Comments by i_syed2000 (Top 7 by date)

Okay. So lets say my encrypted log file is 200 MB Long after months of log writing. I can load the whole file in Memory, decrypt it and show all at once to the user. However, I'm trying avoid hogging user's memory. If I decide to move forward with it, I will probably switch to AES. But first, I need to solve the problem mentioned above.

The problem part is the padding. Padding is only applied when the data block size is less than 64 bits. For some reason, .net code is not updating the 'decrypted' memory stream.
See code below.

Sample String Encrypted: ABCDEFGHI
Total Encrypted Bytes: 16 (because of padding).
Goal: Decrypt and read the first block which contains 'ABCDEFGH'.

string DecryptData(string encryptedData)
{
byte[] dataToDecrypt = Convert.FromBase64String(encryptedData);
byte[] decryptedData = new byte[8];
DESCryptoServiceProvider secProvider = new DESCryptoServiceProvider();
secProvider.Mode = CipherMode.ECB;
secProvider.Padding = PaddingMode.PKCS7;
secProvider.Key = ASCIIEncoding.ASCII.GetBytes(encryptionKey);
ICryptoTransform transform = secProvider.CreateDecryptor();
MemoryStream encryptedDataStream = new MemoryStream(dataToDecrypt);
MemoryStream decryptedDataStream = new MemoryStream();

CryptoStream stream = new
CryptoStream(decryptedDataStream,transform,CryptoStreamMode.Write);
stream.Write(dataToDecrypt, 0, 8);
stream.Flush();
//PROBLEM: doesn't flush bytes to decryptedDataStream unless I change padding to 'None'.
}

Thanks Sergey. When using the ECB mode with DES, each block is encrypted standalone using the same key. So you can read each block and decrypt it independently of all the other blocks. In practice, I would really be reading one 'log line' which may span over mutiple encryption blocks.

For example consider the following log lines.
Line 1: Entering Method ABC\r\n
Line 2: Saving Customer\r\n
Line 3: Exiting Method ABC\r\n

Line 1 may span block 0 to N
Line 2 may span block N+1 to M
Line 3 may span block M+1 to ..