$p = 17;
$q = 19;
$n = $p * $q;
$phi = ($p - 1) * ($q - 1);

$e = mt_rand(2, $phi - 1);
while (gcd($e, $phi) != 1) {
$e = mt_rand(2, $phi - 1);
}

function gcd($a, $b) {
if ($b == 0) {
return $a;
} else {
return gcd($b, $a % $b);
}
}

$d = modInverse($e, $phi);
$public_key = array('e' =>$e, 'n' =>$n);
$private_key = array('d' =>$d, 'n' =>$n);
$plain_text = 'Hello, world!';
$cipher_text = encrypt($plain_text, $public_key);
$recovered_text = decrypt($cipher_text, $private_key);
function encrypt($message, $key) {
$m = str2int($message);
$e = $key['e'];
$n = $key['n'];
$c = bcpowmod($m, $e, $n);
return int2str($c);
}
function decrypt($message, $key) {
$c = str2int($message);
$d = $key['d'];
$n = $key['n'];
$m = bcpowmod($c, $d, $n);
return int2str($m);
}
function str2int($message) {
$result = '';
for ($i = 0; $i< strlen($message); $i++) {
$result .= str_pad(ord($message[$i]), 3, '0', STR_PAD_LEFT);
}
return $result;
}
function int2str($message) {
$result = '';
while ($message >0) {
$result .= chr($message % 1000);
$message = intval($message / 1000);
}
return strrev($result);
}
function modInverse($a, $m) {
$m0 = $m;
$y = 0;
$x = 1;
if ($m == 1) {
return 0;
}
while ($a >1) {
// q is quotient
$q = intval($a / $m);
$t = $m;
// m is remainder now, process
// same as Euclid's algo
$m = $a % $m;
$a = $t;
$t = $y;
// Update x and y
$y = $x - $q * $y;
$x = $t;
}
// Make x positive
if ($x< 0) {
$x += $m0;
}
return $x;
}