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

public void SetCookie(string name, string value, string key)
{
    var data = Encoding.UTF8.GetBytes(value);
    var hmac = GenerateHMAC(data, key);
    var cookie = new HttpCookie(name, Convert.ToBase64String(data) + "|" + hmac);
    HttpContext.Current.Response.Cookies.Add(cookie);
}

public string GetCookie(string name, string key)
{
    var cookie = HttpContext.Current.Request.Cookies[name];
    if (cookie != null)
    {
        var parts = cookie.Value.Split('|');
        if (parts.Length == 2)
        {
            var data = Convert.FromBase64String(parts[0]);
            var hmac = GenerateHMAC(data, key);
            if (hmac == parts[1])
            {
                return Encoding.UTF8.GetString(data);
            }
        }
    }
    return null;
}

private string GenerateHMAC(byte[] data, string key)
{
    var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(key));
    var hash = hmac.ComputeHash(data);
    return Convert.ToBase64String(hash);
}