How to decrypt a string in C# which is encrypted via PowerShell

Question

Is it possible to decrypt a string in C# which is encrypted via PowerShell and how?

The string is encrypted via PowerShell as below:

$pw = read-host "Enter Password" –AsSecureString

ConvertFrom-SecureString $pw | out-file "C:\file.txt"

To convert it back with PowerShell I can use these commands that call C# class System.Runtime.InteropServices.Marshal.

$pwdSec = Get-Content "C:\file.txt" | ConvertTo-SecureString

$bPswd = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($pwdSec)

$pswd = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($bPswd)

File contains the string that has been converted to an encrypted standard string("hello").

So if open the file.txt file, it looks similar to:

01000000d08c9ddf0115d1118c7a00c04fc297eb0100000052ded6c2db80e748933432e19b9de8b10000
000002000000000003660000c00000001000000016dc35885d76d07bab289eb9927cfc1e000000000480
0000a0000000100000003106cde553f45b08d13d89d11336170b280000005cc865c1ee1b57e84ed3d1a2
d3f2d0ec0f189b532e61c18d1f31444d6f119a1e8368477fd2d81f54140000000cb0262e58b08ae14f37
22c14c69684841b6b21c

Answer 1

The output file from the ConvertFrom-SecureString you have is a UTF-16 (password) string protected with the ProtectedData.Protect stored as a hex dump.

To revert the encoding use:

// Read file to string
string exportedData = File.ReadAllText(@"file.txt");

// Remove all new-lines
exportedData = exportedData.Replace(Environment.NewLine, "");

// Convert the hex dump to byte array
int length = exportedData.Length / 2;
byte[] encryptedData = new byte[length];
for (int index = 0; index < length; ++index)
{
    var chunk = exportedData.Substring(2 * index, 2);
    encryptedData[index] =
        byte.Parse(chunk, NumberStyles.HexNumber, CultureInfo.InvariantCulture);
}

// Decrypt the byte array to Unicode byte array
byte[] data = ProtectedData.Unprotect(
    encryptedData, (byte[])null, DataProtectionScope.CurrentUser);

// Convert Unicode byte array to string
string password = Encoding.Unicode.GetString(data);

---

The above code works, when you do not specify the -Key with the ConvertFrom-SecureString. The secure string is then protected with Windows Data Protection API (DPAPI). As such the string has to be decoded on the same machine and account, as it was encoded.

Answer 2

I had a requirement to encrypt a string in power shell and decrypt in .Net Please find the following function to encrypt any string. here (1..16) is a byte array.

function EncriptStringData {
[CmdletBinding()]
param (
    [string] $PlainText        
)
$someSecureString = $PlainText | ConvertTo-SecureString -AsPlainText -Force
$encryptedTextThatIcouldSaveToFile =  ConvertFrom-SecureString -key (1..16) -SecureString $someSecureString

return $encryptedTextThatIcouldSaveToFile
}

Now this encrypt string output i have used as a input for my .Net program and get the same PlainText back as an output of my .Net program. Please find the following function.

using System;    
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Runtime.InteropServices;
using System.Security;
using System.Security.Cryptography;

namespace MyNameSpace
{
    public class DecryptStringData
    {
        public string GetDecryptString(string EncriptData)
        {
            try
            {
                byte[] key = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
            byte[] asBytes = Convert.FromBase64String(EncriptData);
            string[] strArray = Encoding.Unicode.GetString(asBytes).Split(new[] { '|' });

            if (strArray.Length != 3) throw new InvalidDataException("input had incorrect format");

            byte[] magicHeader = HexStringToByteArray(EncriptData.Substring(0, 32));
            byte[] rgbIV = Convert.FromBase64String(strArray[1]);
            byte[] cipherBytes = HexStringToByteArray(strArray[2]);

            SecureString str = new SecureString();
            SymmetricAlgorithm algorithm = SymmetricAlgorithm.Create(); //This for .Net 4.5
//Use this for .Net core //  AesManaged algorithm = new AesManaged();
            ICryptoTransform transform = algorithm.CreateDecryptor(key, rgbIV);
            using (var stream = new CryptoStream(new MemoryStream(cipherBytes), transform, CryptoStreamMode.Read))
            {
                int numRed = 0;
                byte[] buffer = new byte[2]; // two bytes per unicode char
                while ((numRed = stream.Read(buffer, 0, buffer.Length)) > 0)
                {
                    str.AppendChar(Encoding.Unicode.GetString(buffer).ToCharArray()[0]);
                }
            }

            string secretvalue = convertToUNSecureString(str);
            return secretvalue;
        }
        catch (Exception ex)
        {
            return ex.Message;
        }

    }


    public static byte[] HexStringToByteArray(String hex)
    {
        int NumberChars = hex.Length;
        byte[] bytes = new byte[NumberChars / 2];
        for (int i = 0; i < NumberChars; i += 2) bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);

        return bytes;
    }

    public static string convertToUNSecureString(SecureString secstrPassword)
    {
        IntPtr unmanagedString = IntPtr.Zero;
        try
        {
            unmanagedString = Marshal.SecureStringToGlobalAllocUnicode(secstrPassword);
            return Marshal.PtrToStringUni(unmanagedString);
        }
        finally
        {
            Marshal.ZeroFreeGlobalAllocUnicode(unmanagedString);
        }
    }

}

}