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));
      }
    }
  }
}

@doncadavona
I think line 51 and 89 should be Convert.FromBase64String(key) otherwise I don't see how/why one would store bytes[] as string without converting them to Base64.

More specificially using also Aes to generate a key :

Aes aes = Aes.Create();
aes.GenerateIV();
aes.GenerateKey();

var key = Convert.ToBase64String(aes.Key);