在ASP.NET中加密文件有多种方式，以下是一种解决方法的代码示例：

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

public class FileEncryption
{
    public static void EncryptFile(string inputFile, string outputFile, string password)
    {
        byte[] salt = GenerateSalt();
        byte[] passwordBytes = Encoding.UTF8.GetBytes(password);

        using (FileStream fsInput = new FileStream(inputFile, FileMode.Open))
        using (FileStream fsOutput = new FileStream(outputFile, FileMode.Create))
        {
            using (RijndaelManaged aes = new RijndaelManaged())
            {
                aes.KeySize = 256;
                aes.BlockSize = 128;
                aes.Padding = PaddingMode.PKCS7;

                byte[] key = new Rfc2898DeriveBytes(passwordBytes, salt, 1000).GetBytes(aes.KeySize / 8);
                aes.Key = key;
                aes.IV = new Rfc2898DeriveBytes(passwordBytes, salt).GetBytes(aes.BlockSize / 8);

                fsOutput.Write(salt, 0, salt.Length);

                using (CryptoStream cs = new CryptoStream(fsOutput, aes.CreateEncryptor(), CryptoStreamMode.Write))
                {
                    byte[] buffer = new byte[4096];
                    int bytesRead;

                    while ((bytesRead = fsInput.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        cs.Write(buffer, 0, bytesRead);
                    }
                }
            }
        }
    }

    public static void DecryptFile(string inputFile, string outputFile, string password)
    {
        byte[] passwordBytes = Encoding.UTF8.GetBytes(password);
        byte[] salt = new byte[32];

        using (FileStream fsInput = new FileStream(inputFile, FileMode.Open))
        using (FileStream fsOutput = new FileStream(outputFile, FileMode.Create))
        {
            fsInput.Read(salt, 0, salt.Length);

            using (RijndaelManaged aes = new RijndaelManaged())
            {
                aes.KeySize = 256;
                aes.BlockSize = 128;
                aes.Padding = PaddingMode.PKCS7;

                byte[] key = new Rfc2898DeriveBytes(passwordBytes, salt, 1000).GetBytes(aes.KeySize / 8);
                aes.Key = key;
                aes.IV = new Rfc2898DeriveBytes(passwordBytes, salt).GetBytes(aes.BlockSize / 8);

                using (CryptoStream cs = new CryptoStream(fsOutput, aes.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    byte[] buffer = new byte[4096];
                    int bytesRead;

                    while ((bytesRead = fsInput.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        cs.Write(buffer, 0, bytesRead);
                    }
                }
            }
        }
    }

    private static byte[] GenerateSalt()
    {
        byte[] salt = new byte[32];

        using (RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider())
        {
            rngCsp.GetBytes(salt);
        }

        return salt;
    }
}

使用示例：

string inputFile = "path/to/input/file";
string encryptedFile = "path/to/encrypted/file";
string decryptedFile = "path/to/decrypted/file";
string password = "your-password";

FileEncryption.EncryptFile(inputFile, encryptedFile, password);
FileEncryption.DecryptFile(encryptedFile, decryptedFile, password);

请注意，此示例使用Rijndael算法进行文件加密和解密，并采用PBKDF2（基于RFC 2898）来生成密钥和初始化向量。