CODEWITHSUNDEEP
javaphphashshahmac
Marco Gagliardi
Marco Gagliardi

Reputation: 575

php's hmac sha256 implementation mismatches java's one

i'm trying to reproduce in php the test cases mentioned in the official of totp computation reference (https://www.rfc-editor.org/rfc/rfc6238 Appendix A) which are written in java. The reference provides example for sha1, sha256 and sha512 algorithm.

I've found this nice example by Rob Swan (see the 8 digit example) that reproduces fine one test case (with sha1). But if I change the algorithm to sha256 or sha512 (and change the seeds too, according to the reference input data) I got different results from the reference's ones.

May the hmac hash function of php be different from java's one?

Thank you!

Upvotes: 1

Views: 1878

Answers (1)

Marco Gagliardi
Marco Gagliardi

Reputation: 575

* SOLUTION

Here is a copy of the php implementation by Rob Swan I mentioned:

<?php

// Define your secret seed
// NB: this is a hexadecimal representation of the example
// ASCII string which is: 12345678901234567890
$secret_seed = "3132333435363738393031323334353637383930";

// Determine the time window as 30 seconds
$time_window = 30;

// Set the timestamp manually
$exact_time = 1111111109;

// Round the time down to the time window
$rounded_time = floor($exact_time/$time_window);

// Pack the counter into binary
$packed_time = pack("N", $rounded_time);

// Make sure the packed time is 8 characters long
$padded_packed_time = str_pad($packed_time,8, chr(0), STR_PAD_LEFT);

// Pack the secret seed into a binary string
$packed_secret_seed = pack("H*", $secret_seed);

// Generate the hash using the SHA1 algorithm
$hash = hash_hmac ('sha1', $padded_packed_time, $packed_secret_seed, true);

// NB: Note we have change the exponent in the pow function 
// from 6 to 8 to generate an 8 digit OTP not a 6 digit one 

// Extract the 8 digit number fromt the hash as per RFC 6238
$offset = ord($hash[19]) & 0xf;
$otp = (
    ((ord($hash[$offset+0]) & 0x7f) << 24 ) |
    ((ord($hash[$offset+1]) & 0xff) << 16 ) |
    ((ord($hash[$offset+2]) & 0xff) << 8 ) |
    (ord($hash[$offset+3]) & 0xff)
) % pow(10, 8);

// NB: Note that we are padding to 8 characters not 6 for this example

// Add any missing zeros to the left of the numerical output
$otp = str_pad($otp, 8, "0", STR_PAD_LEFT);

// Display the output, which should be 
echo "This should display 07081804: " . $otp;

?>

The key point is this line: 

$offset = ord($hash[19]) & 0xf;

This works fine under the hypotesis of using sha1 algorithm, which returns a 20 chars string.

To abstract the line and make it compatible with any other algorithm change this line to:

$offset = ord($hash[strlen($hash)-1]) & 0xf;

Now you have a general and working php version of RFC 6238 totp calculation!

Upvotes: 1

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