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
/**
 * 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
}