Symmetric File Encryption / Decryption

By: Saurabh Nandu - Friday, January 19, 2001 Add Comment

<div align="center"> <table border="0" width="70%" class="outline"> <tr> <td width="50%" class="outline">Download File</td> <td width="50%" class="outline">SDK</td> </tr> <tr> <td width="50%" class="outline"><a href="../../file/symencryptor.zip" class="wbox">symencryptor.zip</a> (15kb)</td> <td width="50%" class="outline">Beta1</td> </tr> </table> </div> Introduction Cryptography ..... Storing your files in such a way that no-one except you can view the file, does have a lot of importance today. Cryptography consists of 2 types of algorithms Asymmetric and Symmetric.  Asymmetric encryption is based on the concept of Public and Private Key. On the other hand Symmetric encryption is based on a single 'Key' . Hence it up to the person to first send the password to the receiver of the file so that he can decrypt it. In this example we learn how to use 2 Symmetric encryption algorithms namely Data Encryption Standard (DES) and RC2. Both these are single Key based algorithms. Here we use 64 bit encryption in DES and 40 bit encryption in RC2 . Also in this example you will learn how to write your own implementations of the "ICryptoStream" interface.  Usage To encrypt the file the user has to select the file to encrypt and the file name to save the encrypted data in. The file gets encrypted with the password provided by the user using either of the two algorithms selected by the user. Decryption checks the input file for validity and the algorithm used in encryption automatically. If the password provided for decryption is same as the one used for encryption then the file will decrypt. Both binary and text files can be encrypted / decrypted . If you have gone through the .NET SDK (Beta 1) then you will find that I have used the Microsoft's Example on DES encryption example and built my code on it. I will give you a rough idea of how the encryption takes place. First the Password given by the user is Hashed by the "SHA1_CSP" class. By default the SHA1_CSP class returns a Hash of 20 bytes maximum. We are using Symmetric Algorithms which require a 'Key', the size of the Key defers based on the algorithm. In DES we use a 64 bit 'Key' while in 'RC2' we use a 40 bit key. Hence depending on the Key size we store the Hash of the password in a byte array. Also required by the encryption /decryption algorithms is a initialization 'Vector'. The size of the 'Key' and 'Vector' remain the same. So for DES, which requires a 64 bit Key, we use a 64 Bit Vector similarly for 'RC2' we use a 40 bit Vector. Once the Key and Vector are generated, these are feed into a "SymmetricStreamEncryptor" class which encrypts / decrypts the bytes depending on the Algorithm , Key and Vector and writes it out to a File.  Requirements .NET SDK beta1 (Note: This code might not work on the future versions of the SDK). Screen Shot <img border="0" src="../../img/symmetricenc.gif" width="400" height="400"> Figure 1: Symmetric Encryption beta1 Code 1) SymEncryptor.cs :- The File Encryptor / Decryptor (Only relevant code) <table border="0" width="100%" class="code" cellspacing="0" cellpadding="0"> <tr> <td width="100%"> <pre>namespace SaurabhCrypto { using System; using System.Drawing; using System.ComponentModel; using System.WinForms; using System.IO ; using System.Security; using System.Security.Cryptography; // Class which will Encrypt and Decrypt files public class SymEncryptor : System.WinForms.Form { // Required by the Win Forms designer private System.ComponentModel.Container components; private System.WinForms.SaveFileDialog saveFileDialog1; private System.WinForms.OpenFileDialog openFileDialog1; private System.WinForms.StatusBar statusBar1; private System.WinForms.Button decryptb; private System.WinForms.Button encryptb; private System.WinForms.RadioButton radioButton2; private System.WinForms.RadioButton radioButton1; private System.WinForms.Label infol; private System.WinForms.TextBox passt; private System.WinForms.Label passl; private System.WinForms.Button saveb; private System.WinForms.Button openb; private System.WinForms.TextBox savet; private System.WinForms.TextBox opent; private System.WinForms.Label savel; private System.WinForms.Label openl; //these byte arrays will contain the 'KEY' and 'Vector' used in //encryption and decryption . private byte[] symKey ; private byte[] symIV ; //This is the constructor of the class. //It calls only one method 'InitlizeComponent' which