Encrypt and decrypt a file with aes in c#?

Question

I wanted to know if there is a code that encrypts and decrypts a file using AES in C#?I have seen some code about encrypting and decrypting a text in c# using aes but encrypting and decrypting a file in c# ..there was no full code to understand it well..If somebody can help me please?

Answer 1

In general, you don't want to encrypt a file. That is, you don't want to write a file, then encrypt it. The data is probably in a different sector of the storage device, and can likely be recovered. (Of course, if you're trying to write ransomware, by all means write it poorly). What you want to do instead is encrypt contents before they make it to disk.

What you asked for

public static void EncryptFile(string filePath, byte[] key)
{
    string tempFileName = Path.GetTempFileName();

    using (SymmetricAlgorithm cipher = Aes.Create())
    using (FileStream fileStream = File.OpenRead(filePath))
    using (FileStream tempFile = File.Create(tempFileName))
    {
        cipher.Key = key;
        // aes.IV will be automatically populated with a secure random value
        byte[] iv = cipher.IV;

        // Write a marker header so we can identify how to read this file in the future
        tempFile.WriteByte(69);
        tempFile.WriteByte(74);
        tempFile.WriteByte(66);
        tempFile.WriteByte(65);
        tempFile.WriteByte(69);
        tempFile.WriteByte(83);

        tempFile.Write(iv, 0, iv.Length);

        using (var cryptoStream =
            new CryptoStream(tempFile, cipher.CreateEncryptor(), CryptoStreamMode.Write))
        {
            fileStream.CopyTo(cryptoStream);
        }
    }

    File.Delete(filePath);
    File.Move(tempFileName, filePath);
}

public static void DecryptFile(string filePath, byte[] key)
{
    string tempFileName = Path.GetTempFileName();

    using (SymmetricAlgorithm cipher = Aes.Create())
    using (FileStream fileStream = File.OpenRead(filePath))
    using (FileStream tempFile = File.Create(tempFileName))
    {
        cipher.Key = key;
        byte[] iv = new byte[cipher.BlockSize / 8];
        byte[] headerBytes = new byte[6];
        int remain = headerBytes.Length;

        while (remain != 0)
        {
            int read = fileStream.Read(headerBytes, headerBytes.Length - remain, remain);

            if (read == 0)
            {
                throw new EndOfStreamException();
            }

            remain -= read;
        }

        if (headerBytes[0] != 69 ||
            headerBytes[1] != 74 ||
            headerBytes[2] != 66 ||
            headerBytes[3] != 65 ||
            headerBytes[4] != 69 ||
            headerBytes[5] != 83)
        {
            throw new InvalidOperationException();
        }

        remain = iv.Length;

        while (remain != 0)
        {
            int read = fileStream.Read(iv, iv.Length - remain, remain);

            if (read == 0)
            {
                throw new EndOfStreamException();
            }

            remain -= read;
        }

        cipher.IV = iv;

        using (var cryptoStream =
            new CryptoStream(tempFile, cipher.CreateDecryptor(), CryptoStreamMode.Write))
        {
            fileStream.CopyTo(cryptoStream);
        }
    }

    File.Delete(filePath);
    File.Move(tempFileName, filePath);
}

What you really want

Instead of writing the original file via a FileStream, open the file, write the header and IV, create the CryptoStream, and use the CryptoStream for everything. There's no reason to ever let the unencrypted form be on disk.