index.js

/**
 * Encrypts plaintext using AES-GCM with supplied password, for decryption with aes_gcm_decrypt()
 *
 * @param   {string} plaintext plain text to be encrypted
 * @param   {string} password password to use to encrypt plaintext
 * @returns {Promise<string>} encrypted cipher text
 */
export async function aes_gcm_encrypt(plaintext, password) {
	// encode password as UTF-8
	const pwUtf8 = new TextEncoder().encode(password);
	// hash the password
	const pwHash = await crypto.subtle.digest('SHA-256', pwUtf8);
	// get 96-bit random iv
	const iv = crypto.getRandomValues(new Uint8Array(12));
	// iv as utf-8 string
	const ivStr = Array.from(iv).map(b => String.fromCharCode(b)).join('');
	// specify algorithm to use
	const alg = { name: 'AES-GCM', iv: iv };
	// generate key from pw
	const key = await crypto.subtle.importKey('raw', pwHash, alg, false, ['encrypt']);
	// encode plaintext as UTF-8
	const ptUint8 = new TextEncoder().encode(plaintext);
	// encrypt plaintext using key
	const ctBuffer = await crypto.subtle.encrypt(alg, key, ptUint8);
	// ciphertext as byte array
	const ctArray = Array.from(new Uint8Array(ctBuffer));
	// ciphertext as string
	const ctStr = ctArray.map(byte => String.fromCharCode(byte)).join('');
	// iv+ciphertext base64-encoded
	return btoa(ivStr + ctStr);
}

/**
 * Decrypts ciphertext encrypted with aes_gcm_encrypt() using supplied password
 *
 * @param   {string} ciphertext ciphertext to be decrypted
 * @param   {string} password password to use to decrypt ciphertext
 * @returns {Promise<string>} decrypted plaintext
 */
export async function aes_gcm_decrypt(ciphertext, password) {
	// encode password as UTF-8
	const pwUtf8 = new TextEncoder().encode(password);
	// hash the password
	const pwHash = await crypto.subtle.digest('SHA-256', pwUtf8);
	// decode base64 iv
	const ivStr = atob(ciphertext).slice(0, 12);
	// iv as Uint8Array
	const iv = new Uint8Array(Array.from(ivStr).map(ch => ch.charCodeAt(0)));
	// specify algorithm to use
	const alg = { name: 'AES-GCM', iv: iv };
	// generate key from pw
	const key = await crypto.subtle.importKey('raw', pwHash, alg, false, ['decrypt']);
	// decode base64 ciphertext
	const ctStr = atob(ciphertext).slice(12);
	// ciphertext as Uint8Array
	const ctUint8 = new Uint8Array(Array.from(ctStr).map(ch => ch.charCodeAt(0)));

	try {
		// decrypt ciphertext using key
		const plainBuffer = await crypto.subtle.decrypt(alg, key, ctUint8);
		// return plaintext from ArrayBuffer
		return new TextDecoder().decode(plainBuffer);
	} catch (error) {
		throw new Error('decrypt failed');
	}
}