B4R Tutorial ESP32: AES256 encrypted & Base64 encoded post message to and from PHP

Example:

- AES256 encrypt a message with IV and Salt
- Base64 encode it
- send a http post request to a php script
- decrypt in php
- send back a AES256 encrypted and Base64 encoded message to the ESP32

Note:

- change SSID & PW & IP-Address/foldername/scriptname
- message must be padded to a multiple of 16 (done by code here)
- Full message contains: ID(8)+Salt(32)+IV(16)+encrypted_message (16*x)
- on the receivers side ID, Salt and IV will be removed from the message (only the raw message can be decrypted). The given IV must be used.
- Salt and IV are public (not encrypted) and changes every time


PHP-Script:

<?php

$aes256 = file_get_contents("php://input");

//print "Decrypted: ".AES256_Decrypt($aes256, hex2bin(hash('sha256',"Secret"))); //just for a test

print AES256_Encrypt("Hi from PHP", hex2bin(hash('sha256',"Secret")));



function AES256_Decrypt($string, $pw)
{
    $dec = false;
    $encrypt_method = "AES-256-CBC";
    $total=base64_decode($string);
    $id=substr($total, 0, 8);
    $salt=substr($total, 8, 32);
    $iv=substr($total, 40, 16);
    $string=Base64_Encode(substr($total, 56, strlen($total)-56));
    $dec = openssl_decrypt($string, $encrypt_method, $pw, 0,$iv);
    return $dec;
}

function AES256_Encrypt($string, $pw)
{
    $encrypt_method = "AES-256-CBC";
    $id=openssl_random_pseudo_bytes(8, $securityok);
    $IV=openssl_random_pseudo_bytes(16, $securityok);
    $Salt=openssl_random_pseudo_bytes(32, $securityok);
    $enc = openssl_encrypt($string, $encrypt_method, $pw, 0,$IV);       
    $enc=Base64_encode($id.$Salt.$IV.Base64_decode($enc));
    return $enc;
}

?>