In this post I will show you a method I created to encrypt and decrypt a string using an asymmetric encryption method.

The method shown takes into account that you already possess your key pairs.

I created this method so I could implement encryption between an Android app and a web server.

PHP Asymmetric Encryption

Process

Asymmetric encryption has a block size limit. I found the block size for my example implementation is 64 bits. There is an 11 bits that is used reserved for padding. This means that max characters in a data payload to encrypt is 53 bits.

This implementation takes care of creating cipher blocks of 53 bits into an array for encryption and also for decryption.

Code

This is the encryption function used. The method uses PHP openssl_public_encrypt to encrypt with a public key.

The data is split into 53 bits into an array.
A for loop reads each item in array and encrypts it.
Then it reassembles the data into a single stream with a delimiter for each segment that is base64 encoded.

    function encryptString($public_key, $encryptedText){

        $encrypted_arr = str_split($encryptedText, 53);

        $encryptedStr = "";

        for($i = 0; $i < sizeof($encrypted_arr); $i++){

            openssl_public_encrypt($encrypted_arr[$i], $encrypted, $public_key);

            if($i != 0){
                $encryptedStr .= ":";
            }
            $encryptedStr .= base64_encode($encrypted);
        }
        return $encryptedStr;
    }

This is the decryption function used. It uses openssl_private_decrypt to decrypt with a private key.

It looks for the delimiter used in the encryption method and splits the payload into an array.
Then it decrypts the base64 decoded segment.
Finally the payload is reassembled into a single string like the original payload was when sent.

    function decryptString($private_key, $message){

        $dataArr = explode(":", $message);

        $str = "";

        for($i = 0; $i < count($dataArr); $i++){

            openssl_private_decrypt(base64_decode($dataArr[$i]), $test_decrypted, $private_key);

            $str .= $test_decrypted;

        }

        $str = mb_convert_encoding($str, "UTF-8");

        return $str;
    }

Below is a sample of how to implement. I put these functions inside a class named MyUtils.

<?php

class MyUtils {

  function encryptString($public_key, $encryptedText){
  ....
  }

  function decryptString($private_key, $message){
  .....
  }

}
?>

$myUtils = new MyUtils();

$new_test_str = "{'id':45,'message':'Peter Peter Pumpkin eater','host':'localhost'}";
echo "<br /><br /><br />";
echo "This is the text: $new_test_str";
echo "<br /><br /><br />";
$encrpyted_text = $myUtils->encryptPost($myVars, $new_test_str);
echo "<br /><br /><br />";
echo "This is the encrypted text: $encrpyted_text";

$decrypted = $myUtils->decryptString($myVars, $encrpyted_text);

echo "<br /><br /><br />";
echo "This is the decrypted text: $decrypted";

//if you want to access it as a json array
$str = json_decode($decrypted, true);
$message =  $json['message'];
echo "This is the decrypted text: $message";

Hope this was useful.