Xors and byte arrays

[GPO]

Rob Bovey posted a bit of code to http://www.devx.com/tips/Tip/5676

Function szEncryptDecrypt(ByVal szData As String) As String
''' This key value can be changed to alter the
''' encryption, but it must be the same for both
''' encryption and decryption.
Const KEY_TEXT As String = "ScratchItYouFool"
''' The KEY_OFFSET is optional, and may be any
''' value 0-64.
''' Likewise, it needs to be the same coming/going.
Const KEY_OFFSET As Long = 38

Dim bytKey() As Byte
Dim bytData() As Byte
Dim lNum As Long
Dim szKey As String

For lNum = 1 To ((Len(szData) \ Len(KEY_TEXT)) + 1)
szKey = szKey & KEY_TEXT
Next lNum

bytKey = Left$(szKey, Len(szData))
bytData = szData

For lNum = LBound(bytData) To UBound(bytData)
If lNum Mod 2 Then
bytData(lNum) = bytData(lNum) Xor (bytKey(lNum) _
+ KEY_OFFSET)
Else
bytData(lNum) = bytData(lNum) Xor (bytKey(lNum) _
- KEY_OFFSET)
End If
Next lNum

szEncryptDecrypt = bytData
End Function


I'm finding that it generates an awful lot of overflow errors because the
value being returned to bytData(lNum) is not in a byte range. There seem to
be dozens of scenarios where varying the key and the key offset (seemingly
within the expected parameters) that bytData(lNum) Xor (bytKey(lNum) -
KEY_OFFSET) cannot return byte data OR that the final bytData cannot be
converted back to a string.

Am I missing something here?

Do I need to do some preliminary checks before setting the constants and
passing data into the function?

Regards

GPO

 

[Rob Bovey]

Hi GPO,

The general rule for modifying the obfuscation constants in this
function is that the KEY_OFFSET constant must be less than or equal to the
lowest ASCII character value in the KEY_TEXT string. So if KEY_TEXT =
"0123456789abcdefg" then KEY_OFFSET must be less than or equal to 48,
because that's the ASCII value of the character "0" in the KEY_TEXT string.


--
Rob Bovey, Excel MVP
Application Professionals
http://www.appspro.com/

* Take your Excel development skills to the next level.
* Professional Excel Development
http://www.appspro.com/Books/Books.htm

 

[GPO]

By Golly that was quick! Thanks Rob.

Hi GPO,

The general rule for modifying the obfuscation constants in this
function is that the KEY_OFFSET constant must be less than or equal to the
lowest ASCII character value in the KEY_TEXT string. So if KEY_TEXT =
"0123456789abcdefg" then KEY_OFFSET must be less than or equal to 48,
because that's the ASCII value of the character "0" in the KEY_TEXT string.


--
Rob Bovey, Excel MVP
Application Professionals
http://www.appspro.com/

* Take your Excel development skills to the next level.
* Professional Excel Development
http://www.appspro.com/Books/Books.htm

 

[GPO]

Hi again,

Even controlling for the prev issue, it also seems to fall over where the
offset is 0 and they key and data both start with the same char.

Cheers

GPO

 

[Rob Bovey]

Hi again,

Even controlling for the prev issue, it also seems to fall over where the
offset is 0 and they key and data both start with the same char.

Hi GPO,

I'm not seeing this. Can you give me an example where it happens? I set
KEY_OFFSET to zero and used various identical strings for both KEY_TEXT and
the data without any problem.

--
Rob Bovey, Excel MVP
Application Professionals
http://www.appspro.com/

* Take your Excel development skills to the next level.
* Professional Excel Development
http://www.appspro.com/Books/Books.htm

 

[eliano]

Hi Rob.
I confirm that your fine UDF work properly, but not with the
KEY_OFFSET = 0
Fine in all the other situation i tried.

Try:
oggi piove ma forse domani sarà bel tempo; il futuro comunque è nelle
mani di Zeus
0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
ScratchItYouFool

S.O = XP Office = XL2003

Regards
Eliano

 

[eliano]

Hi Rob.
Many thanks for the attention.
I am Italian and i use an Italian Office.
Up tomorrow i cannot try your UDF, but be sure that as soon as
possible i will reply to your courtesy.
Thanks again and regards.
Eliano

 

[eliano]

Hi Rob.
Your last test works perfectly also vith my (Italian) Cod Page.
However also the first version was enough for my necessities.
Thanks again and Best Regards.
Eliano
.

 

[Rob Bovey]

Hi Rob.
Your last test works perfectly also vith my (Italian) Cod Page.
However also the first version was enough for my necessities.

Hi Eliano,

Glad to hear it. Thanks for letting me know.

--
Rob Bovey, Excel MVP
Application Professionals
http://www.appspro.com/

* Take your Excel development skills to the next level.
* Professional Excel Development
http://www.appspro.com/Books/Books.htm