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@fosron
Created July 23, 2015 09:14
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Modern method to encrypt/decrypt a string in PHP
<?php
/**
* Simple string encryption/decryption function.
* CHANGE $secret_key and $secret_iv !!!
**/
function stringEncryption($action, $string){
$output = false;
$encrypt_method = 'AES-256-CBC'; // Default
$secret_key = 'Some#Random_Key!'; // Change the key!
$secret_iv = '!IV@_$2'; // Change the init vector!
// hash
$key = hash('sha256', $secret_key);
// iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a warning
$iv = substr(hash('sha256', $secret_iv), 0, 16);
if( $action == 'encrypt' ) {
$output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
$output = base64_encode($output);
}
else if( $action == 'decrypt' ){
$output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
}
return $output;
}
/**
* Example
*
**/
echo '<pre>';
$info = 'This is really secret!';
echo 'info = '.$info."\n";
$encrypted = stringEncryption('encrypt', $info);
echo 'encrypted = '.$encrypted."\n";
$decrypted = stringEncryption('decrypt', $encrypted);
echo 'decrypted = '.$decrypted."\n";
echo '</pre>';
?>
@paragonie-scott
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  $secret_key = 'Some#Random_Key!';               // Change the key!

You don't want a password, you want a string of 16, 24, or 32 random bytes. Since you specified AES-256, you want 32 bytes.

  $key = hash('sha256', $secret_key);

A single pass of SHA-256 is not a strong KDF. You want hash_pbkdf2() if you're going to use a password to generate a key.

  $secret_iv = '!IV@_$2';  // Change the init vector!

The IV should be randomly generated and stored with the ciphertext.

  // iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a warning
  $iv = substr(hash('sha256', $secret_iv), 0, 16);

AES-256-CBC expects 32 bytes for a key and 16 bytes for an IV. However, mb_substr($str, $start, $length, '8bit'); is the canonical way to count bytes in PHP due to mbstring.func_overload.

  if( $action == 'encrypt' ) {
      $output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
      $output = base64_encode($output);
  }
  else if( $action == 'decrypt' ){
      $output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
  }
  return $output;

Why are you using one function for two inverse operations?

You're encypting a string, but since you're not authenticating your message, this encryption is incredibly brittle. The static IV an weak KDF on $secret_key makes it even worse.

You might want to read our guide to authenticated encryption in PHP.

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