Encryption and Decryption

[John Smith]

Operating System: PocketPC 2003 (Emulator)
Language: VB.NET


I have downloaded the "Pocket PC Signature Application Sample" from
Microsoft's
website(http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnn
etcomp/html/PPCSignatureApp.asp) and have gotten it to work successfully. I
am trying to rip-off the Encryption and Decryption capability from the
sample application and put it into another application that contains
Encrypted Data in it.

My problem is that when I try to decrypt the encrypted value I receive and
error. First, I encrypt the plain text value then convert it from a byte to
a string and store it in my database. Then when I want to view the
encrypted data I call the encrypted string and convert it into a byte array
and call "Crypto.Decrypt()" and pass its parameters. When I do that, then
it fails. I did not change the vb class files whatsoever. I have provided
sample code below.

I have been pulling my hair out on this for a couple of days now.
Any ideas? Thanx in Advance. (Sample Code Below)

-------------------------------------------------
Error:
-------------------------------------------------

An unhandled exception of type 'System.SystemException' occurred in
PasswordDatabase.exe

Additional information: CryptDecrypt failed.
Last error - 0x80000005.
Error message - N/A.

'------------------------------------------------
'Code with Error: (Crypto.vb - Decrypt Function)
'------------------------------------------------

'Decrypt data. Use passphrase to generate the encryption key.
' Returns a byte array that contains the decrypted data.
Public Shared Function Decrypt( _
ByVal passphrase As String, _
ByVal data() As Byte) _
As Byte()

' make a copy of the encrypted data
Dim dataCopy As Byte() = CType(data.Clone(), Byte())

' holds the decrypted data
Dim buffer As Byte() = Nothing

' crypto handles
Dim hProv As IntPtr = IntPtr.Zero
Dim hKey As IntPtr = IntPtr.Zero

Try
' get crypto provider, specify the provider (3rd argument)
' instead of using default to ensure the same provider is
' used on client and server
If Not WinApi.CryptAcquireContext(hProv, Nothing, _
WinApi.MS_DEF_PROV, WinApi.PROV_RSA_FULL, WinApi.CRYPT_VERIFYCONTEXT)
Then
Failed("CryptAcquireContext")
End If

' generate encryption key from the passphrase
hKey = GetCryptoKey(hProv, passphrase)

' decrypt the data
Dim dataLength As Integer = dataCopy.Length

'***********************************************************************
' This next line generated the error.
'***********************************************************************

If Not WinApi.CryptDecrypt(hKey, IntPtr.Zero, True, 0, dataCopy,
dataLength) Then
Failed("CryptDecrypt")
End If

'**** Etc... (Code)


'------------------------------------------------
'Sample Code to call function:
'------------------------------------------------

Private Function DecryptedValue( _
ByVal EncryptedValue As String) _
As String

Dim arrByteEncryptedValue As Byte()

' Get bytes of initialization vector.
If (EncryptedValue Is Nothing) Then
arrByteEncryptedValue = New Byte() {}
Else
arrByteEncryptedValue = Encoding.ASCII.GetBytes(EncryptedValue)
End If

' decrypt the signature data
Dim data As Byte() = Crypto.Decrypt( _
Global.Settings.GetString(SettingKeys.CryptPassphrase), _
arrByteEncryptedValue)

' update the decrypted view
Dim strDecryptedValue As String
strDecryptedValue = Encoding.ASCII.GetString(data, 0, data.Length)

data = Nothing
arrByteEncryptedValue = Nothing

Return strDecryptedValue

End Function



Thanx in Advance,
John Smith

 

[John Smith]

I figured out the problem it was the way I was handling my variables.

 

[Robbe Morris]

Just in case you need it, I pulled out the encryption code from
that sample. My code also shows you how to use the
same encryption class for desktop .net for decryption if needed
in a disconnected environment.

http://www.eggheadcafe.com/articles/compactframeworkencryption.asp

--
2005 Microsoft MVP C#
Robbe Morris
http://www.masterado.net
http://www.mastervb.net