Excel VBA: calculating checksums

[Henry]

I need to be able to calculate checksums (CRC32 algorighm) in Excel VBA
using Excel 2003. I have encrypted passwords and security groups from
another application, and they will have to login to an Excel form. I
cannot export the encrypted info into Excel since I have no way to
translate it back to its original value.

Instead, I thought if I first decrypted the data and replaced it with
checksums, the password wouldn't be brought into Excel, which would add
another measure of security for the application, since even if an
enterprising user went hunting and found both userlist files, the
password and security group would be different in each file, and there
would be no obvious key that would hint at how they are related.

I cannot, however, find any code that shows how to calculate checksums.
I'd be very grateful if someone could point me in the right direction.
Thanks!



Henry
DPM Mellon

 

[Leith Ross]

Hello Henry,

Here is the code for Luhn CRC. I know it isn't the 32 bit CRC
alogrithm. If you lnow the math for the 32 bit CRC then you can use the
Luhn CRC algorithm as a VBA model to construct the 32 bit CRC. Hope this
helps some.


Code:
--------------------
'This is the Luhn Formula as described in ISO/IEC 7812-1:1993.
'Its primary purpose is to ensure accurate entries of the credit card number
'during transactions. You can apply the same technique to other applications
'such as employee numbers or patient numbers. Using check digits for these
'numbers also ensures more accurate data entries.

Public Function CheckDigit(strNum As String) As Integer

Dim i As Integer
Dim iEven As Integer
Dim iOdd As Integer
Dim iTotal As Integer
Dim strOneChar As String
Dim iTemp As Integer

' Add digits in even ordinal positions
' starting from rightmost
For i = Len(strNum) - 1 To 2 Step -2

strOneChar = Mid$(strNum, i, 1)
If IsNumeric(strOneChar) Then
iEven = iEven + CInt(strOneChar)
End If
Next i

' Process digits in odd ordinal positions
' starting from rightmost
For i = Len(strNum) To 1 Step -2
strOneChar = Mid$(strNum, i, 1)
If IsNumeric(strOneChar) Then
' Double it
iTemp = CInt(strOneChar) * 2
If iTemp > 9 Then
' Break the digits (e.g., 19 becomes 1+9)
iOdd = iOdd + (iTemp \ 10) + (iTemp - 10)
Else
iOdd = iOdd + iTemp
End If
End If
Next i

' Add even and odd
iTotal = iEven + iOdd

' Return the 10's complement
CheckDigit = 10 - (iTotal Mod 10)

End Function

 

[Henry]

Thanks, Leith- that was fast.

Unfortunately, I don't know the math for the 32-bit version, and also
unfortunately, I don't have the luxury of being able to do a lot of
research, either. I've just had a project assigned to me that has a
very short deadline, and I have to pull a lot of things together to
enable different modules to draw info from each other. It now seems
that I'll have to resort to another plan - one that I can easily
implement in VBA that can be trusted to accurately decrypt VBA input so
I know that it matches the similarly encrypted data, coming from FoxPro.

Thanks again. I appreciate it.


Henry

 

[mrjames]

Hello Henry,

Here is the code for Luhn CRC. I know it isn't the 32 bit CRC
alogrithm. If you lnow the math for the 32 bit CRC then you can use the
Luhn CRC algorithm as a VBA model to construct the 32 bit CRC. Hope this
helps some.
Code:
--------------------
'Its primary purpose is to ensure accurate entries of the credit card number
'during transactions. You can apply the same technique to other applications
'such as employee numbers or patient numbers. Using check digits for these
'numbers also ensures more accurate data entries.

Public Function CheckDigit(strNum As String) As Integer

Dim i As Integer
Dim iEven As Integer
Dim iOdd As Integer
Dim iTotal As Integer
Dim strOneChar As String
Dim iTemp As Integer

' Add digits in even ordinal positions
' starting from rightmost
For i = Len(strNum) - 1 To 2 Step -2

strOneChar = Mid$(strNum, i, 1)
If IsNumeric(strOneChar) Then
iEven = iEven + CInt(strOneChar)
End If
Next i

' Process digits in odd ordinal positions
' starting from rightmost
For i = Len(strNum) To 1 Step -2
strOneChar = Mid$(strNum, i, 1)
If IsNumeric(strOneChar) Then
' Double it
iTemp = CInt(strOneChar) * 2
If iTemp > 9 Then
' Break the digits (e.g., 19 becomes 1+9)
iOdd = iOdd + (iTemp \ 10) + (iTemp - 10)
Else
iOdd = iOdd + iTemp
End If
End If
Next i

' Add even and odd
iTotal = iEven + iOdd

' Return the 10's complement
CheckDigit = 10 - (iTotal Mod 10)

End Function
--------------------
Sincerely,
Leith Ross

Hi Leith Ross
I hope you are still around, i needed your Luhn digit checksum urgently
but it did not calculate the way i wanted it to when i loaded it to
Excel. I think there are a few ways of doing the sum.

