A sample C# class to encrypt and decrypt strings using the cipher AES-256-CBC used in Laravel.

Aes256CbcEncrypterApp.cs

using System;
using System.Text;
using System.Security.Cryptography;
using System.Web.Script.Serialization;
using System.Collections.Generic;

namespace Aes256CbcEncrypterApp
{
    class MainClass
    {
		public static void Main(string[] args)
		{
			Console.WriteLine("Hello, world!");

            // The sample encryption key. Must be 32 characters.
			string Key = "8UHjPgXZzXCGkhxV2QCnooyJexUzvJrO";

            // The sample text to encrypt and decrypt.
			string Text = "Here is a text to encrypt!";

			// Encrypt and decrypt the sample text via the Aes256CbcEncrypter class.
			string Encrypted = Aes256CbcEncrypter.Encrypt(Text, Key);
			string Decrypted = Aes256CbcEncrypter.Decrypt(Encrypted, Key);

            // Show the encrypted and decrypted data and the key used.
			Console.WriteLine("Original: {0}", Text);
			Console.WriteLine("Key: {0}", Key);
			Console.WriteLine("Encrypted: {0}", Encrypted);
			Console.WriteLine("Decrypted: {0}", Decrypted);
		}
    }

    /**
     * A class to encrypt and decrypt strings using
     * the cipher AES-256-CBC used in Laravel.
     */
    class Aes256CbcEncrypter
    {
        private static readonly Encoding encoding = Encoding.UTF8;

		public static string Encrypt(string plainText, string key)
		{
			try
			{
				RijndaelManaged aes = new RijndaelManaged();
				aes.KeySize = 256;
				aes.BlockSize = 128;
				aes.Padding = PaddingMode.PKCS7;
				aes.Mode = CipherMode.CBC;

				aes.Key = encoding.GetBytes(key);
				aes.GenerateIV();

				ICryptoTransform AESEncrypt = aes.CreateEncryptor(aes.Key, aes.IV);
				byte[] buffer = encoding.GetBytes(plainText);

				string encryptedText = Convert.ToBase64String(AESEncrypt.TransformFinalBlock(buffer, 0, buffer.Length));

				String mac = "";

				mac = BitConverter.ToString(HmacSHA256(Convert.ToBase64String(aes.IV) + encryptedText, key)).Replace("-", "").ToLower();

				var keyValues = new Dictionary<string, object>
				{
					{ "iv", Convert.ToBase64String(aes.IV) },
					{ "value", encryptedText },
					{ "mac", mac },
				};

				JavaScriptSerializer serializer = new JavaScriptSerializer();

				return Convert.ToBase64String(encoding.GetBytes(serializer.Serialize(keyValues)));
			}
			catch (Exception e)
			{
				throw new Exception("Error encrypting: " + e.Message);
			}
		}

		public static string Decrypt(string plainText, string key)
		{
			try
			{
				RijndaelManaged aes = new RijndaelManaged();
				aes.KeySize = 256;
				aes.BlockSize = 128;
				aes.Padding = PaddingMode.PKCS7;
				aes.Mode = CipherMode.CBC;
				aes.Key = encoding.GetBytes(key);

				// Base 64 decode
				byte[] base64Decoded = Convert.FromBase64String(plainText);
				string base64DecodedStr = encoding.GetString(base64Decoded);

				// JSON Decode base64Str
				JavaScriptSerializer ser = new JavaScriptSerializer();
				var payload = ser.Deserialize<Dictionary<string, string>>(base64DecodedStr);

				aes.IV = Convert.FromBase64String(payload["iv"]);

				ICryptoTransform AESDecrypt = aes.CreateDecryptor(aes.Key, aes.IV);
				byte[] buffer = Convert.FromBase64String(payload["value"]);

				return encoding.GetString(AESDecrypt.TransformFinalBlock(buffer, 0, buffer.Length));
			}
			catch (Exception e)
			{
                throw new Exception("Error decrypting: " + e.Message);
			}
		}

		static byte[] HmacSHA256(String data, String key)
		{
			using (HMACSHA256 hmac = new HMACSHA256(encoding.GetBytes(key)))
			{
				return hmac.ComputeHash(encoding.GetBytes(data));
			}
		}
    }
}