Encryption

[01hanstu]

Hi, hope you can help. I would like to know if it is possible to be able to encrypt a file and have my application be able to decrypt it and ammed the file as necessary. and then when the program exits it will encrypt the file back up.

Hope you can help
Stuart

[Jenner]

That's easy. Here's a simple routine that'll strong encrypt and decrypt files.

Be sure to Import these at the top of your class as well:

Imports System.IO
Imports System.Security.Cryptography
Imports System.Text

    Private bytIV() As Byte = {190, 45, 111, 11, 32, 89, 21, 9, 255, 97, 8, 55, 44, 211, 23, 63}
    Private Const chrKeyFill As Char = "X"c
    Private Const intKeySize As Integer = 32

    Private Sub CryptFile(ByVal strInName As String, ByVal strOutName As String, ByVal strKey As String, ByVal booDecrypt As Boolean)
        Dim bytStorage(4096) As Byte
        Dim intTotalBytesWritten As Long = 8

        Dim intPackageSize As Integer

        Dim fin As New FileStream(strInName, FileMode.Open, FileAccess.Read)
        Dim fout As New FileStream(strOutName, FileMode.OpenOrCreate, FileAccess.Write)

        fout.SetLength(0)
        Dim intTotalFileLength As Long = fin.Length
        Dim rij As New System.Security.Cryptography.RijndaelManaged
        Dim crsStream As CryptoStream
        If booDecrypt Then
            crsStream = New CryptoStream(fout, _
                rij.CreateDecryptor(ConvertKeyToBytes(strKey), bytIV), CryptoStreamMode.Write)
        Else
            crsStream = New CryptoStream(fout, _
                rij.CreateEncryptor(ConvertKeyToBytes(strKey), bytIV), CryptoStreamMode.Write)
        End If

        While intTotalBytesWritten < intTotalFileLength
            intPackageSize = fin.Read(bytStorage, 0, 4096)
            crsStream.Write(bytStorage, 0, intPackageSize)
            intTotalBytesWritten = Convert.ToInt32(intTotalBytesWritten + intPackageSize / rij.BlockSize * rij.BlockSize)
        End While
        crsStream.Close()
        fout.Close()
        fin.Close()
    End Sub

    Private Function ConvertKeyToBytes(ByVal strKey As String) As Byte()
         Dim intLength As Integer = strKey.Length

         If intLength < intKeySize Then
             strKey &= Strings.StrDup(intKeySize - intLength, chrKeyFill)
         Else
             strKey = strKey.Substring(0, intKeySize)
         End If

         Return Encoding.UTF8.GetBytes(strKey)
    End Function

If you have any questions as to how it works, feel free to post them.

[01hanstu]

Hi, thanks for the update. your response looks promising but i have never worked with encryption so i dont know where to start. Its basically so information can be stored in a file and not readable by another program like notepad.

[SJWhiteley]

Just remember, that any decryption you perform in your application can be viewed and used to decrypt and encrypt that same file, using your encryption algorithm, unless you take additional steps (such as obfuscation, or storing the key in a "less exposed" location).

[Jenner]

Right, it sounds like all you need is something simple so give the routine I posted above a try.

To encrypt a file:
CryptFile("C:\unencrypted.txt", "C\encrypted.txt", "BlahBlahIAmAKey", False)

And to decrypt it back:
CryptFile("C:\encrypted.txt", "C\unencrypted.txt", "BlahBlahIAmAKey", True)

Now, you probably also want to, after you encrypt your file, delete the unencrypted version of it.
Also, when you decrypt a file, you probably don't want to write it back to the disk, you want to hold it in memory, manipulate it, and then encrypt that memory back to disk. Take a look at my Strong Encryption Class in my signature and you'll see examples of encrypting and decrypting strings.