Encrypting and decrypting data

Encryption software

 

I am seeing a lot of questions people are asking on how to do encryption/decryption. To help those people I have written a simple class encorporating several encryption/decryption functions:

  • byte[] Encrypt(byte[] clearData, byte[] Key, byte[] IV) - encrypts a byte array with a key and an IV and returns a byte array;
  • string Encrypt(string clearText, string Password) - encrypts a string with a password and returns a string;
  • byte[] Encrypt(byte[] clearData, string Password) - encrypts a byte array with a password and returns a byte array;
  • void Encrypt(string fileIn, string fileOut, string Password) - encrypts a file with a password and writes the encrypted bytes into another file.

For each of those there is also a corresponding Decrypt function. The Main method is a simple testing method that exercises some of those functions. The 2nd and the 3rd Encrypt functions call into the 1st function, so you will need to carry the 1st one around if you are using the 2nd or the 3rd. The last Encrypt function (the one that works with files) is standalone. I made it operate in a stream-like manner, without reading the whole file into memory, which makes it possible to encrypt/decrypt gigabytes of data without going out of memory space.

 

I am using Rijndael algorithm in this sample. The reason for this is that it is 100% implemented in managed code in our libraries, so it does not rely on CryptoAPI or any encryption packs and will work everywhere. If you need performance I would suggest replacing it with TripleDES (it is a one line change), and if you do, also do not forget to change the IV size to 8 bytes and the Key size to 16 bytes.

 

I have tried to document the code well, and I would like to encourage you to read through it and understand how it works, it should be pretty easy. You can also grab the whole thing, stick it into a .cs file and it should compile. If you run it you will see it make some test encryption/decryption roundtrip; you can also provide a file name as a parameter, and it will encrypt the file into a <name>.encrypted file and then decrypt it back into a <name>.decrypted.

 

Enjoy!

 

P.S. A crypto-related FAQ can be found at http://www.gotdotnet.com/team/clr/cryptofaq.htm  and there is a good chapter on how to use crypto in "Writing Secure Code" by Michael Howard (2nd edition came out recently). For in depth information on crypto in general "Applied Cryptography" by Bruce Schneier is an excellent resource.

 

 

 

//

//    This sample code is provided "AS IS" with no warranties, and confers no rights.

//

//    ATTENTION: This sample is designed to be more of a tutorial rather than something

//                you can copy and paste in the production code!

//                See the topic on "making it industry strength" in the blog.

//

 

using System;

using System.IO;

using System.Security.Cryptography;

 

//

// Sample encrypt/decrypt functions

//    Parameter checks and error handling are ommited for better readability

//

public class EncDec

{

 

      // Encrypt a byte array into a byte array using a key and an IV

      public static byte[] Encrypt(byte[] clearData, byte[] Key, byte[] IV)

      {

            // Create a MemoryStream that is going to accept the encrypted bytes

            MemoryStream ms = new MemoryStream();

 

            // Create a symmetric algorithm.

            // We are going to use Rijndael because it is strong and available on all platforms.

            // You can use other algorithms, to do so substitute the next line with something like

            //                      TripleDES alg = TripleDES.Create();

            Rijndael alg = Rijndael.Create();

 

            // Now set the key and the IV.

            // We need the IV (Initialization Vector) because the algorithm is operating in its default

            // mode called CBC (Cipher Block Chaining). The IV is XORed with the first block (8 byte)

            // of the data before it is encrypted, and then each encrypted block is XORed with the

            // following block of plaintext. This is done to make encryption more secure.

            // There is also a mode called ECB which does not need an IV, but it is much less secure.

            alg.Key = Key;

            alg.IV = IV;

 

            // Create a CryptoStream through which we are going to be pumping our data.

            // CryptoStreamMode.Write means that we are going to be writing data to the stream

            // and the output will be written in the MemoryStream we have provided.

            CryptoStream cs = new CryptoStream(ms, alg.CreateEncryptor(), CryptoStreamMode.Write);

 

            // Write the data and make it do the encryption

            cs.Write(clearData, 0, clearData.Length);

 

            // Close the crypto stream (or do FlushFinalBlock).

            // This will tell it that we have done our encryption and there is no more data coming in,

            // and it is now a good time to apply the padding and finalize the encryption process.

            cs.Close();

 

            // Now get the encrypted data from the MemoryStream.

