Gnu Privacy Guard (GPG/PGP) for .NET [v1.0]

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Introduction

This article presents GnuPGWrapper v1.0, a wrapper class for GnuPG.

GnuPG stands for GNU Privacy Guard and is GNU's tool for secure communication and data storage. It can be used to encrypt data and to create digital signatures. It includes an advanced key management facility and is compliant with the proposed OpenPGP Internet standard as described in RFC 2440. As such, GnuPG is a complete and free replacement for PGP (Pretty Good Privacy).

This article provides a C# wrapper class (GnuPGWrapper) that will enable use of OpenPGP Internet encryption standard within a .NET world. It is shipped with a demo ASP.NET Web Form (GnuPG.aspx) which calls the wrapper class.

Installation

Prerequisites

• Complete .NET Environment, e.g. Windows XP Professional + IIS 5.0 + .NET Framework SDK
• GnuPG for Windows (more about GnuPG)

Procedure

• Download the zip file (GnuPGDotNet_src.zip)
• Extract the zip file (e.g. in directory C:\Inetpub\wwwroot\)
• Using Administrative Tools / Internet Information Services, create an IIS Application for directory GnuPGDotNet
• Call demo Web Form via URL http://localhost/GnuPGDotNet/GnuPG.aspx

Background

GnuPG ships as a command line program (gpg.exe) acting as a filter (reads from standard input and writes into standard output). Although suitable for scripting on UNIX systems (where calling a command line program from sh or bash is easy), it's pretty hard to integrate this in a production .NET environment.

The GnuPG Wrapper executes the command line program (gpg.exe) in a different process, redirects standard input (stdin), standard output (stdout) and standard error (stderr) streams, and monitors the streams to fetch the results of the encryption/signing operation.

Please note that you must have INSTALLED GnuPG AND generated/imported the appropriate keys before using this class. Refer to the GnuPG manual to do this...

Using the code

In order to use the wrapper class, you need to proceed as follows:

1. Create an instance of the class
2. Set the command property to the requested command (SignAndEncrypt, Encrypt, Decrypt, Sign, Verify)
3. Optionally, set parameters for the command (home directory, originator, recipients, etc...)
4. Call the ExecuteCommand method with input/output strings variables

The next sections show sample source code for the most command operation (SignAndEncrypt, Decrypt, Verify).

Encrypt and Sign

// Reference My GnuPG wrapping class
using Emmanuel.Cryptography.GnuPG;

// Create GnuPG wrapping class
GnuPGWrapper gpg = new GnuPGWrapper();

// Set command
gpg.command = Commands.SignAndEncrypt;

// Set some parameters from on Web.Config file
gpg.homedirectory = Server.MapPath
       (ConfigurationSettings.AppSettings["homedirectory"]);
gpg.passphrase = ConfigurationSettings.AppSettings["passphrase"];

// Set other parameters from Web Controls
gpg.originator = FromTextBox.Text;
gpg.recipient = ToTextBox.Text;

// Declare input/output variables
// (input is also read from a Web control)
string inputText = MessageTextBox.Text;
string outputText = "";

// Execute GnuPG
gpg.ExecuteCommand(inputText, out outputText);

// Display output text
OutputTextBox.Text = outputText;
OutputTextBox.Visible = true;
ErrorMessage.Visible = false;
ExitCodeLabel.Text = gpg.exitcode.ToString();

Decrypt

using Emmanuel.Cryptography.GnuPG;

GnuPGWrapper gpg = new GnuPGWrapper();

gpg.homedirectory = "C:\Inetpub\wwwroot\GnuPGDotNet\GnuPG"
gpg.passphrase = "My passphrase is so cool I can't remember it"
gpg.command = Commands.Decrypt;

// Execute GnuPG
string outputText = "";
gpg.ExecuteCommand("This is a test message.", out outputText);

// Display output text
[...]

Verify

using Emmanuel.Cryptography.GnuPG;
GnuPGWrapper gpg = new GnuPGWrapper();

gpg.homedirectory = "C:\Inetpub\wwwroot\GnuPGDotNet\GnuPG"
gpg.passphrase = "My passphrase is so cool I can't remember it"
gpg.originator = "me@mycompany.com";
gpg.command = Commands.Verify;

// Execute GnuPG
string outputText = "";
gpg.ExecuteCommand("This is a test message.", out outputText);

// Display output text
[...]

Error handling

Error handling is done via a specific Exception class; method ExecuteCommand raises this exception whenever an error occurs. You calling application can handle this exception as follows:

using Emmanuel.Cryptography.GnuPG;

    try 
    {

        GnuPGWrapper gpg = new GnuPGWrapper();

        gpg.homedirectory = "C:\Inetpub\wwwroot\GnuPGDotNet\GnuPG"
        gpg.passphrase = "My passphrase is so cool I can't remember it"
        gpg.originator = "me@mycompany.com";
        gpg.recipient = "you@yourcompany.com";
        gpg.command = Commands.SignAndEncrypt;

        // Execute GnuPG
        string outputText = "";
        gpg.ExecuteCommand("This is a test message.", out outputText);

        // Display output text
        [...]

    }
    catch (GnuPGException gpge)
    {
        // Display error message
        // Contains a clear text error message, 
        // either from the wrapper or from gpg.exe itself
        ErrorMessage.Text = gpge.Message; 
    }

Points of interest

The GnuPG wrapper:

• Doesn't use any temporary files to store results; it directly uses streams/pipes.
• Uses multiple threads to read data from standard input and standard error, preventing any deadlocks.
• Uses configurable timeouts to prevent blocking calling applications in case of a system/program/process crash
• Uses a configurable passphrase, which can be stored in a local configuration file (Web.Config) to prevent disclosure of the phrase

About GnuPG and PGP

This class has been developed and tested with GnuPG v1.2.0 (MingW32).

You can check the command line manual page for gpg.exe.

For more about GNU, please refer to http://www.gnu.org/. For more about GnuPG, please refer to http://www.gnupg.org/. For more about OpenPGP (RFC 2440), please refer to http://www.gnupg.org/rfc2440.html. For more about PGP, please refer to http://www.pgpi.org/.

History

• Date posted: October 30th, 2002
• Updated: September 19th, 2003