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, 27 Nov 2007 CPOL
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LOCK is used to lock and unlock the files using symmetric key cryptography


This article shows a classical approach to lock a file so that it doesn't reveal its original content in any software. For locking a file, a password is required. To view the file again in its original form, it has to be unlocked with the LOCK software with the same password which was used to lock the file.


The basic approach used is the symmetric key cryptography. The file is converted into blocks of 8 bytes and repeatedly XORed with the similar block of 8 bytes obtained by the password. Because the encryption used is symmetric key cryptography, the same process is used for unlocking\decryption.

Using the code

Following is the code for main encryption routine. For more details, look at source code for the comments.

// Replicate the byte array in the key buffer to create
         // an encryption key whose size equals or exceeds the
         // size of the I/O buffer
         int count = (1024 + keyBytes.Length-1)/keyBytes.Length;
                 for( int i=0 ; i<count ; i++)
                 Array.Copy( keyBytes , 0 , keybuf , i*keyBytes.Length , keyBytes.Length);
        // Read the file in bufsize blocks, XOR-encrypt each block,
         // and write the encrypted block back to the file
         long lBytesRemaining = stream.Length;
         while (lBytesRemaining>0)
                 long lPosition = stream.Position ;
                 int nBytesRequested = (int)System.Math.Min(bufSize , lBytesRemaining); 
                 int nBytesRead = reader.Read (buffer, 0, nBytesRequested)

                 for (int i=0; i<nBytesRead; i++)
                          buffer[i] ^= keybuf[i];Stream.Seek (lPosition, SeekOrigin.Begin);
                          writer.Write (buffer, 0, nBytesRead);
                          lBytesRemaining -= nBytesRead;


Screenshot - mainScreenShot.gif

Points of Interest

This is my second article at code project. For the first article which about Handling the Big Numbers, click here. I donot have much experience of article writing. I have just tried to minimize the shortcomings I have faced drafting the First article. Suggestion regarding the clarity, confusion, code, technical writing are welcomed. Comments are also welcomed. You can also mail me at with subject Lock Code Project Article regarding the same.


This is the first version of the program. I have some inputs for the next version of the code. I am expecting some more inputs from the readers of the code. After finalizing all the inputs I would write next version of the code.

If you feel any shortcoming of this code/article, please leave a message about the same, so that these pitfalls may not appear in the subsequent versions.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Software Developer (Senior)
India India
Software Engineer based out in Noida.

Technology skillset – .NET, WPF, WCF, LINQ, XAML.

Started blogging on

Stackoverflow Profile ->

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Comments and Discussions

GeneralYikes. Pin
JohnnyLocust27-Nov-07 10:13
memberJohnnyLocust27-Nov-07 10:13 
GeneralRe: Yikes. Pin
Fun@learn27-Nov-07 18:25
memberFun@learn27-Nov-07 18:25 
you have missed some part of the code while copying and pasting, which contains the real logic

I am posting the part you have missed. Please check the code below in your file

private void button2_Click(object sender, System.EventArgs e)
if(ffield.Text=="" )
MessageBox.Show("File Name is empty");
if(pass.Text=="" )
MessageBox.Show("Password Can't be Null");

MessageBox.Show("password doesnot Match");

const int bufSize =1048576 ;
String fileName = file;
String key = password;
FileStream stream = null;
// open file for reading & writing
stream = File.Open(fileName , FileMode.Open ,FileAccess.ReadWrite ) ;
catch(Exception ex)
// wrap a reader & writer around the file
BinaryReader reader = new BinaryReader(stream);
BinaryWriter writer = new BinaryWriter(stream);

// convert the key to byte array
ASCIIEncoding enc = new ASCIIEncoding ();
byte[] keyBytes = enc.GetBytes (key);

// Allocate an I/O buffer & key buffer
byte[] buffer = new byte[bufSize];
byte[] keybuf = new byte[bufSize + keyBytes.Length - 1];

// Replicate the byte array in the key buffer to create
// an encryption key whose size equals or exceeds the
// size of the I/O buffer
int count = (1024 + keyBytes.Length-1)/keyBytes.Length;
for( int i=0 ; i0)
long lPosition = stream.Position ;
int nBytesRequested = (int)System.Math.Min(bufSize , lBytesRemaining) ;
int nBytesRead = reader.Read (buffer, 0, nBytesRequested);

for (int i=0; i MessageBox.Show(fileName + " is crypted");
catch (Exception x)
Console.WriteLine(x.Message) ;
if (stream != null)
stream.Close ();

destiny is the mirror of the past

GeneralRe: Yikes. Pin
JohnnyLocust27-Nov-07 18:53
memberJohnnyLocust27-Nov-07 18:53 
GeneralRe: Yikes. Pin
Ri Qen-Sin27-Nov-07 19:29
memberRi Qen-Sin27-Nov-07 19:29 
AnswerRe: Yikes. Pin
Fun@learn27-Nov-07 19:49
memberFun@learn27-Nov-07 19:49 
GeneralRe: Yikes. Pin
JohnnyLocust28-Nov-07 7:08
memberJohnnyLocust28-Nov-07 7:08 
GeneralRe: Yikes. Pin
Ri Qen-Sin28-Nov-07 9:22
memberRi Qen-Sin28-Nov-07 9:22 

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