            // Some people make a mistake of using GetBuffer() here, which is not the right way.

            byte[] encryptedData = ms.ToArray();

 

            return encryptedData;

      }

 

 

      // Encrypt a string into a string using a password

      //    Uses Encrypt(byte[], byte[], byte[])

      public static string Encrypt(string clearText, string Password)

      {

            // First we need to turn the input string into a byte array.

            byte[] clearBytes = System.Text.Encoding.Unicode.GetBytes(clearText);

 

            // Then, we need to turn the password into Key and IV

            // We are using salt to make it harder to guess our key using a dictionary attack -

            // trying to guess a password by enumerating all possible words.

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,

                        new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d,  0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

 

            // Now get the key/IV and do the encryption using the function that accepts byte arrays.

            // Using PasswordDeriveBytes object we are first getting 32 bytes for the Key

            // (the default Rijndael key length is 256bit = 32bytes) and then 16 bytes for the IV.

            // IV should always be the block size, which is by default 16 bytes (128 bit) for Rijndael.

            // If you are using DES/TripleDES/RC2 the block size is 8 bytes and so should be the IV size.

            // You can also read KeySize/BlockSize properties off the algorithm to find out the sizes.

            byte[] encryptedData = Encrypt(clearBytes, pdb.GetBytes(32), pdb.GetBytes(16));

 

            // Now we need to turn the resulting byte array into a string.

            // A common mistake would be to use an Encoding class for that. It does not work

            // because not all byte values can be represented by characters.

            // We are going to be using Base64 encoding that is designed exactly for what we are

            // trying to do.

            return Convert.ToBase64String(encryptedData);

           

      }

 

 

      // Encrypt bytes into bytes using a password

      //    Uses Encrypt(byte[], byte[], byte[])

      public static byte[] Encrypt(byte[] clearData, string Password)

      {

            // We need to turn the password into Key and IV.

            // We are using salt to make it harder to guess our key using a dictionary attack -

            // trying to guess a password by enumerating all possible words.

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,

                        new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d,  0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

 

            // Now get the key/IV and do the encryption using the function that accepts byte arrays.

            // Using PasswordDeriveBytes object we are first getting 32 bytes for the Key

            // (the default Rijndael key length is 256bit = 32bytes) and then 16 bytes for the IV.

            // IV should always be the block size, which is by default 16 bytes (128 bit) for Rijndael.

            // If you are using DES/TripleDES/RC2 the block size is 8 bytes and so should be the IV size.

            // You can also read KeySize/BlockSize properties off the algorithm to find out the sizes.

            return Encrypt(clearData, pdb.GetBytes(32), pdb.GetBytes(16));

 

      }

 

 

      // Encrypt a file into another file using a password

      public static void Encrypt(string fileIn, string fileOut, string Password)

      {

            // First we are going to open the file streams

            FileStream fsIn = new FileStream(fileIn, FileMode.Open, FileAccess.Read);

            FileStream fsOut = new FileStream(fileOut, FileMode.OpenOrCreate, FileAccess.Write);

 

            // Then we are going to derive a Key and an IV from the Password and create an algorithm

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,

                        new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d,  0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

 

            Rijndael alg = Rijndael.Create();

 

            alg.Key = pdb.GetBytes(32);

            alg.IV = pdb.GetBytes(16);

 

            // Now create a crypto stream through which we are going to be pumping data.

            // Our fileOut is going to be receiving the encrypted bytes.

            CryptoStream cs = new CryptoStream(fsOut, alg.CreateEncryptor(), CryptoStreamMode.Write);

 

            // Now will will initialize a buffer and will be processing the input file in chunks.

            // This is done to avoid reading the whole file (which can be huge) into memory.

            int bufferLen = 4096;

            byte[] buffer = new byte[bufferLen];

            int bytesRead;

           

            do {

                  // read a chunk of data from the input file

                  bytesRead = fsIn.Read(buffer, 0, bufferLen);

 

                  // encrypt it

                  cs.Write(buffer, 0, bytesRead);

           

            } while(bytesRead != 0);

 

            // close everything

            cs.Close(); // this will also close the unrelying fsOut stream

            fsIn.Close();    

      }

 

 

 

      // Decrypt a byte array into a byte array using a key and an IV

      public static byte[] Decrypt(byte[] cipherData, byte[] Key, byte[] IV)

      {

            // Create a MemoryStream that is going to accept the decrypted bytes

            MemoryStream ms = new MemoryStream();

 

            // Create a symmetric algorithm.

