Encryption & Decryption MD5 C#

Decrypt.cs

public string Decrypt(string ciphertext)
{
    try
    {
        byte[] data = Convert.FromBase64String(ciphertext);
        using var aes = Aes.Create();
        aes.Key = _key;
        aes.Mode = CipherMode.CBC;
        aes.Padding = PaddingMode.PKCS7;

        // Extract IV from the first 16 bytes
        byte[] iv = data.Take(16).ToArray();
        aes.IV = iv;

        using var ms = new MemoryStream();
        using (var cs = new CryptoStream(ms, aes.CreateDecryptor(), CryptoStreamMode.Write))
        {
            cs.Write(data, 16, data.Length - 16); // Skip IV
        }
        return Encoding.UTF8.GetString(ms.ToArray());
    }
    catch (Exception ex)
    {
        throw new CryptographicException("Decryption failed.", ex);
    }
}

Encrypt.cs

public string Encrypt(string plaintext)
{
    try
    {
        using var aes = Aes.Create();
        aes.Key = _key;
        aes.GenerateIV(); // Random IV for each encryption
        aes.Mode = CipherMode.CBC;
        aes.Padding = PaddingMode.PKCS7;

        using var ms = new MemoryStream();
        ms.Write(aes.IV, 0, aes.IV.Length); // Prepend IV to ciphertext
        using (var cs = new CryptoStream(ms, aes.CreateEncryptor(), CryptoStreamMode.Write))
        {
            byte[] data = Encoding.UTF8.GetBytes(plaintext);
            cs.Write(data, 0, data.Length);
        }
        return Convert.ToBase64String(ms.ToArray());
    }
    catch (Exception ex)
    {
        throw new CryptographicException("Encryption failed.", ex);
    }
}

byte[] keys = md5.ComputeHash(UTF8Encoding.UTF8.GetBytes(hash));

whats the value of hash

byte[] keys = md5.ComputeHash(UTF8Encoding.UTF8.GetBytes(hash));

whats the value of hash

static string hash { get; set; } = "A!9HHhi%XjjYY4YP2@Nob009X";

in file Encrypt.cs
line 2: use textBox4.Text instead of textBox3.Text

public static string Encrypt(string value)
{
var md5 = new MD5CryptoServiceProvider();
var hash = md5.ComputeHash(Encoding.UTF8.GetBytes(value));

        //converting the contents of the hash variable from byte[] to string
        var result = string.Empty;
        foreach (var item in hash)
        {
            result += item.ToString("X2"); //X2 -> HEXADECIMAL
        }
        return result.ToUpper();
    }