How to create a password protected file in C#

Question

A bit complementary to, but no way the same as, this question.

How to create a password protected file?

Answer 1

Use the RijndaelManaged class for encryption and Rfc2898DeriveBytes to generate the key (and IV) for the crypto.

Answer 2

encrypt:

private const int SaltSize = 8;

public static void Encrypt( FileInfo targetFile, string password )
{
  var keyGenerator = new Rfc2898DeriveBytes( password, SaltSize );
  var rijndael = Rijndael.Create();

  // BlockSize, KeySize in bit --> divide by 8
  rijndael.IV = keyGenerator.GetBytes( rijndael.BlockSize / 8 );
  rijndael.Key = keyGenerator.GetBytes( rijndael.KeySize / 8 );

  using( var fileStream = targetFile.Create() )
  {
    // write random salt
    fileStream.Write( keyGenerator.Salt, 0, SaltSize );

    using( var cryptoStream = new CryptoStream( fileStream, rijndael.CreateEncryptor(), CryptoStreamMode.Write ) )
    {
      // write data
    }
  }
}

and decrypt:

public static void Decrypt( FileInfo sourceFile, string password )
{
  // read salt
  var fileStream = sourceFile.OpenRead();
  var salt = new byte[SaltSize];
  fileStream.Read( salt, 0, SaltSize );

  // initialize algorithm with salt
  var keyGenerator = new Rfc2898DeriveBytes( password, salt );
  var rijndael = Rijndael.Create();
  rijndael.IV = keyGenerator.GetBytes( rijndael.BlockSize / 8 );
  rijndael.Key = keyGenerator.GetBytes( rijndael.KeySize / 8 );

  // decrypt
  using( var cryptoStream = new CryptoStream( fileStream, rijndael.CreateDecryptor(), CryptoStreamMode.Read ) )
  {
    // read data
  }
}