String encryption function in admin section

Question

I have been using this function locally to encrypt strings for example a username.

If the user successfully logged in, place the encrypted username into a session, decrypt the username in the admin section and use for various things.

Is it a strong method and are there any security concerns with the way I'm using it?

Cipher Function:

 public function cipher($string, $method) {
 $key = pack('H*', "bcb04b7e103a0cd8b54763051cef08bc55abe029fdebae5e1d417e2ffb2a00a3");
 $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
 $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
 if($method === 1) {
 $string = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $string, MCRYPT_MODE_CBC, $iv);
 $string = $iv . $string;
 $encryptedString = base64_encode($string);
 return trim($encryptedString, "0円");
 }
 if($method === 2) {
 $string = base64_decode($string);
 $iv_dec = substr($string, 0, $iv_size);
 $ciphertext_dec = substr($string, $iv_size);
 $decryptedString = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
 return trim($decryptedString, "0円");
 }
 }

Function Usage:

//Encrypt
$_SESSION['username'] = $backend->cipher($username, 1)
// Decrypt
echo $backend->cipher($_SESSION['username'], 2)

Answer 1

It looks great, however, you can improve it as follows:

1. Break out the function into two - encrypt and decrypt. Your functions should do the least amount of things.
2. return on trim - no need to set $encryptedString.
3. In the encrypt, no need to have the second $string when you concat $iv. Just concat on the initial mcrypt call.

In the end, we're left with three functions. One setupCipher which creates the key, iv_size and iv. One encrypt and one decrypt to encrypt and decrypt, as follows:

private function setupCipher(){
 $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
 return array(
 pack('H*', "bcb04b7e103a0cd8b54763051cef08bc55abe029fdebae5e1d417e2ffb2a00a3"),
 $iv_size, 
 mcrypt_create_iv($iv_size, MCRYPT_RAND)
 );
} 
public function decrypt($string){
 list($key, $iv_size) = $this->setupCipher();
 $string = base64_decode($string);
 $iv_dec = substr($string, 0, $iv_size);
 $ciphertext_dec = substr($string, $iv_size);
 return trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec), "0円");
}
public function encrypt($string) {
 list($key, $iv_size, $iv) = $this->setupCipher();
 $string = $iv . mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $string, MCRYPT_MODE_CBC, $iv);
 return trim(base64_encode($string), "0円");
}

You would use them nearly identically to how you used them before, except no need to pass in the $method:

# Encrypt
$_SESSION['username'] = $backend->encrypt($username);
# Decrypt
echo $backend->decrypt($_SESSION['username']);