Question

I have to encrypt a particular URL parameter. If I want the output to be below 6-7 characters, what algorithm should I use?

The inputs are integer only ranging from 1 to 1,000,000.

Answer 1

If you require encryption and need to have the shortest result possible, you must use a stream cipher. Blowfish (what you previously used) is a blockcipher and the result will always have the minimum size of one single block.

Find a comparison of stream ciphers on Wikipedia and the list of supported ciphers in the PHP manual on mcrypt

Also, the result of the encryption may contain special chars, so when putting it into an URL as a parameter, you should use urlencode() or base64_encode()

Using urlencode() or base64_encode() will expand your string, making it longer than the original data. This is necessary to make it transport/URL safe.

However, since your input is always a number you can use base_convert() to shorten your input. On the decoding side, you’d have to do reverse the same thing.

To get even shorter results, you could make use of the enminicode() / deminicode() function provided by Aron Cederholm instead of using base_convert().

Here is an example using the RC4 stream cipher (which is not very strong, by the way) and conversion from base 10 to base 36.

NOTE: this example only works on numbers hence its using base_convert() to shrink the input string!

function my_number_encrypt($data, $key, $base64_safe=true, $shrink=true) {
        if ($shrink) $data = base_convert($data, 10, 36);
        $data = @mcrypt_encrypt(MCRYPT_ARCFOUR, $key, $data, MCRYPT_MODE_STREAM);
        if ($base64_safe) $data = str_replace('=', '', base64_encode($data));
        return $data;
}

function my_number_decrypt($data, $key, $base64_safe=true, $expand=true) {
        if ($base64_safe) $data = base64_decode($data.'==');
        $data = @mcrypt_encrypt(MCRYPT_ARCFOUR, $key, $data, MCRYPT_MODE_STREAM);
        if ($expand) $data = base_convert($data, 36, 10);
        return $data;
}

$data = "15231223";
$key = "&/ASD%g/..&FWSF2csvsq2we!%%";

echo "data: ", $data, "\n";
$encrypted = my_number_encrypt($data, $key);
echo "encrypted: ", $encrypted, "\n";
$decrypted = my_number_decrypt($encrypted, $key);
echo "decrypted: ", $decrypted, "\n";

Result:

data: 15231223
encrypted: BuF3xdE
decrypted: 15231223