Aes256Cipher.cs

using System;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace Gaev.Blog.SecuredAppSettingsJson
{
    /// <summary>
    /// A tiny wrapper around built-in AES-256 algorithm.
    /// For better security it prefixes randomly generated IV to a cipher text.
    /// Yes, it is secure enough https://security.stackexchange.com/a/85778/207381
    /// </summary>
    public class Aes256Cipher
    {
        private readonly byte[] _key;

        public Aes256Cipher(string key)
        {
            if (string.IsNullOrWhiteSpace(key)) throw new NullReferenceException("The key is empty");
            _key = Convert.FromBase64String(key);
        }

        public string Decrypt(string value)
        {
            var ivAndCipherText = Convert.FromBase64String(value);
            using var aes = Aes.Create();
            var ivLength = aes.BlockSize / 8;
            aes.IV = ivAndCipherText.Take(ivLength).ToArray();
            aes.Key = _key;
            using var cipher = aes.CreateDecryptor();
            var cipherText = ivAndCipherText.Skip(ivLength).ToArray();
            var text = cipher.TransformFinalBlock(cipherText, 0, cipherText.Length);
            return Encoding.UTF8.GetString(text);
        }

        public string Encrypt(string value)
        {
            using var aes = Aes.Create();
            aes.Key = _key;
            aes.GenerateIV();
            using var cipher = aes.CreateEncryptor();
            var text = Encoding.UTF8.GetBytes(value);
            var cipherText = cipher.TransformFinalBlock(text, 0, text.Length);
            return Convert.ToBase64String(aes.IV.Concat(cipherText).ToArray());
        }

        public static string GenerateNewKey()
        {
            using var aes = Aes.Create();
            aes.GenerateKey();
            return Convert.ToBase64String(aes.Key);
        }
    }
}