Question

Is there any safe crypting algorithm that you can I can use this way?

<?php

   $message="Hi there!";

   $key1="ablablabla";

   $key2="fooboomoohoo";

   $tmp=encrypt($message,$key1);

   $tmp=encrypt($tmp,$key2);

   $tmp=decrypt($tmp,$key1);

   $result=decrypt($tmp,$key2);

   echo "\"".$message."\" is the same as \"".$result."\"";

?>

It should work like this:

1. User wants to send an encrypted message to server, so he encrypts it with his secret key

2. Server gets an encrypted message, so he encrypts it again with it’s own secret key and sends it back

3. User decrypts the message with his key and sends it back

4. Finally, the server decodes the message

Answer 1

What about a stream cipher like RC4?

Encryption and decryption involve XOR with the output of a PRNG. (Actually decryption is the same as encryption.)

$tmp=encrypt($message,$key1); // tmp = P ^ A
$tmp=encrypt($tmp,$key2);     // tmp = p ^ A ^ B
$tmp=decrypt($tmp,$key1);     // tmp = p ^ A ^ B ^ A = p ^ B
$result=decrypt($tmp,$key2);  // result = p ^ B ^ B = p