CODEWITHSUNDEEP
javaphpsha256
GJK
GJK

Reputation: 37389

Why do I sometimes get different SHA256 hashes in Java and PHP?

So I have an odd little problem with the hashing function in PHP. It only happens some of the time, which is what is confusing me. Essentially, I have a Java app and a PHP page, both of which calculate the SHA256 of the same string. There hasn't been any issues across the two, as they calculate the same hash (generally). The one exception is that every once in a while, PHP's output is one character longer than Java's.

I have this code in PHP:

$token = $_GET["token"];
$token = hash("sha256", $token."<salt>");
echo "Your token is " . $token;

99% of the time, I get the right hash. But every once in a while, I get something like this (space added to show the difference):

26be60ec9a36f217df83834939cbefa33ac798776977c1970f6c38ba1cf92e92 # PHP
26be60ec9a36f217df83834939cbefa33ac798776977c197 f6c38ba1cf92e92 # Java

As you can see, they're nearly identical. But the top one (computed by PHP) has one more 0 for some reason. I haven't really noticed a rhyme or reason to it, but it's certainly stumped me. I've tried thinking of things like the wrong encoding, or wrong return value, but none of them really explain why they're almost identical except for that one character.

Any help on this issue would be much appreciated.

EDIT: The space is only in the bottom one to highlight where the extra 0 is. The actual hash has no space, and is indeed a valid hash, as it's the same one that Java produces.

EDIT2: Sorry about that. I checked the lengths with Notepad++, and since it's different than my normal text editor, I misread the length by 1. So yes, the top one is indeed right. Which means that it's a bug in my Java code. I'm going to explore Ignacio's answer and get back to you.

Upvotes: 2

Views: 2949

Answers (2)

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799240

The top hash is the correct length; the bottom hash is output because the hexadecimal values were not zero-filled on output (note that it's the MSn of a byte). So, a bug in the Java program unrelated to the hash algorithm.

>>> '%04x %02x%02x %x%x' % (0x1201, 0x12, 0x01, 0x12, 0x01)
'1201 1201 121'

Upvotes: 4

LSerni
LSerni

Reputation: 57418

Actually it's the SECOND hash which seems to have an incorrect length (63). Could it be that it is generated by assembling two different tokens, and maybe the last one - which should be 16 characters - gets the initial zero removed?

Upvotes: 2

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