            // We are going to use Rijndael because it is strong and available on all platforms.

            // You can use other algorithms, to do so substitute the next line with something like

            //                      TripleDES alg = TripleDES.Create();

            Rijndael alg = Rijndael.Create();

 

            // Now set the key and the IV.

            // We need the IV (Initialization Vector) because the algorithm is operating in its default

            // mode called CBC (Cipher Block Chaining). The IV is XORed with the first block (8 byte)

            // of the data after it is decrypted, and then each decrypted block is XORed with the previous

            // cipher block. This is done to make encryption more secure.

            // There is also a mode called ECB which does not need an IV, but it is much less secure.

            alg.Key = Key;

            alg.IV = IV;

 

            // Create a CryptoStream through which we are going to be pumping our data.

            // CryptoStreamMode.Write means that we are going to be writing data to the stream

            // and the output will be written in the MemoryStream we have provided.

            CryptoStream cs = new CryptoStream(ms, alg.CreateDecryptor(), CryptoStreamMode.Write);

 

            // Write the data and make it do the decryption

            cs.Write(cipherData, 0, cipherData.Length);

 

            // Close the crypto stream (or do FlushFinalBlock).

            // This will tell it that we have done our decryption and there is no more data coming in,

            // and it is now a good time to remove the padding and finalize the decryption process.

            cs.Close();

 

            // Now get the decrypted data from the MemoryStream.

            // Some people make a mistake of using GetBuffer() here, which is not the right way.

            byte[] decryptedData = ms.ToArray();

 

            return decryptedData;

      }

 

 

      // Decrypt a string into a string using a password

      //    Uses Decrypt(byte[], byte[], byte[])

      public static string Decrypt(string cipherText, string Password)

      {

            // First we need to turn the input string into a byte array.

            // We presume that Base64 encoding was used

            byte[] cipherBytes = Convert.FromBase64String(cipherText);

 

            // Then, we need to turn the password into Key and IV

            // We are using salt to make it harder to guess our key using a dictionary attack -

            // trying to guess a password by enumerating all possible words.

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,

                        new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d,  0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

 

            // Now get the key/IV and do the decryption using the function that accepts byte arrays.

            // Using PasswordDeriveBytes object we are first getting 32 bytes for the Key

            // (the default Rijndael key length is 256bit = 32bytes) and then 16 bytes for the IV.

            // IV should always be the block size, which is by default 16 bytes (128 bit) for Rijndael.

            // If you are using DES/TripleDES/RC2 the block size is 8 bytes and so should be the IV size.

            // You can also read KeySize/BlockSize properties off the algorithm to find out the sizes.

            byte[] decryptedData = Decrypt(cipherBytes, pdb.GetBytes(32), pdb.GetBytes(16));

 

            // Now we need to turn the resulting byte array into a string.

            // A common mistake would be to use an Encoding class for that. It does not work

            // because not all byte values can be represented by characters.

            // We are going to be using Base64 encoding that is designed exactly for what we are

            // trying to do.

            return System.Text.Encoding.Unicode.GetString(decryptedData);

           

      }

 

 

      // Decrypt bytes into bytes using a password

      //    Uses Decrypt(byte[], byte[], byte[])

      public static byte[] Decrypt(byte[] cipherData, string Password)

      {

            // We need to turn the password into Key and IV.

            // We are using salt to make it harder to guess our key using a dictionary attack -

            // trying to guess a password by enumerating all possible words.

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,

                        new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d,  0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

 

            // Now get the key/IV and do the Decryption using the function that accepts byte arrays.

            // Using PasswordDeriveBytes object we are first getting 32 bytes for the Key

            // (the default Rijndael key length is 256bit = 32bytes) and then 16 bytes for the IV.

            // IV should always be the block size, which is by default 16 bytes (128 bit) for Rijndael.

            // If you are using DES/TripleDES/RC2 the block size is 8 bytes and so should be the IV size.

