Iterate through n bit chunks of a byte string

December 4, 2023

I have a code like this:

$alphabet = array('0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
$alphabetSize = count($alphabet);
$alphabetBitSize = ceil(log($alphabetSize, 2));
$bitSize = $length * $alphabetBitSize;
$bytes = random_bytes(ceil($bitSize/8));

What I need is reading $bitSize bits in a loop to generate a latin1 string using the alphabet. Now I am totally lost about how to do this with the $bytes I have. Most of the answers are using string functions to do this, but I guess I need something binary. Another option is doing it with hex maybe. Any hints?

>Solution :

I generally will use PHP’s built-in function unpack() to convert the bytes into a hexadecimal string and then to a binary string. I hope this helps.

$alphabet = array('0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
$alphabetSize = count($alphabet);
$alphabetBitSize = ceil(log($alphabetSize, 2));
$bitSize = $length * $alphabetBitSize;
$bytes = random_bytes(ceil($bitSize/8));

$binaryStr = '';
foreach (unpack('C*', $bytes) as $byte) {
    $binaryStr .= sprintf("%08b", $byte);
}

$resultString = '';
for ($i = 0; $i < strlen($binaryStr); $i += $alphabetBitSize) {
    $bitSegment = substr($binaryStr, $i, $alphabetBitSize);
    $index = bindec($bitSegment) % $alphabetSize;
    $resultString .= $alphabet[$index];
}

echo $resultString;