.NET Symmetric File Encryption Object

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Abstract

I would have paid a month's worth of salary to get a library as extensive as the .NET Framework class library. The reason why I say that is because using the .NET Framework library, I can easily shave off weeks in development time on a single project. But, as with any other library you still need to develop your own objects to abstract the complexity of the underlying framework. The .NET Framework is no exception.

The CCrypto object I am demonstrating shows how to use the .NET Framework Cryptographic Service classes to encrypt and decrypt files. This article documents the object, and includes code samples on implementing it.

With anything new there is a learning curve. For me, the .NET Framework and Managed Extensions for C++ does not change that rule. With encryption near and dear to my heart I decided to begin developing crypto objects that hide the complexity of the framework, this article shows how easy it is done.

This article is the first in a three part series on symmetric encryption with the .NET Framework using Managed Extensions for C++. I'll follow this article with a command line utility implementing the object, and an article covering an overview of symmetric encryption and the classes in the .NET Cryptographic Services.

You will need the Visual C++.NET to compile and test the code. I have assumed you are familiar with Managed Extensions for C++.

The CCrypto Object

The purpose of CCrypto is to abstract all the .NET Framework classes to encrypt, decrypt, wipe, and fingerprint (hash) files.

To use the CCrypto object you need to include the definition header for the object and reference the namespace. The CCrypto object consumes any derived encryption algorithm of the SymmetricAlgorithm class. When encrypting or decrypting, you also need to consume a HashAlgorithm derived class for hashing.

The selection of the hashing function must have a similar key space as the encryption function. So, if you are using RC2 which produces uses a 128 bit key, use the MD5CryptoServiceProvider() object which has a 128 bit key space. Mix and matching the key spaces with throw an exception.

The CCrypto object does not directly use the passphrase or key file as the encryption key. Rather, it creates a hash of the passphrase or key file and uses the hash as the key.

The object's definition and implementation code is listed after the code snippets demonstrating the use of the object.

Issues

I am not aware of any issues with the object. I was not able to find any test vectors to test Microsoft's implementation of the symmetric encryption algorithms. I have also not tested the quality of the cryptographic random number generator. I would have liked it if the encryption / decryption worked faster. With my P4 1.6Ghz, ATA100 drive, XP Pro, I achieved 7.6 Mb/s encryption on a 20 Mb file using the RC2 algorithm.

Constructors

CCrypto (SymmetricAlgorithm * CryptoServiceProvider, HashAlgorithm * HashServiceProvider);

CCrypto (HashAlgorithm * HashServiceProvider);

CCrypto ()

Public Methods

Encrypt	Encrypts a file
Decrypt	Decrypts a file
WipeFile	Securely removes the file
HashFile	Gets the file's fingerprint
CreateKeyFile	Creates a file with random data

Properties

get_HashProvider	Sets the hash provider
set_SetKeyPhrase	Sets the Key phrase for encryption/decryption
set_SetFileKey	Sets the Key file for encryption/decryption

Protected Methods

Crypt	Performs the encryption/decryption
HandleException	Exception handler

Implementations:

bool Encrypt (String * inFilePath, String * OutfilePath)
bool Decrypt (String * inFilePath, String * OutfilePath)
bool WipeFile (String * inFilePath, int nWipes )
bool CreateKeyFile (String * outfilepath, unsigned int nBytes)
Byte HashFile (String * inFile )[]
bool Crypt (int Direction, String * inFilePath, String * OutfilePath);
virtual void HandleException (Exception * e)
__property void set_SetKeyPhrase (String * Phrase)
__property HashAlgorithm * get_HashProvider ()
__property void  set_HashProvider (HashAlgorithm * myHashServiceProvider)
__property void  set_SetFileKey (String * FileKeyPath)

Code Examples

I have referenced the namespaces of System::Security::Cryptography and MyCrypto so that the samples are more readable. To use the object, you need to include the Crypto.h header. Below is an example of the include and namespace references.

#using <system.dll>
#include "Crypto.h"

using namespace System::Security::Cryptography ;
using namespace MyCrypto ;

This code snippet to retrieve a file's fingerprint and displays that to the console.

CCrypto * myCrypto = new CCrypto (new MD5CryptoServiceProvider());	
Console::WriteLine(  BitConverter::ToString (myCrypto->HashFile ("c:\\encnotes2.txt")) );

This code example creates a file with random data to be used as a key. The random data comes from the random number generator that is part of the .NET Cryptographic Services. This is the preferred way to encrypt the data. The key file can reside on disk, smart card, or USB key.

CCrypto * myCrypto = new CCrypto ();
// Create key file of c:\mykey.key with 512 bytes of random data

myCrypto->CreateKeyFile ("c:\\mykey.key",512);

This example demonstrates how to wipe a file that is no longer needed. Before deleting the file from the device the file will be over written with ten patterns.

CCrypto * myCrypto = new CCrypto ();
// Wipe file with 10 patterns before deleting

myCrypto->WipeFile ("c:\\PersonalNotes.txt",10);

Encrypting a file using the 256 bit Rijndael algorithm and a file key.

CCrypto * myCrypto = new CCrypto (new RijndaelManaged(),new SHA256Managed());
myCrypto->CreateKeyFile ("c:\\mykey.key",512);
myCrypto->SetFileKey = "c:\\mykey.key";
myCrypto->Encrypt ("c:\\PersonalNotes.txt","c:\\PersonalNotes.txt.enc");

To decrypt the file we need to use the same algorithm and key.

CCrypto * myCrypto = new CCrypto (new RijndaelManaged(),new SHA256Managed());
myCrypto->SetFileKey = "c:\\mykey.key";
myCrypto->Decrypt ("c:\\PersonalNotes.txt.enc","c:\\PersonalNotes.txt");

Here is an example of encrypting a file using a phrase.

CCrypto * myCrypto = new CCrypto (new RijndaelManaged(),new SHA256Managed());
myCrypto->SetKeyPhrase = S"You can place any valid string here.";
myCrypto->Encrypt ("c:\\PersonalNotes.txt","c:\\PersonalNotes.txt.enc");

I am interested in hearing what you have to say regarding the CCrypto object or this article. If you have any questions, comments, or criticisms, don't hesitate to let me know.