draws the WinForm . public SymEncryptor() { // Call the method to make the WinForm InitializeComponent(); } //This method is called when the WinForm exits and it //Cleans up any resources being used public override void Dispose() { base.Dispose(); components.Dispose(); } // The main entry point for the application. public static void Main(string[] args) { Application.Run(new SymEncryptor()); } //Method called from the constructor //It initializes all the WinForm components private void InitializeComponent() { //Here the WinForm components are placed //I have removed this code for explanation purpose //You can get the full code when you download the example } //This method is called when the 'Decrypt' button is clicked by the user. //It first checks the file type and then calls the required methods to //decrypt the file . protected void decryptb_click(object sender, System.EventArgs e) { //call the method 'ValidateBoxes()' which checks all the fields //of the WinForm and returns a 'true' if everything is filled //properly. If there are some errors it returns a 'false' if(ValidateBoxes()) { //if all the Fields of the WinForm are valid then call the method //DecryptData(), this method takes care of all the decrypting to do DecryptData(); //after decrypting set all the TextBoxes to their default values //this is not necessary but more user friendly... opent.Text=""; savet.Text="" ; passt.Text="" ; radioButton1.Checked=true ; } } //This method is called when the 'Encrypt' button is pressed. //It checks the file and encrypts it according to the algorithm //specified by the user. protected void encryptb_click(object sender, System.EventArgs e) { //a method 'ValidateBoxes()' is called which returns a true only if //all the fields of the WinForm are properly filled if(ValidateBoxes()) { //Here we make a call the a method 'GenerateKee(bool variable)' //this method takes in a bool value and generates the Hash 'Key' //and 'Initialization Vector' which will be used in encryption /// decryption. It returns a 'true' if everything is proper //Here we Pass the 'radioButton1.Checked' as an argument since the //method generates 'key' and 'vector' depending on which algorithm to //use so if 'radioButton1.Checked=true' it means we are using the //'DES' algorithm else we are using the 'RC2' algorithm if(GenerateKee(radioButton1.Checked)) { //call the method 'EncryptData()' //which will take care of all the encrypting EncryptData() ; //Set the TextBoxes to their default values //this can be omitted , but its more user friendly opent.Text=""; savet.Text="" ; passt.Text="" ; radioButton1.Checked=true ; } } } //This method is called when the 'Save' button is clicked // It opens a 'SaveFileDialog' and takes the file to be saved to protected void saveb_click(object sender, System.EventArgs e) { //different parameters of the 'SaveFileDialog' are set here saveFileDialog1.Filter="All files (*.*)|*.* | Encryptor files (*.enc)|*.enc"; saveFileDialog1.FilterIndex = 2 ; saveFileDialog1.RestoreDirectory = true ; //check if the user has already typed a file to save in the TextBox if(savet.Text!="") { //if the user has typed a filename in the TextBox then assign that //to the dialogs 'FileName' property saveFileDialog1.FileName=savet.Text ; } if(saveFileDialog1.ShowDialog() == DialogResult.OK) { //when the user presses 'Ok' on the 'SaveFileDialog' then set the //text of the TextBox 'savet' to the file selected savet.Text = saveFileDialog1.FileName ; } } //This method is called when the 'Open' button is clicked by the user protected void openb_click(object sender, System.EventArgs e) { //Set the various properties of the 'OpenFileDialog' openFileDialog1.Filter="Encryptor files (*.enc)|*.enc|All files (*.*)|*.*"; openFileDialog1.FilterIndex = 2 ; openFileDialog1.RestoreDirectory = true ; //check if the user has already typed the file to open in the textbox if(opent.Text!