Credit card security

[ntsa]

Brazil 2 - England 1

Oh well. I'll try cheering myself up with a tutorial...

Outline

In this tutorial I am going to cover two types of credit card validation. The first type of validation is a LUHN check. The second validation method that I cover examines the use of a cards.ini text file for credit card type validation.

LUHN checking

Most credit card numbers (including Visa and Mastercard) conform to the LUHN checksum algorithm. You can calculate the last digit of a card number, by examining all the preceeding digits. You cannot tell from this if the card is stolen, but it can be used to check if the user has made a mistake when entering a card number.

In this article I am publishing both my ASP and PHP LUHN checking functions.

Card Type validation

The second type of validation is checking that the selected card type is correct. Each card type - Visa, mastercard etc - has a given length and prefix. For exampe Visa cards have a prefix of 4 and a length of either 13 or 16 characters. Because of this you can validate that the card type that has been selected by the user is accurate.

This part of the code is only provided in ASP - Sorry

A explaination of the LUHN algorithm

The following steps are involved in this calculation:

Step 1: Double the value of alternate digits beginning with the first
right-hand digit (low order).

Step 2: Add the individual digits comprising the products obtained in
step 1 to each of the unaffected digits in the original number.

Step 3: Subtract the total obtained in step 2 from the next higher
number ending in 0 [this in the equivalent of calculating the
"tens complement" of the low order digit (unit digit) of the total].
If the total obtained in step 2 is a number ending in zero
(30, 40 etc.), the check digit is 0.

Example:

Account number without check digit: 4992 73 9871

4 9 9 2 7 3 9 8 7 1
x2 x2 x2 x2 x2
----------------------------
18 4 6 16 2

4 + 1 + 8 + 9 + 4 + 7 + 6 + 9 + 1 + 6 + 7 + 2 = 64

70 - 64 = 6

Account number with check digit 4992 73 9871 6

From: http://staff.semel.fi/~kribe/document/luhn.htm

LUHN Check - ASP version

Example Usage

CardIsValid = true
if card.luhncheck(p_CardNo) = true then
debug "luhn ok"
else
debug "luhn failed"
CardIsValid = false
end if

The ASP Code!

'ASP ISO 7812:1987(E) LUHN Checker
	'This function was written by Simon Barnett. (si@ntsa.org.uk)
	'Use and distribution of this software is covered
	'by open source licence (http://www.opensource.org/osd.html).
	'Please include these comments in any distribution.  

Function LuhnCheck(p_cardno)

   session("debug") = "false"
   
   if isnumber(p_cardno) = false then
   	luhncheck = false
   else
   	if len(p_cardno) = 0 then
   		luhncheck = false
   	else
   		
   		tot = 0
   		chk = right(p_cardno,1)
   		p_cardno = left(p_cardno,len(p_cardno)-1)
   
   		for n = len(p_cardno) to 1 step - 2
   			x = cint(mid(p_cardno,n,1))  * 2
   			if len(x) > 1 then
   				tot = tot + cint(mid(x,1,1))
   				'debug mid(x,1,1)
   				tot = tot + cint(mid(x,2,1))
   				'debug mid(x,2,1)
   			else
   				tot = tot + cint(mid(x,1,1))
   				'debug mid(x,1,1)
   			end if		
   		next
   
   		for n = len(p_cardno)-1 to 1 step - 2
   			tot = tot + cint(mid(p_cardno,n,1))
   			'debug mid(p_cardno,n,1)
   		next
   
   		'debug tot
   
   		checkdigit = cint(cstr(cint(left(cstr(tot),1))+1) & "0") - tot
   
   		debug "Checkdigit should be "&checkdigit
   		debug "Actual Check Digit "&chk
   
   		if int(chk) = int(checkdigit) then
   			luhncheck = true
   		else
   			luhncheck = false
   		end if
   
   	end if
   end if

End Function

LUHN Check - PHP version

Example Usage

// <EXAMPLE USAGE>

if(ValidCard($credit_card)) // Pass credit card details without spaces.
{
echo "Valid Card";
}
else
{
echo "Invalid Card";
}

The PHP Code!

