How to encrypt and decrypt a file by using Visual C#

Article translations Article translations
Article ID: 307010 - View products that this article applies to.
This article was previously published under Q307010
For a Microsoft Visual Basic .NET version of this article, see 301070.
This article refers to the following Microsoft .NET Framework Class Library namespaces:
  • System.IO
  • System.Security
  • System.Security.Cryptography
Note This article does not apply to the Microsoft .NET Framework 2.0.
Expand all | Collapse all

On This Page

Summary

This article describes how to use the cryptography classes that are provided by the Microsoft .NET Framework to encrypt a text file to an unreadable state, and then to decrypt that text file back to its original format.

Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that you must have:
  • Microsoft Windows 2000 Professional, Windows 2000 Server, Windows 2000 Advanced Server, Windows NT 4.0 Server or Microsoft Windows XP Professional
  • Microsoft Visual Studio 2005 or Microsoft Visual Studio .NET

Encryption and decryption

The System.Security.Cryptographic namespace in the Microsoft .NET Framework provides a variety of tools to help you with encryption and with decryption. The CryptoStream class is one of the many classes that is provided. The CryptoStream class is designed to encrypt or to decrypt content as it is streamed out to a file.

Encrypt a file

To encrypt a file, follow these steps:
  1. Start Visual Studio 2005 or Visual Studio .NET.
  2. Click Visual C# under Projects, and then click Console Application under Templates. Visual C# .NET creates a Static class for you, together with an empty Main() procedure.
  3. Use the using statement (as indicated in the sample code that follows) on the following namespaces:
    • System
    • System.Security
    • System.Security.Cryptography
    • System.Text
    • System.IO
    so that you do not have to qualify declarations from these namespaces later in your code. You must use these statements before any other declarations.
    using System;
    using System.IO;
    using System.Security;
    using System.Security.Cryptography;
    using System.Runtime.InteropServices;
    using System.Text;
    					
  4. Generate a secret key to encrypt and to decrypt the data. The DESCryptoServiceProvider is based on a symmetric encryption algorithm. The symmetric encryption requires a key and an initialization vector (IV) to encrypt the data. To decrypt the data, you must have the same key and the same IV. You must also use the same encryption algorithm. You can generate the keys by using either of the following methods:
    • Method 1 You can prompt the user for a password. Then, use the password as the key and the IV.
    • Method 2 When you create a new instance of the symmetric cryptographic classes, a new key and IV are automatically created for the session. Use the key and IV that are generated by the managed symmetric cryptographic classes to encrypt and to decrypt the file.

      For more information about how to generate and distribute keys, see the Microsoft .NET Framework SDK Documentation, or see the following Microsoft Developer Network (MSDN) Web site:
      Generating keys for encryption and decryption
  5. Add the following function to generate a new key for a session (as noted in Method 2 of step 4):
    //  Call this function to remove the key from memory after use for security.
    [System.Runtime.InteropServices.DllImport("KERNEL32.DLL", EntryPoint="RtlZeroMemory")]
    public static extern bool ZeroMemory(ref string Destination, int Length);
    		
    // Function to Generate a 64 bits Key.
    static string GenerateKey() 
    {
    	// Create an instance of Symetric Algorithm. Key and IV is generated automatically.
    	DESCryptoServiceProvider desCrypto =(DESCryptoServiceProvider)DESCryptoServiceProvider.Create();
    
    	// Use the Automatically generated key for Encryption. 
    	return ASCIIEncoding.ASCII.GetString(desCrypto.Key);
    }
  6. Create a method in your class that is named EncryptFile. The EncryptFile class must have the following three parameters:
    • sInputFilename
    • sOutputFilename
    • sKey (The secret key that is used to encrypt and decrypt the file.)
    static void EncryptFile(string sInputFilename,
    		string sOutputFilename,
    		string sKey)
    					
  7. In the EncryptFile procedure, create an input FileStream object and an output FileStream object. These objects can be read from and written to the target files.
    FileStream fsInput = new FileStream(sInputFilename, 
    				FileMode.Open, 
    				FileAccess.Read);
    
    FileStream fsEncrypted = new FileStream(sOutputFilename, 
    				FileMode.Create, 
    				FileAccess.Write);
    					
  8. Declare an instance of the DESCryptoServiceProvider class. This represents the actual encryption and the actual decryption technology that is used on the files. At this point, you can create a different provider if you prefer to use RSAsecutiry or another cryptographic technique.
    DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
    					
  9. The cryptographic provider must be provided with your secret key as an array of bytes. The System.Text namespace provides a function that is named GetBytes(). As part of its encoding features, the GetBytes() function takes a string, and then returns an array of bytes. The size of the key is different for each cryptographic technique. For example, Data Encryption Standard (DES) takes a 64-bit key that is equal to 8 bytes or to 8 characters.