="") { //Then assign the Dialog the Filename from the TextBox openFileDialog1.FileName = opent.Text ; } if(openFileDialog1.ShowDialog() == DialogResult.OK) { //if the user presses 'Ok' in the 'OpenFileDialog' then assign the //file selected to the textbox opent.Text = openFileDialog1.FileName ; } } //this method checks if any of the TextBoxes are left empty by the user //and it generates the error required //it returns 'true' if all values are proper private bool ValidateBoxes() { //check the 'Open' File textbox if(opent.Text=="") { MessageBox.Show("Please Enter the file to Encrypt / Decrypt !") ; return false ; } //check the 'Save' file textbox if(savet.Text=="") { MessageBox.Show("Please Enter the filename to save !") ; return false ; } //check the password textbox if(passt.Text=="") { MessageBox.Show("Please Enter the Password Encrypt/Decrypt your file with."); return false ; } //if everything is alright return true return true ; } //This Method computes the 'Key' and the 'Vector' to be used in //encrypting / decrypting. It generates the key according to the algorithm. //It returns is key generation is successful //If the algorithm to be used is 'DES' then it generates a 64bit Key and //64bit Vector from the provided 'Password' .Then this is stored in to a //'byte' array having length '8'. Since 1 byte = 8 bits , hence a byte //array having length '8' will contain a key of 8 x 8 =64 bits. //We use a 64bit key since DES supports a minimum 64 bit key //If the algorithm to be used is 'RC2' then it generates a 40bit Key //and a 40 bit Vector from the provided 'Password'. //We use a 40 bit key here since RC2 supports a minimum 40 bit key //(it does not support a 64bit key). private bool GenerateKee(bool isDES) { //try-catch block try { //store the password in a string string pass =passt.Text ; int i ; int len ; //convert the password in to a Char array char[] cp = pass.ToCharArray() ; len = cp.GetLength(0) ; //initialize a byte array byte[] bt = new byte[len] ; //convert the Char array of the Password to a byte array for(i=0 ; i<len ;i++) { bt[i] =(byte) cp[i]; } //if we are producing a Key-Vector for the 'DES' algorithm if(isDES) { //initialize the byte arrays which will contain the 'Key' and //the 'Vector' to a length of '8' //(see above why we use a array of length '8') symKey=new byte[8] ; symIV = new byte[8] ; //make a instance of the class 'SHA1_CSP()' //this class is useful in converting 'byte' into 'Hash'. SHA1_CSP sha = new SHA1_CSP() ; //write the Hash of the byte array containing the password sha.Write(bt) ; //close the stream sha.CloseStream() ; //now Initialize the 'Key' array with the lower 64bits of the //Hash of the 'Password' provided by the user for(i=0 ; i<8 ; i++) { symKey[i] = sha.Hash[i] ; } //initialize the 'Vector' array with the higher 64 bits of //Hash of the 'Password' provided by the user for(i=8 ; i<16 ; i++) { symIV[i-8]= sha.Hash[i] ; } } else { //if the algorithm is 'RC2' then generate the following Key //initialize the Key and Vector arrays to a length of '5' //(see above why we use '5') symKey=new byte[5] ; symIV = new byte[5] ; //make a instance of the class 'SHA1_CSP' //this class writes the hash of a given byte SHA1_CSP sha = new SHA1_CSP() ; //write the Hash of the byte array containing the user password sha.Write(bt) ; //close the stream sha.CloseStream() ; //now Initialize the Key array to lower 40 bits of the Hash //of the 'Password'. for(i=0 ; i<5 ; i++) { symKey[i] = sha.Hash[i] ; } //initilize the Vector array to 40bits of hash for(i=5 ; i<10 ; i++) { symIV[i-5]= sha.Hash[i] ; } } //since every thing went properly return 'true' return true ; } catch(Exception e) { MessageBox.Show("A Exception Occurred in Generating Keys :"+e.ToString()) ; //return false since there was a error return false ; } } //This method encrypts the given input file in either of //the 2 algorithms 'DES' or 'RC2'. Then it writes out the encrypted file //This method first reads the input file to check to see if its already //encrypted by this same program. If its already encrypted it gives a error. //According to the algorithm specified it then encrypts the file. //Also in the first 8 bytes of the new Encrypted it writes //out "[saudes]" or "[saurc2]". This is done so the while decrypting //the program can know which algorithm was used to encrypt the file. //Also it used to check if the file has already encrypted private void EncryptData() { //try-catch block try { bool algo ; //a boolean variable to check which algorithm to use in encrypting //open the 'FileStream' on the file to be encrypted FileStream fin = new FileStream(opent.Text , FileMode.Open ,FileAccess.Read) ; //Make a file to save the encrypted data to and open a 'FileStream' FileStream fout = new FileStream(savet.Text , FileMode.OpenOrCreate , FileAccess.Write); //set the position of the 'cursor' to the start of the file fout.SetLength(0) ; //make a byte array of the size 64 bits //this is called the 'Buffer Size' of the algorithm //i.e. while encrypting blocks of the size 64bits are processed //at a single time later other blocks are of the same