I found the formula that calculates the way i wanted although it looks
to be the same as the one you done a whle ago:
Luhn Check Digit




Calculation

Step 1 Double the values of the odd labelled digits of the IMEI.
Step 2a Add together all the individual digits obtaind in step 1.
Step 2b Add the sum obtained in Step 2a together with the sum of all
even labelled digits of the IMEI
Step 3 If the number obtained in Step 2b end with '0' then the Luhn
Check Digit will be 0, otherwise the Luhn Check Digit will be the
substraction of the result from Step 2b with the next higher number
ending with a zero.


Example

Enter IMEI :

D14 D13 D12 D11 D10 D09 D08 D07 D06 D05 D04 D03 D02 D01
4 9 0 1 5 4 2 0 3 2 3 7 5 1

Calculation
Step 1:
18 2 8 0 4 14 2
Step 2a+b
( 1+8 + 2 + 8 + 0 + 4 + 1+4 + 2 ) + ( 4 + 0 + 5 + 2 + 3 + 3 + 5 )
= 52
Step 3:
Luhn Digit : 8
IMEI :
490154-20-323751-8


Please help me as i am not very good with computer stuff.

 

[Leith Ross]

Hello Mrjames,

Here are 2 macros that should help you out. One will verify if a
checksum sequence is correct, and the other will generate a checksum
digit for a sequence, and if the sequence is valid, no checksum digit
needed, it will return a -1.

Using the number you provided we will get the following result:
Cell A1 formula: =IsLuhnChecksumOK("49015420323751")
Returns TRUE. This sum of the digits equals 50, not 52.

How the Luhn algorithm functions:
1) Starting with the second to last digit, multiply every other digit
by 2.

2) If the product is greater than 10 then subtract 9 from the product.
This produces the same result as adding the digits of number greater
than 9. Sum these values.

3) Sum all the unmultiplied digits and add the sum from step 2. The
result is valid if it is an even mutliple of 10.

4 9 0 1 5 4 2 0 3 2 3 7 5 1

Step 1 - double the digits
5(2) 3(2) 3(2) 2(2) 5(2) 0(2) 4(2)

Step 2 - sum and adjust the doubled digits
(10 - 9) + 6 + 6 + 4 + (10 - 9) + 0 + 8 = 26

Step 3 - sum the digits not doubled and the doubled digits. Test is
total is evenly divided by 10
1 + 7 + 2 + 0 + 4 + 1 + 9 = 24
24 + 26 = 50
10 Mod 50 = 0


Code:
--------------------

Public Function IsLuhnChecksumOK(Number_String As String) As Boolean

Dim Digit As Integer
Dim i As Integer
Dim N As Integer
Dim Result As Integer
Dim SumDigits As Integer
Dim nDigits As Integer

SumDigits = 0
nDigits = Len(Number_String)

For i = nDigits To 1 Step -1
Digit = CInt(Mid(Number_String, i, 1))
N = N + 1
If N Mod 2 = 0 Then Digit = Digit * 2
If Digit > 9 Then Digit = Digit - 9
SumDigits = SumDigits + Digit
Next i

Result = SumDigits Mod 10

If Result = 0 Then
IsLuhnChecksumOK = True
End If

End Function

Public Function GetLuhnCheckDigit(Number_To_Check As String)

Dim J As Integer
Dim X

X = IsLuhnChecksumOK(Number_To_Check)
If X = False Then
For J = 0 To 9
X = IsLuhnChecksumOK(Number_To_Check & J)
If X = True Then
'Check digit found
GetLuhnCheckDigit = J
Exit Function
End If
Next J
Else
'No check digit needed returns -1
GetLuhnCheckDigit = -1
End If

End Function

--------------------


If you have any further questions, you can emial me at
(e-mail address removed).

Sincerely,
Leith Ross