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Ruby Programming Language

Checking Credit Cards (#122)


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if you can.

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- =-=-=

Before a credit card is submitted to a financial institution, it generally makes
sense to run some simple reality checks on the number.  The numbers are a good
length and it's common to make minor transcription errors when the card is not
scanned directly.

The first check people often do is to validate that the card matches a known
pattern from one of the accepted card providers.  Some of these patterns are:

        +============+=============+===============+
        | Card Type  | Begins With | Number Length |
        +============+=============+===============+
        | AMEX       | 34 or 37    | 15            |
        +------------+-------------+---------------+
        | Discover   | 6011        | 16            |
        +------------+-------------+---------------+
        | MasterCard | 51-55       | 16            |
        +------------+-------------+---------------+
        | Visa       | 4           | 13 or 16      |
        +------------+-------------+---------------+

All of these card types also generate numbers such that they can be validated by
the Luhn algorithm, so that's the second check systems usually try.  The steps
are:

        1. Starting with the next to last digit and continuing with every other
           digit going back to the beginning of the card, double the digit
        2. Sum all doubled and untouched digits in the number
        3. If that total is a multiple of 10, the number is valid

For example, given the card number 4408 0412 3456 7893:

        Step 1:  8 4 0 8 0 4 2 2 6 4 10 6 14 8 18 3
        Step 2:  8+4+0+8+0+4+2+2+6+4+1+0+6+1+4+8+1+8+3 = 70
        Step 3:  70 % 10 == 0

Thus that card is valid.

Let's try one more, 4417 1234 5678 9112:

        Step 1:  8 4 2 7 2 2 6 4 10 6 14 8 18 1 2 2
        Step 2:  8+4+2+7+2+2+6+4+1+0+6+1+4+8+1+8+1+2+2 = 69
        Step 3:  69 % 10 != 0

That card is not valid.

This week's Ruby Quiz is to write a program that accepts a credit card number as
a command-line argument.  The program should print the card's type (or Unknown)
as well a Valid/Invalid indication of whether or not the card passes the Luhn
algorithm.

On Apr 27, 2007, at 8:59 PM, Ruby Quiz wrote:

For completeness and to make this Quiz exercise valid to more people,  
can anyone include the information for other major credit cards from  
major countries? Japan: JCB,  et. al., U.K.: Barclay, et. al. , etc...
This would be good to expand the exercise and return more to our  
friends in various areas!
Discover is itself not found outside of North America, AFIK.

> For completeness and to make this Quiz exercise valid to more people,
> can anyone include the information for other major credit cards from
> major countries? Japan: JCB,  et. al., U.K.: Barclay, et. al. , etc...

UK had Switch and Solo till recently which are now rebranded under the
Maestro umbrella. They're not really credit cards and basically no
rules exist, you have to check tables to determine valid prefixes and
card number length. Additionally, Maestro card numbers may also be
Mastercard numbers, so they can't be uniquely identified as being
Maestro.

For JCB and Diner's:

JCB       3528-2589  Length: 16
Diners   3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14

Cheers,
   -Tim

Ruby Quiz <j@grayproductions.net> writes:
> This week's Ruby Quiz is to write a program that accepts a credit
> card number as a command-line argument.  The program should print
> the card's type (or Unknown) as well a Valid/Invalid indication of
> whether or not the card passes the Luhn algorithm.

May I make an additional suggestion?  Treat spaces in the card number
as perfectly valid, and simply strip them before processing.  One of
the most irritating features found in many online stores is the
requirement that the credit card number be typed in with no spaces.
(The other biggie is using a select box for the state abbreviation)

Now granted, for accepting the card number from the command line this
will mean that on the command line the card number was quoted, but
presumably most people's programs will be a command-line wrapper
around a function/class that does the real work.

--
s=%q(  Daniel Martin -- mar@snowplow.org
       puts "s=%q(#{s})",s.to_a[1]       )
       puts "s=%q(#{s})",s.to_a[1]      

On Apr 27, 2007, at 7:36 AM, Daniel Martin wrote:

> Now granted, for accepting the card number from the command line this
> will mean that on the command line the card number was quoted...