size. //we use 64bits because both 'DES' and 'RC2' both algorithms have //64 bit 'Buffer Size' byte[] bin = new byte[4096] ; //set the total length of the file to me encrypted to a variable long totlen = fin.Length ; long rdlen=0; int len ; //the code below is used to check if the file has already been //encrypted make a byte array of length '4' byte[] tag = new byte[4]; //read the first 4 bytes from the file to be encrypted fin.Read(tag,0,tag.Length); //if it contains the chars "[sau" then it has been already //encrypted by this program if ((tag[0]==(byte)'[')&&(tag[1]==(byte)'S')&& (tag[2]==(byte)'a')&&(tag[3]==(byte)'u')) { //generate a error to let the user know of the error MessageBox.Show("This file is already Encrypted or in Invalid format!") ; statusBar1.Text="Error - Invalid File Format !!" ; } else { //if the file if ok the proceed with encryption statusBar1.Text="Encrypting..."; //set the file read cursor back to the 'Beginning' fin.Seek(0, SeekOrigin.Begin); } //make a object of the class 'SymmetricAlgorithm' SymmetricAlgorithm des ; if(radioButton1.Checked) { //if the algorithm to be used is 'DES' then initialize the //'SymmetricAlgorithm' to 'DES_CSP' des = new DES_CSP(); //set the variable to true because we are using 'DES' algorithm. algo=true ; } else { //if the algorithm to be used is 'RC2' then initialize the variable //'des' to 'RC2_CSP' des=new RC2_CSP() ; //set the key size of the algorithm to 40 bits since we are //using a 40 bit key des.KeySize=40 ; //uncomment the below code to find out the bits of keys supported //by RC2 algorithm /*KeySizes[] ks = des.LegalKeySizes ; Console.WriteLine("Key Sizes Supported :") ; Console.WriteLine("Minimum Size:" +ks[0].MinSize) ; Console.WriteLine("Skip size of key: "+ks[0].SkipSize) ; Console.WriteLine("Maximum Size: "+ks[0].MaxSize) ; */ //set the bool variable to false since we are using the RC2 algorithm algo=false ; } //Make a object of the inner class 'StoreCryptoStream' we pass the //bool variable containing the algorithm information and the FileStream StoreCryptoStream scs = new StoreCryptoStream(algo,fout); //make an object of the 'SymmetricStreamEncryptor' class and pass //it the 'Key' and the 'Vector' this stream helps to encrypt data //according to the algorithm SymmetricStreamEncryptor sse = des.CreateEncryptor(symKey, symIV); // a little extra feature here to show how to compose crypto // components that support ICryptoStream SHA1_CSP sha = new SHA1_CSP(); // wire up the encryptor - hash - StoreCryptoStream sse.SetSink(sha); sha.SetSource(sse); sha.SetSink(scs); scs.SetSource(sha); //read from the file to encrypt while (rdlen < totlen) { //set the number of bytes read len = fin.Read(bin,0,4096); //write the encrypted data sse.Write(bin,0,len); //increase the size of bytes read rdlen = rdlen + len; } //free up the resources sse.CloseStream(); fin.Close(); fout.Close() ; statusBar1.Text="Encryption Complete !" ; } catch(Exception e) { MessageBox.Show("An exception occurred while encrypting :"+e.ToString()) ; statusBar1.Text="Error" ; } } //This method decrypts the given input file in either of the //2 algorithms 'DES' or 'RC2'. Then it writes out the decrypted file //This method first reads the input file to check to see if //its encrypted by which algorithm. then it automatically decrypts the file. //It reads the first 8 bytes of the encrypted file to find the custom //tag that we place while encrypting. //If it finds "[saudes]" it uses the 'DES' algorithm to decrypt //If it finds "[saurc2]" it uses the 'RC2' algorithm to decrypt private void DecryptData() { //try-catch block try { statusBar1.Text="Decrypting...." ; //open file streams to the input and outfiles FileStream fin = new FileStream(opent.Text , FileMode.Open , FileAccess.Read) ; FileStream fout = new FileStream(savet.Text ,FileMode.OpenOrCreate ,FileAccess.Write); fout.SetLength(0) ; //a variable to check the validity of the input file bool filecheck = false ; //make a byte array of the size 64 bits //this is called the 'Buffer Size' of the algorithm //i.e. while encrypting blocks of the size 64bits are processed //at a single time later other blocks are of the same size. //we use 64bits because both 'DES' and 'RC2' both algorithms //have 64 bit 'Buffer Size' byte[] bin = new byte[4096] ; long totlen = fin.Length ; long rdlen=8; int len ; // declare a object of type 'SymmetricAlgorithm' SymmetricAlgorithm des ; //set a temporary variable to length '8' (we use '8' since the size of //the tag put in the