<?php

function ValidCard($credit_card) 
/*	PHP ISO 7812:1987(E) LUHN Checker
	This function was written by Simon Barnett. (si@ntsa.org.uk)
	Use and distribution of this software is covered
	by open source licence (http://www.opensource.org/osd.html).
	Please include these comments in any distribution.  
*/

{ 
	$checksum = substr ($credit_card, -1);    // returns checksum
	$restnum = substr($credit_card, 0, strlen($credit_card)-1); 
                // returns rest of credit card number

	$calcvals = "";

	for ($i = strlen($restnum)+1; $i >= 0; $i=$i-2) {	
		$calcvals = $calcvals . strval(intval(substr($restnum,$i,1))*2);
	}

	for ($i = strlen($restnum) -2; $i >= 0; $i=$i-2) {	
		$calcvals = $calcvals . strval(intval(substr($restnum,$i,1))*1);
	} 

	$calcvals = substr($calcvals,1);
	$count = 0;

	for ($i = 0; $i <= strlen($calcvals); $i++) {
		$count = $count + intval(substr($calcvals,$i,1));
	}

	if(intval(substr(strval((10*(intval(substr(strval($count),0,1))+1)-$count)),-1))==$checksum)
	{
		return true;
	}
	else
	{
		return false;
	}
} 

?>

Checking the type of card (ASP)

I didn't want to hard code the card type information for obvious reasons, not least amongst which is the fact that I'm using code similar to this over a large number of sites, each with their own list of accepted cards. So I stored all the information about the fornat of each of the different credit card companies in an ini file. This ini file is shown below.

The cards.ini file

[Visa]
prefix=4
length=13,16

[Mastercard]
prefix=51,52,53,54,55
length=16

[American Express]
prefix=34,37
length=15

[Dinners Club]
prefix=300,301,302,303,304,305,36,38
length=14

[JCB]
prefix=3
length=16

[Discover]
prefix=6011
length=16

Displaying the list of accepted cards

The CardList routine below reads the cards.ini file given above and displays each of the credit card types in a drop down combo box. The returned 'CardType' variable is used validating the card type in the FormatCheck Function.

Sub CardList()

   cards = split(getcards(),",")
   
   response.write " <select name='CardType' size='1'>"
   response.write "     <option selected value=''>&lt;-Please Select-&gt;</option>"
   
   n = 0
   on error resume next
   while err.number = 0 
   	response.write "     <option value='"& cards (n) &"'>"& cards (n) &"</option>"
   	n = n + 1
   wend
   
   response.write "   </select>"

End Sub

Function GetCards()

   Dim fso, MyFile
   Set fso = CreateObject("Scripting.FileSystemObject")
   Set MyFile = fso.OpenTextFile(replace(request("inipath"),"*","\"), ForReading)
   
   getcards = ""
   while not myfile.atendofstream
   	l = MyFile.ReadLine
   	if left(l,1) = "[" then
   		if getcards = "" then
   			getcards = mid(l,2,len(l)-2)
   		else
   			getcards = getcards & ","  & mid(l,2,len(l)-2)
   		end if
   	end if
   wend

End Function

Validating the selected card type

Example Usage

CardIsValid = true
if card.formatcheck(p_CardNo,p_CardType) = true then
debug "Format ok"
else
debug "Format failed"
CardIsValid = false
end if

The Code!