It doesn't have to mean that:

$ ruby -e 'p ARGV.join' 1111 2222 3333 4444
"1111222233334444"

James Edward Gray II

On Apr 27, 2007, at 9:22 PM, Tim Becker wrote:

Wow, those are some broad ranges.

> > JCB       3528-2589  Length: 16

should be 3528-3589, obviously.

> > Diners   3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14
> Wow, those are some broad ranges.

Well, those ranges DO include Carte Blanche :) I haven't seen a
physical Diners Card in at least 20 years to be honest, not sure who
still uses them.

If you think those ranges are broad, the last UK-Switch/Solo table I
had has over 180 card prefixes with different card lengths...

On Apr 27, 2007, at 10:37 PM, Tim Becker wrote:

>> > JCB       3528-2589  Length: 16

> should be 3528-3589, obviously.

>> > Diners   3000-3029, 3040-3059, 36, 3815-3889, 389 Length: 14

>> Wow, those are some broad ranges.

> Well, those ranges DO include Carte Blanche :) I haven't seen a
> physical Diners Card in at least 20 years to be honest, not sure who
> still uses them.

> If you think those ranges are broad, the last UK-Switch/Solo table I
> had has over 180 card prefixes with different card lengths...

My grandmother used to have a Diners Club card. Last time I saw that  
was about 20 years ago too.
But of the cards accepted here in Tokyo, Diners Club is one that is  
often listed! (in bars and restaurants of course)

Oh, I can only imagine. Processing cards with less deeply thought out  
numbering schemes. The big boys planned that stuff back when  
computing power was a premium.

>         +============+=============+===============+
>         | Card Type  | Begins With | Number Length |
>         +============+=============+===============+
>         | AMEX       | 34 or 37    | 15            |
>         +------------+-------------+---------------+
>         | Discover   | 6011        | 16            |
>         +------------+-------------+---------------+
>         | MasterCard | 51-55       | 16            |
>         +------------+-------------+---------------+
>         | Visa       | 4           | 13 or 16      |
>         +------------+-------------+---------------+

So is a card number like "4012 3456 7890" a valid Unknown, or an invalid Visa?
Similarly, for Unknown, should we accept any length, or go for the common 16?
On Apr 27, 2007, at 12:58 PM, Matthew Moss wrote:

>>         +============+=============+===============+
>>         | Card Type  | Begins With | Number Length |
>>         +============+=============+===============+
>>         | AMEX       | 34 or 37    | 15            |
>>         +------------+-------------+---------------+
>>         | Discover   | 6011        | 16            |
>>         +------------+-------------+---------------+
>>         | MasterCard | 51-55       | 16            |
>>         +------------+-------------+---------------+
>>         | Visa       | 4           | 13 or 16      |
>>         +------------+-------------+---------------+

> So is a card number like "4012 3456 7890" a valid Unknown, or an  
> invalid Visa?

Visa Invalid

Always match the type first.

> Similarly, for Unknown, should we accept any length, or go for the  
> common 16?

Unknown is for any type that doesn't match, so it could be any length.

James Edward Gray II

So I took this as an opportunity to try doing a little TDD, something
which is appealing but I've never really tried.  It _is_ fun to see
failure first and then fix.  =)  There are probably other tests I
could have included, but here is a start, in case people want to check
a few numbers or add more tests:

Obviously, you may need to change this to match your solution...

require "test/unit"
require "ccard"

class CreditCardCheckTest < Test::Unit::TestCase

   def test_valid_numbers
      assert CreditCard.new("4408041234567893").valid?
      assert CreditCard.new("6011111111111117").valid?
   end

   def test_invalid_nubers
      assert !CreditCard.new("4417123456789112").valid?
   end

   def test_valid_numbers_with_spaces
      assert CreditCard.new("4408 0412 3456 7893").valid?
      assert CreditCard.new("6011 1111 1111 1117").valid?
   end

   def test_invalid_nubers_with_spaces
      assert !CreditCard.new("4417 1234 5678 9112").valid?
   end

   def test_valid_types
      assert_equal CreditCard.new("4408041234567893").type,    :visa
      assert_equal CreditCard.new("4408 0412 3456 7893").type, :visa
      assert_equal CreditCard.new("6011 1111 1111 1117").type, :discover
      assert_equal CreditCard.new("123456789").type,           :unknown
   end

   def test_invalid_types
      assert_nil CreditCard.new("4408 0412 3456 789").type
   end

end

James Edward Gray II wrote:

> On Apr 27, 2007, at 12:58 PM, Matthew Moss wrote:
>> So is a card number like "4012 3456 7890" a valid Unknown, or an
>> invalid Visa?

