Cryptomoji - Encrypt messages as emoji

cryptomoji.js

var emoji = require('node-emoji')

var emojiList = emoji.search('');

function emojiNum(emoji) {
    for (var i = 0; i < emojiList.length; i++) {
        if (emojiList[i].key === emoji) {
            return i;
        }
    }

    return -1;
}

function numEmoji(index) {
    for (var i = 0; i < emojiList.length; i++) {
        if (i === index) {
            return emojiList[i].emoji;
        }
    }

    return -1;
}

function decryptChar(z, p) {
    return (z + p) % 26;
}

function encryptChar(c, p) {
    var lo = -1;
    var hi = -1;
    var n = 0;

    while (true) {
        var z = 26 * n + c - p;

        if (lo < 0 && z > 0) {
            lo = n;
        }

        if (z > emojiList.length) {
            hi = n - 1;
            break;
        }

        n++;
    }

    // One letter can map to multiple emoji, pick one at random
    return 26 * Math.floor(Math.random() * (hi-lo+1) + lo) + c - p;
}

// Split the emoji string into an emoji list
function emojiSplit(key) {
    var unemoji = emoji.unemojify(key);
    var emojis = [];

    var regex = /:([^:]+):/g;
    var match = regex.exec(unemoji);

    while (match != null) {
        emojis.push(match[1]);
        match = regex.exec(unemoji);
    }

    return emojis;
}

function decrypt(cypher, key) {
    var cypherList = emojiSplit(cypher);
    var keyList = emojiSplit(key);
    var output = '';

    for (var i = 0; i < cypherList.length; i++) {
        var cypherVal = emojiNum(cypherList[i]);
        var keyVal = emojiNum(keyList[i % keyList.length]);

        var d = decryptChar(cypherVal, keyVal);

        output += String.fromCharCode(d + 97);
    }

    return output;
}

function encrypt(msg, key) {
    var keyList = emojiSplit(key);
    var cypher = '';

    for (var i = 0; i < msg.length; i++) {
        var plaintextChar = msg[i].charCodeAt(0) - 97;
        var keyEmoji = keyList[i % keyList.length];

        var cyphermojiNum = encryptChar(plaintextChar, emojiNum(keyEmoji));
        var cyphermoji = numEmoji(cyphermojiNum);

        cypher += cyphermoji;
    }

    return cypher;
}

var msg = "test";
var key = "🤔😰";

var e = encrypt(msg, key);
console.log(e);

var d = decrypt(e, key);
console.log(d);