在ASP.NET中，可以使用AES加密算法来进行表单身份验证。常用的AES模式有CBC（Cipher Block Chaining）和ECB（Electronic Codebook）。

以下是使用CBC模式进行表单身份验证的代码示例：

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

public class AESHelper
{
    private static byte[] key = Encoding.UTF8.GetBytes("YourSecretKey123"); // 16字节长的密钥
    private static byte[] iv = Encoding.UTF8.GetBytes("YourInitializationVector"); // 16字节长的初始化向量

    public static string Encrypt(string plainText)
    {
        using (Aes aes = Aes.Create())
        {
            aes.Key = key;
            aes.IV = iv;
            aes.Mode = CipherMode.CBC;

            ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

            byte[] encryptedBytes = null;

            using (var ms = new System.IO.MemoryStream())
            {
                using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                {
                    using (var sw = new System.IO.StreamWriter(cs))
                    {
                        sw.Write(plainText);
                    }

                    encryptedBytes = ms.ToArray();
                }
            }

            string encryptedText = Convert.ToBase64String(encryptedBytes);

            return encryptedText;
        }
    }

    public static string Decrypt(string encryptedText)
    {
        byte[] encryptedBytes = Convert.FromBase64String(encryptedText);

        using (Aes aes = Aes.Create())
        {
            aes.Key = key;
            aes.IV = iv;
            aes.Mode = CipherMode.CBC;

            ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);

            string decryptedText = null;

            using (var ms = new System.IO.MemoryStream(encryptedBytes))
            {
                using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                {
                    using (var sr = new System.IO.StreamReader(cs))
                    {
                        decryptedText = sr.ReadToEnd();
                    }
                }
            }

            return decryptedText;
        }
    }
}

你可以使用AESHelper.Encrypt()方法来加密表单中的敏感信息，然后使用AESHelper.Decrypt()方法来解密该信息。在调用这些方法之前，你需要将密钥和初始化向量替换为你自己的值。