Decrypting a GPG string from command line

Question

I'm trying to write a console application that will decrypt a gpg signature on request. Everything's going fine, EXCEPT for the part where it prompts for my GPG password. How do I call gpg --decrypt from the command line without a password dialog?

Here's my code so far:

var startInfo = new ProcessStartInfo("gpg.exe");
startInfo.Arguments = "--decrypt"; //this is where I want to insert "--passphrase MyFakePassword"
startInfo.CreateNoWindow = true;
startInfo.UseShellExecute = false;
startInfo.RedirectStandardInput = true;
startInfo.RedirectStandardOutput = true;
startInfo.RedirectStandardError = true;
startInfo.WorkingDirectory = @"C:\Program Files (x86)\GNU\GnuPG";

var proc = Process.Start(startInfo);
var sCommandLine = stringData + "\n"+(char)26+"\n"; //stringData is the encrypted string
proc.StandardInput.WriteLine(sCommandLine); 
proc.StandardInput.Flush();
proc.StandardInput.Close();

var result = proc.StandardOutput.ReadToEnd();

I've tried using --passphrase MyFakePassword, --passphrase-fd MyFakePassword and even --passphrase-fd 0 with my password on the first line of input. I'd like to avoid putting my password in a txt file on the machine that's running this code, if at all possible.

Thanks in advance for any help.

Solution 1

I did a bit more digging. A few months ago someone reported this as a bug on Gpg4Win's forums. The only solutions at this time are to roll back from 2.1.0 to a previous version (not an option in my case), disable the password for the key, or pipe it in from text. Here's the forum post: http://wald.intevation.org/forum/forum.php?thread_id=1116&forum_id=21&group_id=11 There is no comment from the development team.

Solution 2

Use the --batch --passphrase-fd options together, .eg gpg2 --batch --passphrase-fd 0 --armor --decrypt /path/to/encrypted_file.pgp

In your code, after proc.StandardInput.WriteLine(sCommandLine); add this:

proc.StandardInput.WriteLine("your passphrase here");
proc.StandardInput.Flush();

Solution 3

To avoid the dialog password try this method, I use it and it worked perfectly, you will find more details.

http://www.systemdeveloper.info/2013/11/decrypt-files-encrypted-with-gnupg-from.html

    public static string DecryptFile(string encryptedFilePath)
    {
        FileInfo info = new FileInfo(encryptedFilePath);
        string decryptedFileName = info.FullName.Substring(0, info.FullName.LastIndexOf('.')) + "Dec.TXT";
        string encryptedFileName = info.FullName;

        string password = System.Configuration.ConfigurationManager.AppSettings["passphrase"].ToString();

        System.Diagnostics.ProcessStartInfo psi = new System.Diagnostics.ProcessStartInfo("cmd.exe");

        psi.CreateNoWindow = true;
        psi.UseShellExecute = false;
        psi.RedirectStandardInput = true;
        psi.RedirectStandardOutput = true;
        psi.RedirectStandardError = true;
        psi.WorkingDirectory = @System.Configuration.ConfigurationManager.AppSettings["WorkingDirectory"].ToString();

        System.Diagnostics.Process process = System.Diagnostics.Process.Start(psi);
        string sCommandLine = @"echo " + password + "|gpg.exe --passphrase-fd 0 --batch --verbose --yes --output " + decryptedFileName + @" --decrypt " + encryptedFileName;

        process.StandardInput.WriteLine(sCommandLine);
        process.StandardInput.Flush();
        process.StandardInput.Close();
        process.WaitForExit();
        //string result = process.StandardOutput.ReadToEnd();
        //string error = process.StandardError.ReadToEnd();
        process.Close();
        return decryptedFileName;
    }