CIpher sample from https://stackoverflow.com/a/10177020 optimized for UWP.

Cipher.cs

using System;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using Windows.Storage;

namespace Elorucov.Laney.Helpers.Security {
    // https://stackoverflow.com/a/10177020 
    // optimized for UWP: using 128 bits instead 256
    // and save entropy (salt/iv) in local settings.
    public static class Cipher {
        public static string EncryptToBase64(this string plaintext, string passPhrase) {
            return Convert.ToBase64String(plaintext.Encrypt(passPhrase));
        }

        public static string DecryptFromBase64(this string cipherText, string passPhrase) {
            return Convert.FromBase64String(cipherText).Decrypt(passPhrase);
        }

        // This constant is used to determine the keysize of the encryption algorithm in bits.
        // We divide this by 8 within the code below to get the equivalent number of bytes.
        private const int Keysize = 128;

        // This constant determines the number of iterations for the password bytes generation function.
        private const int DerivationIterations = 1237;

        public static byte[] Encrypt(this string plainText, string passPhrase) {
            // Salt and IV is randomly generated each time, but is preprended to encrypted cipher text
            // so that the same Salt and IV values can be used when decrypting.  
            var saltStringBytes = Generate128BitsOfRandomEntropy();
            var ivStringBytes = Generate128BitsOfRandomEntropy();
            var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
            using(var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations)) {
                var keyBytes = password.GetBytes(Keysize / 8);
                using(var symmetricKey = new RijndaelManaged()) {
                    symmetricKey.BlockSize = 128;
                    symmetricKey.Mode = CipherMode.CBC;
                    symmetricKey.Padding = PaddingMode.PKCS7;
                    using(var encryptor = symmetricKey.CreateEncryptor(keyBytes, ivStringBytes)) {
                        using(var memoryStream = new MemoryStream()) {
                            using(var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write)) {
                                cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
                                cryptoStream.FlushFinalBlock();
                                // Create the final bytes as a concatenation of the random salt bytes, the random iv bytes and the cipher bytes.
                                var cipherTextBytes = saltStringBytes;
                                cipherTextBytes = cipherTextBytes.Concat(ivStringBytes).ToArray();
                                cipherTextBytes = cipherTextBytes.Concat(memoryStream.ToArray()).ToArray();
                                memoryStream.Close();
                                cryptoStream.Close();
                                return cipherTextBytes;
                            }
                        }
                    }
                }
            }
        }

        public static string Decrypt(this byte[] cipherTextBytesWithSaltAndIv, string passPhrase) {
            // Get the saltbytes by extracting the first 16 bytes from the supplied cipherText bytes.
            var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(Keysize / 8).ToArray();
            // Get the IV bytes by extracting the next 16 bytes from the supplied cipherText bytes.
            var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(Keysize / 8).Take(Keysize / 8).ToArray();
            // Get the actual cipher text bytes by removing the first 32 bytes from the cipherText string.
            var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((Keysize / 8) * 2).Take(cipherTextBytesWithSaltAndIv.Length - ((Keysize / 8) * 2)).ToArray();

            using(var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations)) {
                var keyBytes = password.GetBytes(Keysize / 8);
                using(var symmetricKey = new RijndaelManaged()) {
                    symmetricKey.BlockSize = 128;
                    symmetricKey.Mode = CipherMode.CBC;
                    symmetricKey.Padding = PaddingMode.PKCS7;
                    using(var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes)) {
                        using(var memoryStream = new MemoryStream(cipherTextBytes)) {
                            using(var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read)) {
                                var plainTextBytes = new byte[cipherTextBytes.Length];
                                var decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
                                memoryStream.Close();
                                cryptoStream.Close();
                                return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
                            }
                        }
                    }
                }
            }
        }

        private static byte[] Generate128BitsOfRandomEntropy() {
            var randomBytes = new byte[16]; // 16 Bytes will give us 128 bits.
            object val = ApplicationData.Current.LocalSettings.Values["iv"];

            if(val != null && val is string s) {
                byte[] b = Convert.FromBase64String(s);
                return b;
            } else {
                using(var rngCsp = new RNGCryptoServiceProvider()) {
                    // Fill the array with cryptographically secure random bytes.
                    rngCsp.GetBytes(randomBytes);
                    ApplicationData.Current.LocalSettings.Values["iv"] = Convert.ToBase64String(randomBytes);
                }
            }
            return randomBytes;
        }
    }
}