Encryption and Decryption of File

Question

I am trying to encrypt and decrypt a file. The file can be any file like txt, jpg, doc, pdf. The IV is prepended to the cipher text. but while decrypting if i read first 8 bytes it gives me different value then the value of IV while encryption.

Here is my code to encrypt the file

fsOut = new FileStream(savepath, FileMode.OpenOrCreate, FileAccess.Write);
                des = new DESCryptoServiceProvider();
                des.Key = ASCIIEncoding.ASCII.GetBytes(secretkey);                 

                des.GenerateIV();                

                desencrypt = des.CreateEncryptor();
                cryptoStream = new CryptoStream(fsOut, desencrypt, CryptoStreamMode.Write);

                //write the IV to beginning of encrypted data
                BinaryWriter bw = new BinaryWriter(cryptoStream);
                bw.Write(des.IV, 0, des.IV.Length);

                // Now 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 = filestream.Read(buffer, 0, bufferLen);
                    // Encrypt it 
                    cryptoStream.Write(buffer, 0, bytesRead);
                }
                while (bytesRead != 0);

                cryptoStream.FlushFinalBlock();

This produces an encrypted file to the specified location.

Now below is my decryption code

des = new DESCryptoServiceProvider();
                des.Key = Encoding.ASCII.GetBytes(secretkey);

                byte[] iv = new byte[8];
                sourcefile.Read(iv, 0, 8);
                des.IV = iv;

                desdecrypt = des.CreateDecryptor();
                msOut = new MemoryStream();
                cryptoStream = new CryptoStream(sourcefile, desdecrypt, CryptoStreamMode.Read);

                StreamWriter fsDecrypted = new StreamWriter("D:\\tempDecrypted.jpg");
                fsDecrypted.Write(new StreamReader(cryptoStream).ReadToEnd());
                fsDecrypted.Flush();
                fsDecrypted.Close();

Some more notes: 1) DES isn't secure. Use AES instead. 2) You shouldn't use ASCII for a proper key. Either it's a proper key, then you need to use hex or Base64. Or it's a password, then you need to use a salted password hash to turn it into a key. — CodesInChaos, Sep 20 '13 at 10:05
well, i am totally new to this and confused which is best and how to implement. basically i want to encrypt & decrypt files rather than string data — Ronak Patel, Sep 20 '13 at 10:07

score 0 · Answer 1 · answered Sep 22 '13 at 12:13

0

You are writing your IV to the CryptoStream so it will be transformed to ciphertext. The IV should be prepended to the ciphertext in plain, it should not be part of the ciphertext.

answered Sep 22 '13 at 12:13

Maarten Bodewes

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ya but how to do that. I mean how to write the IV as plain text to the beginning of my cipher text – Ronak Patel Sep 23 '13 at 07:08
by writing it to `fsOut` maybe? Sheesh. – Maarten Bodewes Sep 23 '13 at 09:50

Encryption and Decryption of File

1 Answers1