Uses the SubtleCrypto interface of the Web Cryptography API to encrypt and decrypt text using AES-GCM (AES Galois counter mode).

crypto-aes-gcm.js

/*
Forked from https://gist.github.com/chrisveness/43bcda93af9f646d083fad678071b90a
License: MIT
This version at: https://gist.github.com/sohrabsaran/5f1260a5410ee38b9e88dc186af37f70
Changes: 
1. Fixed regex bug in ctStr.match (fixed later in the original in https://gist.github.com/chrisveness/43bcda93af9f646d083fad678071b90a/revisions#diff-d559f25d942e0fc455a7207c04b00d94)
2. Added polyfill for TextEncoder
*/

/**
 * Encrypts plaintext using AES-GCM with supplied password, for decryption with aesGcmDecrypt().
 *                                                                      (c) Chris Veness MIT Licence
 *
 * @param   {String} plaintext - Plaintext to be encrypted.
 * @param   {String} password - Password to use to encrypt plaintext.
 * @returns {String} Encrypted ciphertext.
 *
 * @example
 *   const ciphertext = await aesGcmEncrypt('my secret text', 'pw');
 *   aesGcmEncrypt('my secret text', 'pw').then(function(ciphertext) { console.log(ciphertext); });
 */
async function aesGcmEncrypt(plaintext, password) {
    const pwUtf8 = new TextEncoder().encode(password);                                 // encode password as UTF-8
    const pwHash = await crypto.subtle.digest('SHA-256', pwUtf8);                      // hash the password

    const iv = crypto.getRandomValues(new Uint8Array(12));                             // get 96-bit random iv

    const alg = { name: 'AES-GCM', iv: iv };                                           // specify algorithm to use

    const key = await crypto.subtle.importKey('raw', pwHash, alg, false, ['encrypt']); // generate key from pw

    const ptUint8 = new TextEncoder().encode(plaintext);                               // encode plaintext as UTF-8
    const ctBuffer = await crypto.subtle.encrypt(alg, key, ptUint8);                   // encrypt plaintext using key
 
    const ctArray = Array.from(new Uint8Array(ctBuffer));                              // ciphertext as byte array
    const ctStr = ctArray.map(byte => String.fromCharCode(byte)).join('');             // ciphertext as string
    const ctBase64 = btoa(ctStr);                                                      // encode ciphertext as base64

    const ivHex = Array.from(iv).map(b => ('00' + b.toString(16)).slice(-2)).join(''); // iv as hex string

    return ivHex+ctBase64;                                                             // return iv+ciphertext
}


/**
 * Decrypts ciphertext encrypted with aesGcmEncrypt() using supplied password.
 *                                                                      (c) Chris Veness MIT Licence
 *
 * @param   {String} ciphertext - Ciphertext to be decrypted.
 * @param   {String} password - Password to use to decrypt ciphertext.
 * @returns {String} Decrypted plaintext.
 *
 * @example
 *   const plaintext = await aesGcmDecrypt(ciphertext, 'pw');
 *   aesGcmDecrypt(ciphertext, 'pw').then(function(plaintext) { console.log(plaintext); });
 */
async function aesGcmDecrypt(ciphertext, password) {
    const pwUtf8 = new TextEncoder().encode(password);                                 // encode password as UTF-8
    const pwHash = await crypto.subtle.digest('SHA-256', pwUtf8);                      // hash the password

    const iv = ciphertext.slice(0,24).match(/.{2}/g).map(byte => parseInt(byte, 16));  // get iv from ciphertext

    const alg = { name: 'AES-GCM', iv: new Uint8Array(iv) };                           // specify algorithm to use

    const key = await crypto.subtle.importKey('raw', pwHash, alg, false, ['decrypt']); // use pw to generate key

    const ctStr = atob(ciphertext.slice(24));                                          // decode base64 ciphertext
    const ctUint8 = new Uint8Array(ctStr.match(/[\s\S]/g).map(ch => ch.charCodeAt(0)));  // ciphertext as Uint8Array
    // note: why doesn't ctUint8 = new TextEncoder().encode(ctStr) work?

    const plainBuffer = await crypto.subtle.decrypt(alg, key, ctUint8);                // decrypt ciphertext using key
    const plaintext = new TextDecoder().decode(plainBuffer);                           // decode password from UTF-8

    return plaintext;                                                                  // return the plaintext
}

//from https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder
if (typeof TextEncoder === "undefined") {
    TextEncoder=function TextEncoder(){};
    TextEncoder.prototype.encode = function encode(str) {
        "use strict";
        var Len = str.length, resPos = -1, resArr = new Uint8Array(Len * 3);
        for (var point=0, nextcode=0, i = 0; i !== Len; ) {
            point = str.charCodeAt(i), i += 1;
            if (point >= 0xD800 && point <= 0xDBFF) {
                if (i === Len) {
                    resArr[resPos += 1] = 0xef/*0b11101111*/; resArr[resPos += 1] = 0xbf/*0b10111111*/;
                    resArr[resPos += 1] = 0xbd/*0b10111101*/; break;
                }
                // https://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
                nextcode = str.charCodeAt(i);
                if (nextcode >= 0xDC00 && nextcode <= 0xDFFF) {
                    point = (point - 0xD800) * 0x400 + nextcode - 0xDC00 + 0x10000;
                    i += 1;
                    if (point > 0xffff) {
                        resArr[resPos += 1] = (0x1e/*0b11110*/<<3) | (point>>>18);
                        resArr[resPos += 1] = (0x2/*0b10*/<<6) | ((point>>>12)&0x3f/*0b00111111*/);
                        resArr[resPos += 1] = (0x2/*0b10*/<<6) | ((point>>>6)&0x3f/*0b00111111*/);
                        resArr[resPos += 1] = (0x2/*0b10*/<<6) | (point&0x3f/*0b00111111*/);
                        continue;
                    }
                } else {
                    resArr[resPos += 1] = 0xef/*0b11101111*/; resArr[resPos += 1] = 0xbf/*0b10111111*/;
                    resArr[resPos += 1] = 0xbd/*0b10111101*/; continue;
                }
            }
            if (point <= 0x007f) {
                resArr[resPos += 1] = (0x0/*0b0*/<<7) | point;
            } else if (point <= 0x07ff) {
                resArr[resPos += 1] = (0x6/*0b110*/<<5) | (point>>>6);
                resArr[resPos += 1] = (0x2/*0b10*/<<6)  | (point&0x3f/*0b00111111*/);
            } else {
                resArr[resPos += 1] = (0xe/*0b1110*/<<4) | (point>>>12);
                resArr[resPos += 1] = (0x2/*0b10*/<<6)    | ((point>>>6)&0x3f/*0b00111111*/);
                resArr[resPos += 1] = (0x2/*0b10*/<<6)    | (point&0x3f/*0b00111111*/);
            }
        }
        resArr = new Uint8Array(resArr.buffer.slice(0, resPos+1));
        return resArr;
    };
    TextEncoder.prototype.toString = function(){return "[object TextEncoder]"};
    if (Object.defineProperty) {
      Object.defineProperty(TextEncoder.prototype,"encoding",{get:function(){if(Object.getPrototypeOf
      (this)!==TextEncoder.prototype)throw TypeError("Illegal invocation");else return"utf-8"}});
    } else {
      TextEncoder.prototype.encoding = "utf-8";
    }
    if(typeof Symbol!=="undefined")TextEncoder.prototype[Symbol.toStringTag]="TextEncoder";
}