PowerShell LUHN validation

 

I came across an interesting security post on Detecting Credit Card Numbers in Network Traffic. Section 2.3 caught my eye and 2 seconds later I was on WIKI's Luhn algorithm page. Soon enough I found myself converting the code to PowerShell. So, pull out your credit card and check its number.

 

#######################################################
## Returns true if the given array of digits represents 
## a valid Luhn number, and false otherwise.

function Test-LuhnNumber([int[]]$digits){
     
    [int]$sum=0
    [bool]$alt=$false

    for($i = $digits.length - 1; $i -ge 0; $i--){
        if($alt){
            $digits[$i] *= 2
            if($digits[$i] -gt 9) { $digits[$i] -= 9 }
        }

        $sum += $digits[$i]
        $alt = !$alt
    }

    return ($sum % 10) -eq 0
}


#######################################################
## Generates a number that passes the Luhn algorithm

function New-LuhnNumber([int]$length){
    
    $random = new-object random
    $digits = new-object int[] $length

    for($i = 0; $i -lt $length - 1; $i++){
        $digits[$i] = $random.next(10)
    }    

    [int]$sum = 0;
    [bool]$alt = $true

    for($i = $length - 2; $i -ge  0; $i--){
        
        if($alt){
            [int]$temp = $digits[$i]
            $temp *= 2
            if($temp -gt 9){ $temp -= 9 }
            $sum += $temp
        } else {
            $sum += $digits[$i]
        }
    
        $alt = !$alt
    }
    
    [int]$modulo = $sum % 10
    if($modulo -gt 0) { $digits[$length-1] = (10 - $modulo) }

    return $digits
}

## testing...
PS > Test-LuhnNumber (0..9)
False

PS > Test-LuhnNumber (New-LuhnNumber 10)
True

PS > Test-LuhnNumber (New-LuhnNumber 15)
True

PS > Test-LuhnNumber (4,4,6,6,6,7,6,5,1)
True

PS > Test-LuhnNumber @(2,8,6,3,7,7,1,1,2)
False

PS > Test-LuhnNumber @(0,6,6,2,4,9,1,8,9,2,3)
True

PS > Test-LuhnNumber "0","6","6","2","4","9","1","8","9","2","3"
True

PS > Test-LuhnNumber ([string[]][char[]]"67276171468")
True