Function FormatCheck(p_CardNo, p_CardType)

   Dim fso, MyFile
   Set fso = CreateObject("Scripting.FileSystemObject")
   Set MyFile = fso.OpenTextFile(replace(request("inipath"),"*","\"), ForReading)
   
   f=0
   debug "Opening Cards.ini file..."
   while not myfile.atendofstream
   	l = MyFile.ReadLine
   	if left(l,1) = "[" then
   		if mid(l,2,len(l)-2) = p_cardtype then
   			f = 1
   			debug "found " & mid(l,2,len(l)-2)
   			prefix = split(trim(replace(MyFile.ReadLine,"prefix=","")),",")
   			length = split(trim(replace(MyFile.ReadLine,"length=","")),",")
   		end if
   	end if
   wend
   
   myfile.close
   
   if f = 0 then 
   	formatcheck = false
   else
   	prefixok = false
   	n = 0
   	on error resume next
   	while err.number = 0
   		debug "Check Prefix:" & prefix(n)
   		if left(p_cardno,len(prefix(n))) = prefix(n) then
   			debug left(p_cardno,len(prefix(n))) &"="& prefix(n)
   			if err.number = 0 then
   				prefixok = true
   			end if
   		end if
   		n = n +1
   	wend
   
   	lengthok = false
   	n = 0
   	on error resume next
   	while err.number = 0
   		debug "check len: " &  int(length(n))-1
   		
   		if len(p_cardno) = int(length(n))-1 then
   			if err.number = 0 then
   				debug len(p_cardno) &"="& int(length(n))-1
   				lengthok = true
   			end if
   		end if
   		n = n +1
   	wend
   
   	if lengthok = true and prefixok = true then
   		formatcheck = true
   	else
   		formatcheck = false
   	end if
   
   end if

End Function

If you liked this tutorial check out these others by ntsa :
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Forced shutdown of a remote nt/2k server
Securing an installation of IIS 4. (No, seriously)
Remote DSN Connections, using WinAPIs and the registry
Scripting Internet Connections Under Window$

[ntsa]

This is the article and code zipped for your ofline pleasure. The zip contains.
1...Credit card security.htm (The above doc)
2...CreditCardCls.inc (the ASP class the above routines are from)
3...cc.php (the PHP LUHN checking algorithm)
4...cards.ini (the ini file for card types show in the article above)

[5150]

you have too many good tutorials. i would give you greenies but it wont let me.

so i am guessing that if you use this alogorthm to generate a number it could possibly be a valid number.

Thats pretty nuts seeing that someone actually cracked credit card numbers

[ntsa]

Yep - you could write this the other way to generate a list of valid credit card numbers, so these checks described here can't be the /only/ checks you make. But they're useful routines none the less for form validation. For example I reccomend that my customers check the name / postcode / address against the electoral register (which is availible on CD) and then PDQ the card itself prior to despatching any goods.

Just to speculate... if you were to write it the other way you would be better to list all of the valid numbers starting from the number of a real credit card. Such a piece of code would look like this:

range =1
for x = 1 to cardlen -1
   range = range * 10
next

for n = realcardnum to realcardnum + range
   if LuhnCheck(p_cardno) = true then
      echo n
   end if
next n

That way you'd be much more likley to hit number that won't be rejected as an invalid card.

As I say...just speculation .

[chsh]

Nice tut NTSA. You write very nice code.

It should be noted that generating credit card numbers will not let you buy things anywhere, and that this algorithm is publicly available. As anyone with a CC # knows, you must know the expiration date of the card in order to purchase something, this is an often overlooked detail when people say "a cracker broke into our database and stole all the credit card numbers". It's not just the numbers, it's the Expiry date, and the owner, etc..

Most CC validation services nowadays also require you to enter your phone number and address. For instance, I ordered some BOFH stuff from the Register a few weeks ago, and it required not only my credit card number and expiry date, but also my Address and phone number.

[ntsa]

I don't believe it! Someone gave me reds for this!

germany won,**** pom!!!!!!!!!!!! - contact user about this assignment

Well, thank you for the inciteful and literate comments - NOT.

I will not lower myself to your level by responding with any similar racial or cultural slur, because such is not in my nature, but I would go so far as to say that you, sir, are an arse.

[ntsa]

Originally posted here by chsh
Nice tut NTSA. You write very nice code.

Thanks