    If you do not provide a key, the provider randomly generates one. This successfully encrypts the file, but there is no way to decrypt the file. Note that you must also provide the initialization vector (IV). This value is used as part of the encryption. Like the key, the IV is randomly generated if you do not provide the value. Because the values must be the same for the encryption and the decryption, you must not permit random generation of these values.
    DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
    DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
    					
  10. Create an instance of the CryptoStream class by using the cryptographic provider to obtain an encrypting object (CreateEncryptor) and the existing output FileStream object as a part of the constructor.
    ICryptoTransform desencrypt = DES.CreateEncryptor();
    CryptoStream cryptostream = new CryptoStream(fsEncrypted, 
    					desencrypt, 
    					CryptoStreamMode.Write);
    					
  11. Read in the input file, and then write out to the output file. Pass through the CryptoStream object where the file is encrypted by using the key that you provided.
    byte[] bytearrayinput = new byte[fsInput.Length - 1];
    fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
    cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
    					

Decrypt a file

To decrypt a file, follow these steps:
  1. Create a method, and then name it DecryptFile. The decryption process is similar to the encryption process, however, the DecryptFile procedure has two key differences from the EncryptFile procedure.
    • CreateDecryptor is used instead of CreateEncryptor to create the CryptoStream object, that specifies how the object can be used.
    • When the decrypted text is written to the destination file, the CryptoStream object is now the source instead of the destination stream.
    static void DecryptFile(string sInputFilename, 
    	                string sOutputFilename,
    	                string sKey)
    {
    	DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
    	//A 64 bit key and IV is required for this provider.
    	//Set secret key For DES algorithm.
    	DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
    	//Set initialization vector.
    	DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
    
    	//Create a file stream to read the encrypted file back.
    	FileStream fsread = new FileStream(sInputFilename, 
    		                           FileMode.Open, 
    		                           FileAccess.Read);
    	//Create a DES decryptor from the DES instance.
    	ICryptoTransform desdecrypt = DES.CreateDecryptor();
    	//Create crypto stream set to read and do a 
    	//DES decryption transform on incoming bytes.
    	CryptoStream cryptostreamDecr = new CryptoStream(fsread, 
    		                                         desdecrypt,
    		                                         CryptoStreamMode.Read);
    	//Print the contents of the decrypted file.
    	StreamWriter fsDecrypted = new StreamWriter(sOutputFilename);
    	fsDecrypted.Write(new StreamReader(cryptostreamDecr).ReadToEnd());
    	fsDecrypted.Flush();
    	fsDecrypted.Close();
    }
    					
  2. Add the following lines to the Main() procedure to call both EncryptFile and DecryptFile:
    static void Main()
    {
          // Must be 64 bits, 8 bytes.
          // Distribute this key to the user who will decrypt this file.
          string sSecretKey;
             
          // Get the key for the file to encrypt.
          sSecretKey = GenerateKey();
    
          // For additional security pin the key.
          GCHandle gch = GCHandle.Alloc( sSecretKey,GCHandleType.Pinned );
             
          // Encrypt the file.        
          EncryptFile(@"C:\MyData.txt", 
             @"C:\Encrypted.txt", 
             sSecretKey);
    
          // Decrypt the file.
          DecryptFile(@"C:\Encrypted.txt", 
             @"C:\Decrypted.txt", 
             sSecretKey);
    
          // Remove the key from memory. 
          ZeroMemory(gch.AddrOfPinnedObject(), sSecretKey.Length * 2);
          gch.Free();
    }
  3. Save the file. Run your application. Make sure that the path that is used for the input file name points to an existing file.

Test the procedure

Test this code with a text (.txt) file to confirm that the code encrypted and decrypted the file correctly. Make sure that you decrypt the file to a new file (as in the Main() procedure in this article) instead of to the original file. Examine the decrypted file, and then compare it to the original file.

