I was having a heck of a time finding help on making asynchronous encryption/decryption using private key/public key systems working, and I had to have it for creating a credit card module that uses recurring billing.
You'd be a fool to use normal, 'synchronous' or two-way encryption for this, so the whole mcrypt library won't help.
But, it turns out OpenSSL is extremely easy to use...yet it is so sparsely documented that it seems it would be incredibly hard.
So I share my day of hacking with you - I hope you find it helpful!
if (isset($_SERVER['HTTPS']) )
echo "SECURE: This page is being accessed through a secure connection.<br><br>";
echo "UNSECURE: This page is being access through an unsecure connection.<br><br>";
echo "Private Key:<BR>$privatekey<br><br>Public Key:<BR>$publickey<BR><BR>";
$cleartext = '1234 5678 9012 3456';
echo "Clear text:<br>$cleartext<BR><BR>";
openssl_public_encrypt($cleartext, $crypttext, $publickey);
echo "Crypt text:<br>$crypttext<BR><BR>";
openssl_private_decrypt($crypttext, $decrypted, $privatekey);
echo "Decrypted text:<BR>$decrypted<br><br>";
Many thanks to other contributors in the docs for making this less painful.
Note that you will want to use these sorts of functions to generate a key ONCE - save your privatekey offline for decryption, and put your public key in your scripts/configuration file. If your data is compromised you don't care about the encrypted stuff or the public key, it's only the private key and cleartext that really matter.