> Visa Invalid

> Always match the type first.

Nope thats no Visa number but a unknown one. Visa Cards have a certain
length (13 or 16) and if that length doesn't fit (Matthew's number here
had just 12 digits)it's unknown :) otherwise the length would be
obsolete don't you think?
Both length and startingbytes have equal weight in the decision wether a
card belongs to a known Company or not.

--
greets
                                       (
                                        )
                                       (
                                 /\  .-"""-.  /\
                                //\\/  ,,,  \//\\
                                |/\| ,;;;;;, |/\|
                                //\\\;-"""-;///\\
                               //  \/   .   \/  \\
                              (| ,-_| \ | / |_-, |)
                                //`__\.-.-./__`\\
                               // /.-(() ())-.\ \\
                              (\ |)   '---'   (| /)
                               ` (|           |) `
                         jgs     \)           (/

  one must still have chaos in oneself to be able to give birth to a
dancing star

On Apr 27, 2007, at 3:10 PM, anansi wrote:

> James Edward Gray II wrote:
>> On Apr 27, 2007, at 12:58 PM, Matthew Moss wrote:
>>> So is a card number like "4012 3456 7890" a valid Unknown, or an  
>>> invalid Visa?
>> Visa Invalid
>> Always match the type first.

> Nope thats no Visa number but a unknown one. Visa Cards have a  
> certain length (13 or 16) and if that length doesn't fit (Matthew's  
> number here had just 12 digits)it's unknown :) otherwise the length  
> would be obsolete don't you think?
> Both length and startingbytes have equal weight in the decision  
> wether a card belongs to a known Company or not.

You are right.  I didn't read well.  It's Unknown.

James Edward Gray II

There's no way to differentiate between a JCB starting with 34 and an
American Express card that starts with 34 given those starting numbers.

Raj

On Apr 27, 2007, at 8:39 PM, Raj Sahae wrote:

Tim corrected his typo:

>> > JCB       3528-2589  Length: 16

> should be 3528-3589, obviously.

So I don't think a leading 34 is ambiguous.  It would be American  
Express, with an expected length of 15 digits.

-Mark

Sorry, didn't see that email for some reason, even though it's right
here in my inbox. . .

Raj

On Apr 27, 2007, at 23:06 , James Edward Gray II wrote:

Do we display Valid/Invalid for unknown cards?

- And by the way: 4012 3456 7890 isn't valid is it? So should that  
not be "invalid Unknown" or "invalid Visa"?

/C

Christoffer Lern wrote:
>> James Edward Gray II

> Do we display Valid/Invalid for unknown cards?

We desplay unknown for unkown cards :) and valid/invalid for valid or
invalid cards, the first has nothing to do with the second.

Check first if it's a known one which depends on both the length and the
  starting bytes.  Then output if it's a known or unknown one.

After that, check the known or unknown number for validation and output
that result.

> - And by the way: 4012 3456 7890 isn't valid is it? So should that not
> be "invalid Unknown" or "invalid Visa"?

> /C

That's a unknown and invalid one. To be a Visa one it would need to be
13 or 15 bytes long.

--
greets
                                       (
                                        )
                                       (
                                 /\  .-"""-.  /\
                                //\\/  ,,,  \//\\
                                |/\| ,;;;;;, |/\|
                                //\\\;-"""-;///\\
                               //  \/   .   \/  \\
                              (| ,-_| \ | / |_-, |)
                                //`__\.-.-./__`\\
                               // /.-(() ())-.\ \\
                              (\ |)   '---'   (| /)
                               ` (|           |) `
                         jgs     \)           (/

  one must still have chaos in oneself to be able to give birth to a
dancing star

anansi wrote:

  > That's a unknown and invalid one. To be a Visa one it would need to be

> 13 or 15 bytes long.

