Restore base64 after it was accedentally lowercased (InnoCTF task)

README.md

This is a solution to one of the tasks in InnoCTF (2018, I think?)
The task is as follows:

$s1 and $s2 strings were gotten in the following way:

$s1 = strtolower(base64_encode($secret))
$s2 = strtolower(base64_encode(base64_encode($secret)))

Restore $secret.

Lowercase_base64_restore.php

<?php

//$s1 = 'sw5mb3jtyxrpb24gd2fudhmgdg8gymugsw5ub0nurntmcjmzxzfuzjbybtq3mtbufsbmcmvlica=';
//$s2 = 'u1c1bwizsnrzwfjwyji0z2qyrnvkse1nzec4z1ltvwdtvzv1yjbovvjudg1jak16whpgdvpqqnlivfeztvrcdwztqm1jbvzssunbpq==';

//$s1 == strtolower(base64_encode($secret))
//$s2 == strtolower(base64_encode(base64_encode($secret)))
//return $secret

function restore_base64($s1, $s2) {
	$res = '';
	for ($i = 0; $i<=strlen($s1)/3; $i++) {
		$sx = substr($s1, $i*3, 3);
		
		$s = $sx;
		$mode = 0;
		while (strtolower(base64_encode($s)) != substr($s2, $i*4, 4)) {
			//echo $s;
			switch ($mode) {
				case 0:
					$s = strtoupper($sx[0]) . $sx[1] . $sx[2];
				break;
				case 1:
					$s = $sx[0] . strtoupper($sx[1]) . $sx[2];
				break;
				case 2:
					$s = $sx[0] . $sx[1] . strtoupper($sx[2]);
				break;
				case 3:
					$s = strtoupper($sx[0]) . strtoupper($sx[1]) . $sx[2];
				break;
				case 4:
					$s = $sx[0] . strtoupper($sx[1]) . strtoupper($sx[2]);
				break;
				case 5:
					$s = strtoupper($sx[0]) . $sx[1] . strtoupper($sx[2]);
				break;
				case 6:
					$s = strtoupper($sx[0]) . strtoupper($sx[1]) . strtoupper($sx[2]);
				break;
			}
			$mode++;
		}
		$res .= $s;
	}
	return base64_decode($res);
}

Lowercate_base64_server_restore.php

<pre><?php
ini_set('max_execution_time', '0');
$sock = fsockopen("innoctf.ru:5005");

while ($in = fread($sock,255)) {
	echo "> $in\r\n";
	
	$in = explode("\n", $in);
	$s1 = trim($in[0]);
	$s2 = strtolower(trim($in[1]));
	$res = '';
	for ($i = 0; $i<=strlen($s1)/3; $i++) {
		$sx = substr($s1, $i*3, 3);
		
		$s = $sx;
		$mode = 0;
		while (strtolower(base64_encode($s)) != substr($s2, $i*4, 4)) {
			echo $s;
			switch ($mode) {
				case 0:
					$s = strtoupper($sx[0]) . $sx[1] . $sx[2];
				break;
				case 1:
					$s = $sx[0] . strtoupper($sx[1]) . $sx[2];
				break;
				case 2:
					$s = $sx[0] . $sx[1] . strtoupper($sx[2]);
				break;
				case 3:
					$s = strtoupper($sx[0]) . strtoupper($sx[1]) . $sx[2];
				break;
				case 4:
					$s = $sx[0] . strtoupper($sx[1]) . strtoupper($sx[2]);
				break;
				case 5:
					$s = strtoupper($sx[0]) . $sx[1] . strtoupper($sx[2]);
				break;
				case 6:
					$s = strtoupper($sx[0]) . strtoupper($sx[1]) . strtoupper($sx[2]);
				break;
			}
			$mode++;
		}
		$res .= $s;
	}
	$out = base64_decode($res);
	
	fwrite($sock, "$out\n");
	echo "< $out\r\n";
	echo '<!--' . str_repeat('x', 999) . '-->';
	flush();
}
fclose($sock);
echo '</pre>';