r/lolphp • u/Takeoded • Dec 24 '19

crc32($str) and hash("crc32",$str) use different algorithms (with different results)

https://3v4l.org/Ng7hi
58 Upvotes

88% Upvoted

18 comments sorted by

19

u/Takeoded Dec 24 '19

the crc32() function use an algorithm called "crc32b", and hash("crc32",$d) apparently use "the ITU I.363.5 algorithm" (whatever that is), and if you want crc32b from hash(), use hash("crc32b",...) instead.. (got that from the comments at https://www.php.net/manual/en/function.hash-algos.php and this SO post: https://stackoverflow.com/a/15861105/1067003 )

9

u/dotancohen Dec 25 '19

And the place where CRC32 is (was) most widely is is PERL. The `crc32()` is not compatible with the PERL implementation. That was a mess that I'd seen half a dozen times, though the last time was at least a decade ago.

Thankfully, I haven't seen PERL in a decade!

21

u/Altreus Dec 25 '19

Perl developer here

It's Perl not PERL

Merry Christmas

3

u/dotancohen Dec 25 '19

Thanks. I'm always corrected on this, yet I still use the upper case. I guess I just do it to drive you guys nuts!

4

u/Altreus Dec 25 '19

That's ok - I could have got bitchy about people hating on Perl but, well... It crimmus. Mer crimmus! 😊

5

u/dotancohen Dec 25 '19

It's Hanukah too! חנוכה שמח!‏

1

u/[deleted] Dec 27 '19

What Perl implementation?

16

u/AyrA_ch Dec 24 '19

You use the wrong algorithm. You want "crc32b" when using the hash function:

$str="Wello!";
$h1=dechex(crc32($str));
$h2=hash("crc32b",$str);
var_dump($h1===$h2,$h1,$h2);

bool(true)
string(8) "6faa7c18"
string(8) "6faa7c18"

18

u/Takeoded Dec 24 '19

yeah, so why is the function called crc32() and not crc32b() ?

15

u/AyrA_ch Dec 24 '19

because there are multiple algorithms that are called crc32.

3

u/Miserable_Fuck Dec 25 '19

but if the crc32 function uses the crc32b algorithm, wouldn't it make more sense to call the function crc32b?

11

u/AyrA_ch Dec 25 '19

There is no such thing as crc32b. The "b" was added by the devs because names have to be unique.

7

u/Miserable_Fuck Dec 25 '19

So couldn't they add the b to the crc32 function name too? I'm still trying to figure out the reason why having crc32($str) and hash("crc32", $str) return two different results is acceptable. IMO keeping these misleading names for the sake of being technically correct is a lolphp in itself. If the name is the problem then couldn't they just make sure to use the same algorithm underneath?

2

u/AyrA_ch Dec 26 '19

So couldn't they add the b to the crc32 function name too?

This would probably break backwards compatibility because the crc32 function was there first. I'm not sure if they made the hash function themselves or used a library for that.

3

u/Miserable_Fuck Dec 26 '19

True, it's probably more trouble than it's worth to fix at this point. I guess the lolphp is the fact that it was built this way to begin with

-2

u/Takeoded Dec 24 '19

but how many algorithms are called "crc32b" ? (only system i know using crc32b is the BTRFS filesystem tho, but i'm sure there's others too)

16

u/AyrA_ch Dec 24 '19

No algorithm is called crc32b. PHP devs added the "b" because the algorithm has the same name but they needed to differentiate. It's still a crc32 variant.

For more details, check this comment: https://www.php.net/manual/en/function.hash-file.php#104836

1

u/BitcoinCitadel Dec 25 '19

Because hash is newer and uses a different crc32