File encryption and decryption with random salt.

Encrypter.cs

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

namespace Dalstroem
{
    public class Encrypter
    {
        private const int SaltLength = 32;
        private const int Iterations = 1000;

        [DllImport("KERNEL32.DLL", EntryPoint = "RtlZeroMemory")]
        public static extern bool ZeroMemory(IntPtr destination, int length);

        private byte[] GenerateRandomSalt()
        {
            var data = new byte[SaltLength];

            using (var rng = new RNGCryptoServiceProvider())
            {
                for (var i = 0; i < 10; i++)
                {
                    rng.GetBytes(data);
                }
            }

            return data;
        }

        private RijndaelManaged CreateEncryptor() => new RijndaelManaged
        {
            KeySize = 256,
            BlockSize = 128,
            Padding = PaddingMode.PKCS7,
            Mode = CipherMode.CFB
        };

        public byte[] Encrypt(byte[] plainBytes, string passphrase)
        {
            var passphraseBytes = Encoding.UTF8.GetBytes(passphrase);
            var salt = GenerateRandomSalt();

            using (var encryptor = CreateEncryptor())
            {
                using (var key = new Rfc2898DeriveBytes(passphraseBytes, salt, Iterations))
                {
                    encryptor.Key = key.GetBytes(encryptor.KeySize / 8);
                    encryptor.IV = key.GetBytes(encryptor.BlockSize / 8);
                }

                using (var encryptedStream = new MemoryStream())
                {
                    encryptedStream.Write(salt, 0, salt.Length);

                    using (var cryptoStream = new CryptoStream(encryptedStream, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
                    {
                        cryptoStream.Write(plainBytes, 0, plainBytes.Length);
                        cryptoStream.FlushFinalBlock();
                    }

                    return encryptedStream.ToArray();
                }
            }
        }

        public byte[] Decrypt(byte[] encryptedBytes, string passphrase)
        {
            var passphraseBytes = Encoding.UTF8.GetBytes(passphrase);
            var salt = new byte[SaltLength];

            for (var i = 0; i < SaltLength; i++)
            {
                salt[i] = encryptedBytes[i];
            }

            using (var encryptor = CreateEncryptor())
            {
                using (var key = new Rfc2898DeriveBytes(passphraseBytes, salt, Iterations))
                {
                    encryptor.Key = key.GetBytes(encryptor.KeySize / 8);
                    encryptor.IV = key.GetBytes(encryptor.BlockSize / 8);
                }

                using (var decryptedStream = new MemoryStream())
                {
                    using (var cryptoStream = new CryptoStream(decryptedStream, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                    {
                        var length = encryptedBytes.Length - SaltLength;
                        cryptoStream.Write(encryptedBytes, SaltLength, length);
                        cryptoStream.FlushFinalBlock();
                    }

                    return decryptedStream.ToArray();
                }
            }
        }
    }
}

Original source: https://ourcodeworld.com/articles/read/471/how-to-encrypt-and-decrypt-files-using-the-aes-encryption-algorithm-in-c-sharp