PHP: md5('240610708') == md5('QNKCDZO') • Hacker News Board

Re: PHP: md5('240610708') == md5('QNKCDZO')

#51

Post by eurleif » Mon, May 04, 2015, 10:22 AM UTC

WalterGR wrote: post #45

eurleif wrote: post #38

Earlier quoted context omitted.

PHP: var_export(0 == "hello"); // true JavaScript: console.log(0 == "hello"); // false

Actually, I was hoping for something more than a single example. Or, did you mean that PHP and JavaScript were neck-and-neck all the way up to that one example, and ultimately it's the very one that proves PHP's type coercion is worse?

You're in a thread about how PHP's type coercion can easily cause a serious vulnerability. So, the title of this thread is your second example. If you want a third example, find it yourself.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#52

Post by viraptor » Mon, May 04, 2015, 10:23 AM UTC

seanp2k2 wrote: post #46

viraptor wrote: post #20

Earlier quoted context omitted.

Well, either in the example. Parent was saying "Reminds me on bash" For sh version, I'd go with super-safe: if test "x$1" = "x$2"

If you're doing that, even better to use "x${1}" to be safer. Also, conditional expressions ( [[ instead of [ or `test`) are generally a bit more well-behaved. See http://wiki.bash-hackers.org/syntax/ccmd/conditional_express... for more info.

But [[ is a bashism - it won't work on bare sh.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#53

Post by [deleted] » Mon, May 04, 2015, 10:28 AM UTC

drvortex wrote: post #29

So this is a PHP fail. But all the same, MD5 has been shown to fail collision resistance several times now.

[deleted]

Re: PHP: md5('240610708') == md5('QNKCDZO')

#54

Post by DoubleMalt » Mon, May 04, 2015, 10:29 AM UTC

dietrichepp wrote: post #6

I'm not exactly clear on how PHP == works, but you can see the MD5 for yourself: $ echo -n 240610708 | md5sum 0e462097431906509019562988736854 - $ echo -n QNKCDZO | md5sum 0e830400451993494058024219903391 - $ echo -n aabg7XSs | md5sum 0e087386482136013740957780965295 - All of them start with 0e, which makes me think that they're being parsed as floats and getting converted to 0.0. This is why "magic" operators like =…

Once I wrote a little PHP application to manage a clan in a browser game. I used an MD5 hash as session id that I checked with if(session_id)

When users started reporting that their logins would sometimes not work at the first time, I found out that strings that start with zero are coerced to 0 and then interpreted as false.

Never used PHP for anything important since.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#55

Post by dietrichepp » Mon, May 04, 2015, 10:33 AM UTC

VMG wrote: post #40

dietrichepp wrote: post #6

I'm not exactly clear on how PHP == works, but you can see the MD5 for yourself: $ echo -n 240610708 | md5sum 0e462097431906509019562988736854 - $ echo -n QNKCDZO | md5sum 0e830400451993494058024219903391 - $ echo -n aabg7XSs | md5sum 0e087386482136013740957780965295 - All of them start with 0e, which makes me think that they're being parsed as floats and getting converted to 0.0. This is why "magic" operators like =…

Same goes for the `0E` prefix with an uppercase E The likelihood of generating a hash value with that kind of prefix is 2 in 65536. Finding a collision `hash(a) == hash(b)` with this "weak" equality comparison is approximately 1 in 256 if I'm not mistaken.

You are mistaken: my guess is that you are taking the square root because of the birthday paradox, but that is incorrect, and the birthday paradox does not apply here anyway.

The probability of generating a hash with the right prefix is 10 in 16^3, or about 0.25%. Finding a 0e... == 0e... collision has probability ~6e-6, if both inputs are random. The chance that two hashes collide in this way given N random inputs is 1-(1-p)^(N-1), for N>0.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#56

Post by pdpi » Mon, May 04, 2015, 10:34 AM UTC

z92 wrote: post #39

Rygu wrote: post #21

Earlier quoted context omitted.

> in_array(.., .., $strict) I think you're aware of the third parameter but for anyone who reads this post, it disables the type coercion of the in_array call.

Spoiling a good rant with facts.

The fact that you need to specify a third, optional parameter to get sane output out of a really basic function is still pretty rant-worthy.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#57

Post by dietrichepp » Mon, May 04, 2015, 10:35 AM UTC

masklinn wrote: post #50

VMG wrote: post #40

Earlier quoted context omitted.

Same goes for the `0E` prefix with an uppercase E The likelihood of generating a hash value with that kind of prefix is 2 in 65536. Finding a collision `hash(a) == hash(b)` with this "weak" equality comparison is approximately 1 in 256 if I'm not mistaken.

> The likelihood of generating a hash value with that kind of prefix is 2 in 65536. The prefix is not sufficient though, the suffix must be entirely decimal otherwise it's not a valid number in scientific notation.

The prefix is sufficient. Any hash matching /0e[0-9].*/ works.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#58

Post by masklinn » Mon, May 04, 2015, 10:36 AM UTC

drvortex wrote: post #29

So this is a PHP fail. But all the same, MD5 has been shown to fail collision resistance several times now.

The problem is this is an issue with any comparison of hex digit strings. It's a possible issue with any hash function not just MD5.

Re: PHP: md5('240610708') == md5('QNKCDZO')

#59

Post by masklinn » Mon, May 04, 2015, 10:43 AM UTC

dietrichepp wrote: post #57

masklinn wrote: post #50

Earlier quoted context omitted.

> The likelihood of generating a hash value with that kind of prefix is 2 in 65536. The prefix is not sufficient though, the suffix must be entirely decimal otherwise it's not a valid number in scientific notation.

The prefix is sufficient. Any hash matching /0e[0-9].*/ works.

As far as I can see the prefix is not sufficient, a single non-digit character in the tail fails the conversion (and the equality check): http://3v4l.org/ctASF (vs http://3v4l.org/5FvJu, exact same strings but for the last character replaced by a digit)

Re: PHP: md5('240610708') == md5('QNKCDZO')

#60

Post by Arkanosis » Mon, May 04, 2015, 10:48 AM UTC

seanp2k2 wrote: post #47

Arkanosis wrote: post #17

Earlier quoted context omitted.

Actually, you don't prefix with “x” to handle empty vars, but special characters, as Stephane Chazelas recently reminded: http://www.zsh.org/mla/workers/2015/msg00797.html

Again here conditional expressions should make this a non-issue ( [[ instead of [ ) since the stuff inside doesn't get parsed the same as general input. See http://wiki.bash-hackers.org/syntax/ccmd/conditional_express...

Yes, but then you need either bash or zsh. It won't work on bare sh (or on dash, which is the default /bin/sh on Debian and derivatives like Ubuntu).