let key = await window.crypto.subtle.generateKey(
        {
            name: "AES-GCM",
            length: 256,
        },
        true,
        ["encrypt", "decrypt"]
    );
    let iv = await window.crypto.getRandomValues(new Uint8Array(12));
    let ciphertext = await window.crypto.subtle.encrypt(
        {
            name: "AES-GCM",
            iv: iv
        },
        key,
        encoded
    );
    let keyBuffer = await window.crypto.subtle.exportKey('raw', key);
    return toHexString(keyBuffer) + ','
        + toHexString(iv) + ','
        + toHexString(ciphertext);
}
async function decryptString(str) {
    let [keyStr, ivStr, ciphertextStr] = str.split(',');
    let keyBuffer = fromHexString(keyStr);
    let iv = fromHexString(ivStr);
    let ciphertext = fromHexString(ciphertextStr);

    let key = await window.crypto.subtle.importKey('raw', keyBuffer,
        { name: "AES-GCM" }, false, ['decrypt']);
    let decrypted = await window.crypto.subtle.decrypt(
        {
            name: "AES-GCM",
            iv: iv
        },
        key,
        ciphertext
    );
    let plaintext = new TextDecoder().decode(decrypted);
    return plaintext;
}

async function _scrambleString(str, kind) {
    if (kind[0] == 'B') {
        return 'B' + btoa(await _scrambleString(str, kind.substring(1)));
    } else if (kind[0] == 'E') {
        return 'E' + await encryptString(
                        await _scrambleString(str, kind.substring(1)));
    } else if (kind[0] == 'A') {
        return 'A' + await encryptStringNoKey(
                        await _scrambleString(str, kind.substring(1)));
    } else if(!kind) {
        return str;
    } else {
        throw new Exception("Unknown scramble kind: " + kind);
    }
}
function scrambleString(str, scrambleKinds) {
    let res = null;
    for (let i in scrambleKinds) {
        let kind = scrambleKinds[i];
        if (res === null) {
            res = _scrambleString(str, kind);
        } else {
            res = res.catch(_ => _scrambleString(str, kind));
        }
    }
    return res;
}
async function unscrambleString(str) {
    try {
        if (str[0] == 'B') {
            return await unscrambleString(atob(str.substring(1)));
        } else if (str[0] == 'E') {
            return await unscrambleString(
                    await decryptString(str.substring(1)));
        } else if (str[0] == 'A') {
            return await unscrambleString(
                    await decryptStringNoKey(str.substring(1)));
        } else {
            return str;
        }
    }
    catch(e) {
        return null;
    }
}