Complete code listing

using System;
using System.IO;
using System.Security;
using System.Security.Cryptography;
using System.Runtime.InteropServices;
using System.Text;

namespace CSEncryptDecrypt
{
   class Class1
   {
      //  Call this function to remove the key from memory after use for security
      [System.Runtime.InteropServices.DllImport("KERNEL32.DLL", EntryPoint="RtlZeroMemory")]
      public static extern bool ZeroMemory(IntPtr Destination, int Length);
		
      // Function to Generate a 64 bits Key.
      static string GenerateKey() 
      {
         // Create an instance of Symetric Algorithm. Key and IV is generated automatically.
         DESCryptoServiceProvider desCrypto =(DESCryptoServiceProvider)DESCryptoServiceProvider.Create();

         // Use the Automatically generated key for Encryption. 
         return ASCIIEncoding.ASCII.GetString(desCrypto.Key);
      }

      static void EncryptFile(string sInputFilename,
         string sOutputFilename, 
         string sKey) 
      {
         FileStream fsInput = new FileStream(sInputFilename, 
            FileMode.Open, 
            FileAccess.Read);

         FileStream fsEncrypted = new FileStream(sOutputFilename, 
            FileMode.Create, 
            FileAccess.Write);
         DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
         DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
         DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
         ICryptoTransform desencrypt = DES.CreateEncryptor();
         CryptoStream cryptostream = new CryptoStream(fsEncrypted, 
            desencrypt, 
            CryptoStreamMode.Write); 

         byte[] bytearrayinput = new byte[fsInput.Length];
         fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
         cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
         cryptostream.Close();
         fsInput.Close();
         fsEncrypted.Close();
      }

      static void DecryptFile(string sInputFilename, 
         string sOutputFilename,
         string sKey)
      {
         DESCryptoServiceProvider DES = new DESCryptoServiceProvider();
         //A 64 bit key and IV is required for this provider.
         //Set secret key For DES algorithm.
         DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
         //Set initialization vector.
         DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey);

         //Create a file stream to read the encrypted file back.
         FileStream fsread = new FileStream(sInputFilename, 
            FileMode.Open, 
            FileAccess.Read);
         //Create a DES decryptor from the DES instance.
         ICryptoTransform desdecrypt = DES.CreateDecryptor();
         //Create crypto stream set to read and do a 
         //DES decryption transform on incoming bytes.
         CryptoStream cryptostreamDecr = new CryptoStream(fsread, 
            desdecrypt,
            CryptoStreamMode.Read);
         //Print the contents of the decrypted file.
         StreamWriter fsDecrypted = new StreamWriter(sOutputFilename);
         fsDecrypted.Write(new StreamReader(cryptostreamDecr).ReadToEnd());
         fsDecrypted.Flush();
         fsDecrypted.Close();
      } 

      static void Main()
      {
         // Must be 64 bits, 8 bytes.
         // Distribute this key to the user who will decrypt this file.
         string sSecretKey;
         
         // Get the Key for the file to Encrypt.
         sSecretKey = GenerateKey();

         // For additional security Pin the key.
         GCHandle gch = GCHandle.Alloc( sSecretKey,GCHandleType.Pinned );
         
         // Encrypt the file.        
         EncryptFile(@"C:\MyData.txt", 
            @"C:\Encrypted.txt", 
            sSecretKey);

         // Decrypt the file.
         DecryptFile(@"C:\Encrypted.txt", 
            @"C:\Decrypted.txt", 
            sSecretKey);

         // Remove the Key from memory. 
         ZeroMemory(gch.AddrOfPinnedObject(), sSecretKey.Length * 2);
         gch.Free();
      }
   }
}

References

For more information about cryptography, and about using the cryptographic features of .NET, see the following MSDN Web sites:
System.Security.Cryptography namespace
Microsoft .NET Framework Developer Center

Properties

Article ID: 307010 - Last Review: November 15, 2012 - Revision: 9.0
Applies to
  • Microsoft Visual C# 2005
  • Microsoft Visual C# .NET 2003 Standard Edition
  • Microsoft Visual C# .NET 2002 Standard Edition
Keywords: 
kbsecurity kbio kbcrypt kbhowtomaster KB307010

Give Feedback

 

Contact us for more help

Contact us for more help
Connect with Answer Desk for expert help.
Get more support from smallbusiness.support.microsoft.com