file is 8) byte[] tag = new byte[8]; //read the first 8 bytes from the input file to check which algorithm //is used to encrypt the file fin.Read(tag,0,tag.Length); if ((tag[0]==(byte)'[')&&(tag[1]==(byte)'S')&&(tag[2]==(byte)'a') &&(tag[3]==(byte)'u')&&(tag[4]==(byte)'d')&&(tag[5]==(byte)'e') &&(tag[6]==(byte)'s')&&(tag[7]==(byte)']')) { //If this is true then the 'DES' algorithm has been used to encrypt //the file so set the variable 'des' to new 'DES_CSP' des = new DES_CSP() ; //set the variable to true since the file is encrypted filecheck=true ; //generate the Key and Vector from the given password //we send 'true' here since the algorithm used is 'DES' GenerateKee(true) ; } else if((tag[0]==(byte)'[')&&(tag[1]==(byte)'S')&&(tag[2]==(byte)'a') &&(tag[3]==(byte)'u')&&(tag[4]==(byte)'r')&&(tag[5]==(byte)'c') &&(tag[6]==(byte)'2')&&(tag[7]==(byte)']')) { //if this is true then the 'RC2' algorithm has been used to //encrypt the file so we set the variable 'des' to new 'RC2_CSP' des = new RC2_CSP() ; //set the keysize of the algorithm des.KeySize=40 ; //set the variable to true since it is encrypted filecheck=true ; //generate the Key and Vector from the password given by the user //we pass false here since we are using 'RC2' algorithm GenerateKee(false) ; } else { MessageBox.Show("File Error !! File is not encrypted by SymEncryptor !!") ; //set the des to null so no encryption occurs des=null ; statusBar1.Text="Error "; } //if the file is encrypted then decrypt it if(filecheck){ //create a object of the inner class 'StoreCryptoStream' //we pass it the FileStream StoreCryptoStream scs = new StoreCryptoStream(fout); //make a object of this stream passing the Key and Vector //we use this stream since it helps us decrypt data from a //encrypted file SymmetricStreamDecryptor ssd = des.CreateDecryptor(symKey, symIV); //set up the streams ssd.SetSink(scs); scs.SetSource(ssd); //read the full encrypted file and decrypt while (rdlen < totlen) { //set the length of the number of bytes read len = fin.Read(bin,0,4096); //write the decrypted data ssd.Write(bin,0,len); //increase the total read bytes variable rdlen = rdlen + len; } //free up the resources ssd.CloseStream(); } fin.Close(); fout.Close(); statusBar1.Text="Decryption Complete" ; } catch(Exception e) { MessageBox.Show("An exception occurred while decrypting :"+e.ToString()) ; statusBar1.Text="Error" ; } } //This is a inner class which Implements the 'ICryptoStream' interface //we write this class so that it will provide us with a custom stream //which will write and read the way we want it to public class StoreCryptoStream : ICryptoStream { //two byte arrays containing the bytes to be written while encrypting static byte[] tag1 = {(byte)'[',(byte)'S',(byte)'a',(byte)'u' ,(byte)'d' ,(byte)'e',(byte)'s' ,(byte)']'}; static byte[] tag2= {(byte)'[',(byte)'S',(byte)'a',(byte)'u' ,(byte)'r' ,(byte)'c',(byte)'2' ,(byte)']'}; FileStream fs; //This is the constructor of this class //it takes 2 parameters //1)a bool variable to indicate which algorithm has called it //if it 'true' then 'DES' has called it , if its 'false' then //'RC2' has called it. //2)the FileStream to the output file public StoreCryptoStream(bool algo, FileStream fout) { fs = fout; if (algo){ //we are here since the variable 'algo' is 'true' indicating //that 'DES' algorithm is being used //hence it writes the byte array 'tag1' to the output file fs.Write(tag1,0,8); } else { //we are here since the variable 'algo' is 'false' indicating that //'RC2'algorithm is being used //hence it writes the byte array 'tag2' to the output file fs.Write(tag2,0,8) ; } } //This is another constructor which takes one parameter "FileStream" //It is called from the decrypting method public StoreCryptoStream(FileStream fout) { fs=fout ; } //other methods which have to be implemented public virtual void CloseStream() {fs.Close();} public virtual void CloseStream(Object obj) {fs.Close();} public virtual void SetSink(ICryptoStream pstm) {} public virtual void SetSource(CryptographicObject co) {} public virtual ICryptoStream GetSink () {return null;} // Write routines just copy output to the target file public virtual void Write(byte[] bin) { int len = bin.GetLength(0); Write(bin, 0, len); } public virtual void Write(byte[] bin, int start, int len ) { fs.Write(bin,start,len); } }//StoreCryptoStream class }//SymEncryptor class }</pre> </td> </tr> </table>

Symmetric File Encryption / Decryption

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