            // You can also read KeySize/BlockSize properties off the algorithm to find out the sizes.

            return Decrypt(cipherData, pdb.GetBytes(32), pdb.GetBytes(16));

 

      }

 

 

      // Decrypt a file into another file using a password

      public static void Decrypt(string fileIn, string fileOut, string Password)

      {

            // First we are going to open the file streams

            FileStream fsIn = new FileStream(fileIn, FileMode.Open, FileAccess.Read);

            FileStream fsOut = new FileStream(fileOut, FileMode.OpenOrCreate, FileAccess.Write);

 

            // Then we are going to derive a Key and an IV from the Password and create an algorithm

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(Password,

                        new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d,  0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76});

 

            Rijndael alg = Rijndael.Create();

 

            alg.Key = pdb.GetBytes(32);

            alg.IV = pdb.GetBytes(16);

 

            // Now create a crypto stream through which we are going to be pumping data.

            // Our fileOut is going to be receiving the Decrypted bytes.

            CryptoStream cs = new CryptoStream(fsOut, alg.CreateDecryptor(), CryptoStreamMode.Write);

 

            // Now will will initialize a buffer and will be processing the input file in chunks.

            // This is done to avoid reading the whole file (which can be huge) into memory.

            int bufferLen = 4096;

            byte[] buffer = new byte[bufferLen];

            int bytesRead;

           

            do {

                  // read a chunk of data from the input file

                  bytesRead = fsIn.Read(buffer, 0, bufferLen);

 

                  // Decrypt it

                  cs.Write(buffer, 0, bytesRead);

           

            } while(bytesRead != 0);

 

            // close everything

            cs.Close(); // this will also close the unrelying fsOut stream

            fsIn.Close();    

      }

 

 

 

      //

      // Testing function

      //    I am sure you will be able to figure out what it does!

      //

      public static void Main(string[] args)

      {

            if (args.Length == 0) {

                  string plainText = "This is some plain text";

                  string Password = "Password";

                  Console.WriteLine("Plain text: \"" + plainText + "\", Password: \"" + Password + "\"");

                  string cipherText = Encrypt(plainText, Password );

                  Console.WriteLine("Encrypted text: " + cipherText);

                  string decryptedText = Decrypt(cipherText, Password);

                  Console.WriteLine("Decrypted: " + decryptedText);

            } else {

                  Encrypt(args[0], args[0]+".encrypted", "Password");

                  Decrypt(args[0]+".encrypted", args[0]+".decrypted", "Password");

            }

            Console.WriteLine("Done.");

      }

 

}

 

 

Making it "industry strength"

The encryption sample above had a very defined purpose - being extremely easy to read and understand. While it explains how to use symmetric encryption classes and gives some ideas on how to start implementing encryption in your applications, there are things you will have to do before it becomes a shippable piece of code. One of them I have already mentioned in the posting below - parameter checking and error handling. Check the parameters for being valid, wrap calls that can potentially fail into try/catch blocks, use finally blocks to release resources (close files) if something goes wrong, etc.

Some cryptography specific considerations should also be there. For example, the salt values in PasswordDeriveBytes should better be random rather than hardcoded (sometimes it is ok to have them hardcoded, for example, when encryption happens rarely and the code is not accessible by attackers). If the salt is random and changed frequently, you don't even have to keep it secret. Also, when possible, use byte[] keys as opposed to passwords. Because of the human factor, password-based encryption is not the most secure way to protect information. In order to get 128bit of key information out of a password it has to be long. If you are using just small letters that gives you about 5 bits of information per character and your password will have to be over 25 characters long to get to 128bit. If you are using capital letters and some symbols you can get to about 7 bits per character and your password minimum length to around 18 characters (how long is your password? ;-)).

Upd: Note, that the IV we are using in this sample comes from PasswordDeriveBytes and is statically linked to the password. This may be ok when you only encrypt a few files or use different passwords for every file you encrypt, but the moment you start mass file encryption with the same password this scheme isn't secure anymore. To make it more secure make IVs random and distribute them along with your ciphertext (thanks to Mitch Gallant for drawing attention to this).

 

(c) Ivan Medvedev    2003-2007

 

At out software archive you may find a wide variety of security software from antispam and antivirus software to encryption tools, firewalls or password managers

 

Home