13 or 16 of course...

--
greets
                                       (
                                        )
                                       (
                                 /\  .-"""-.  /\
                                //\\/  ,,,  \//\\
                                |/\| ,;;;;;, |/\|
                                //\\\;-"""-;///\\
                               //  \/   .   \/  \\
                              (| ,-_| \ | / |_-, |)
                                //`__\.-.-./__`\\
                               // /.-(() ())-.\ \\
                              (\ |)   '---'   (| /)
                               ` (|           |) `
                         jgs     \)           (/

  one must still have chaos in oneself to be able to give birth to a
dancing star

On Apr 28, 2007, at 7:43 AM, Christoffer Lern wrote:

I would go ahead and display Valid/Invalid normally.  That way people  
can use the program on cards we didn't cover, as long as they conform  
to the standard.

James Edward Gray II

On Fri, Apr 27, 2007 at 08:59:49PM +0900, Ruby Quiz wrote:
> The first check people often do is to validate that the card matches a known
> pattern from one of the accepted card providers.  Some of these patterns are:

>    +============+=============+===============+
>    | Card Type  | Begins With | Number Length |
>    +============+=============+===============+
>    | AMEX       | 34 or 37    | 15            |
>    +------------+-------------+---------------+
>    | Discover   | 6011        | 16            |
>    +------------+-------------+---------------+
>    | MasterCard | 51-55       | 16            |
>    +------------+-------------+---------------+
>    | Visa       | 4           | 13 or 16      |
>    +------------+-------------+---------------+

Wikipedia has a great chart showing all the prefixes of most known
credit cards, along with lengths and overlap between cards (currently
none).

http://en.wikipedia.org/wiki/Credit_card_number

enjoy,

-jeremy

--
========================================================================
 Jeremy Hinegardner                              jer@hinegardner.org

On Apr 27, 2007, at 7:59 AM, Ruby Quiz wrote:
<snippage>

>    1. Starting with the next to last digit and continuing with every  
> other
>       digit going back to the beginning of the card, double the digit
>    2. Sum all doubled and untouched digits in the number
>    3. If that total is a multiple of 10, the number is valid

> For example, given the card number 4408 0412 3456 7893:

>    Step 1:  8 4 0 8 0 4 2 2 6 4 10 6 14 8 18 3
>    Step 2:  8+4+0+8+0+4+2+2+6+4+1+0+6+1+4+8+1+8+3 = 70
>    Step 3:  70 % 10 == 0

> Thus that card is valid.

Uh, this is probably just affecting me but....

In his example, after he doubles the second to last digit (call it  
d), he uses mod10 on it (9*2 = 18 %10 = 8). That is the way to get  
his numbers, but a) where does he say that and b) where do the 10 and  
14 come from?

Help me, hyperactive ruby posters!
~ Ari
English is like a pseudo-random number generator - there are a  
bajillion rules to it, but nobody cares.

You move back to the beginning after doubling the 2nd to last. You  
double every other one on the way back to the beginning. The 10 is  
5*2 and the 14 is 7*2

On Apr 28, 2007, at 8:04 PM, Ari Brown wrote:

On Apr 28, 9:04 pm, Ari Brown <a@aribrown.com> wrote:

The confusing part (that I didn't catch when I read it) is that step 2
is to sum all the *digits*, not the numbers.

So
step 1) 9 * 2 = 18
step 2) 1 + 8

He's not modding the result of the multiplication by 10, but rather
adding up the resulting component digits. The same occurs with the 10
and 14 (which Philip pointed out are the result of 5*2 and 7*2,
respectively).

On 4/27/07, Ruby Quiz <j@grayproductions.net> wrote:

> This week's Ruby Quiz is to write a program that accepts a credit card number as
> a command-line argument.  The program should print the card's type (or Unknown)
> as well a Valid/Invalid indication of whether or not the card passes the Luhn
> algorithm.

This Ruby Quiz has been translated into Japanese.
Maybe there will be more people participating.

http://d.hatena.ne.jp/nappa_zzz/20070429

Harry

--
http://www.kakueki.com/ruby/list.html
A Look into Japanese Ruby List in English

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