Set Byte Array Position To Arbitrary Integer Value

Question

I'm implementing an AES encryption algorithm, and I need to come up with a way to increment my iv. I'm stuck on a pretty simple question:

If I have a 16-byte, randomly generated array, how can I set a specific byte to an arbitrary value?

For example, say that I just want to arbitarily set my least-significant byte to 0xff:

<?php

$bytes = openssl_random_pseudo_bytes(16);
echo bin2hex($bytes) . "<br>";

$bytes[15] = 0xff; // arbitrarily set this byte
echo bin2hex($bytes) . "<br>";

?>

This yields output like this (clearly wrong):

9299dd089611fa47f130c4e92aaa09dc
9299dd089611fa47f130c4e92aaa0932

I'm trying to get to this output:

9299dd089611fa47f130c4e92aaa09ff

I've been at this for hours, and I just can't figure it out. Can anyone help? Thank you.

EDIT: I've also tried to output the array like this: echo var_dump(unpack('C*', $bytes));

This yields output like this (these numbers won't cross reference to what you see above, but just focus on the last byte):

array(16) { [1]=> int(242) [2]=> int(106) [3]=> int(88) [4]=> int(109) [5]=> int(145) [6]=> int(251) [7]=> int(38) [8]=> int(54) [9]=> int(39) [10]=> int(61) [11]=> int(175) [12]=> int(183) [13]=> int(27) [14]=> int(98) [15]=> int(13) [16]=> int(106) }

array(16) { [1]=> int(242) [2]=> int(106) [3]=> int(88) [4]=> int(109) [5]=> int(145) [6]=> int(251) [7]=> int(38) [8]=> int(54) [9]=> int(39) [10]=> int(61) [11]=> int(175) [12]=> int(183) [13]=> int(27) [14]=> int(98) [15]=> int(13) [16]=> int(50) }

Shouldn't the last byte show

[16]=> int(255)

@Tarik - that gives me the same output as $bytes[15] = 0xff — AndroidDev
– AndroidDev, Commented Feb 20, 2015 at 18:12

Community · Accepted Answer · 2017-05-23 12:20:26Z

2

You should use string manipulation routines to manipulate raw byte arrays. See Byte manipulation in PHP

Try:

$bytes = substr_replace ($bytes, chr(0xFF), 15 , 1);

Also try:

$bytes[15] =  chr(0xff)

edited May 23, 2017 at 12:20

CommunityBot

11 silver badge

answered Feb 20, 2015 at 18:20

Tarik

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1 Comment

Tarik Over a year ago

So it was a matter of converting the integer to a character.

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