ASCII HEX decode?

[l0ve2hat3]

this is a comment on http://php.net/manual/en/function.hash-hmac.php

 

HOTP Algorithm that works according to the RCF http://tools.ietf.org/html/draft-mraihi-oath-hmac-otp-04
The test cases from the RCF document the ASCII string as "123456787901234567890".
But the hex decoded to a string is "12345678901234567890".
Secret="12345678901234567890";
Count:
0 755224
1 287082
<?php
function oath_hotp($key,$counter) {

   // Convert to padded binary string
   $data = pack ('C*', $counter);
   $data = str_pad($data,8,chr(0),STR_PAD_LEFT);

   // HMAC
   return hash_hmac('sha1',$data,$key);
}

function oath_truncate($hash, $length = 6) {

   // Convert to dec
   foreach(str_split($hash,2) as $hex) {
      $hmac_result[]=hexdec($hex);
   }

   // Find offset
   $offset = $hmac_result[19] & 0xf;

   // Algorithm from RFC
   return (
         (($hmac_result[$offset+0] & 0x7f) << 24 ) |
         (($hmac_result[$offset+1] & 0xff) << 16 ) |
         (($hmac_result[$offset+2] & 0xff) << 8 ) |
         ($hmac_result[$offset+3] & 0xff)
         ) % pow(10,$length);
}

print "<pre>";
print "Compare results with:"
print " http://tools.ietf.org/html/draft-mraihi-oath-hmac-otp-04\n";
print "Count\tHash\t\t\t\t\t\tPin\n";
for($i=0;$i<10;$i++)
   print $i."\t".($a=oath_hotp("12345678901234567890",$i))
   print "\t".oath_truncate($a)."\n";

                                                                 

How do they get "123456787901234567890" from "12345678901234567890"??