Answer by mgeek for Swift - Encrypt and decrypt a string using a users password

One of the most popular libraries out there is CryptoSwift (more than 4000 stars!). It provides a lot of crypto related functionality that might interest you.

For your question, I am assuming you want to do AES based encryption, and the message is small (but further down, there is a example for larger files etc.). Using the library mentioned above, the code (taken from the library's page itself--please scroll down):

do {
    let aes = try AES(key: "passwordpassword", iv: "drowssapdrowssap") 
    let ciphertext = try aes.encrypt(Array("Nullam quis risus eget urna mollis ornare vel eu leo.".utf8))
} catch { }

One of the questions asked is how to get IV (Initialization Vector). In this example, the IV is simply the reverse of the password, which may not be a good idea in real world applications. The going principle is that the IV doesn't need to be secret. It just needs to be random, and if possible, unique. So, you can save the IV with the file, in a database with the cipher text.