A sample C# class to encrypt and decrypt texts 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 some 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.
	 */
	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 serializer = new JavaScriptSerializer();
				var payload = serializer.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));
			}
		}
	}
}

This is an answer to my own question at https://stackoverflow.com/questions/45368840/how-to-encrypt-in-c-sharp-that-is-compatible-with-laravels-encryption/45633123#45633123

Hey, thank you so much for this routine. You can't imagine how much time I spent looking for this.

Just to give something back: Here's a version that works on .NET 6 (it replaces the JavaScriptSerializer by the JsonSerializer from the System.Text.Json)

using System;
using System.Text;
using System.Security.Cryptography;
using System.Collections.Generic;
using System.Text.Json;

namespace Junkyard
{
    /**
	 * A class to encrypt and decrypt strings using the cipher AES-256-CBC.
	 */
    public 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 },
                };

                var serializedKeyValues = JsonSerializer.Serialize(keyValues);

                return Convert.ToBase64String(encoding.GetBytes(serializedKeyValues));
            }
            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
                var payload = JsonSerializer.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));
            }
        }
    }
}

Thank you.

Glad this helped. Thank you too @danarrib.

I used the above code but got an error when decoding it on laravel, or an error about "The MAC is invalid." or the error "unserialize(): Error at offset 0 of 34 bytes". Can someone help me fix it. thanks!