Encryption, authentication and data integrity in PHP 7

by Enrico Zimuel / @ezimuel
Senior Software Engineer
Zend Technologies a Rogue Wave Company

21th October 2016 - ZendCon - Las Vegas

About me

Encryption in PHP

  • Mcrypt extension, symmetric encryption (outdated)
  • OpenSSL extension, public key and symmetric encryption

Mcrypt (outdated)

  • Ciphers: rijndael (AES), Twofish, Blowfish, DES, 3DES, etc
  • Block modes: CBC, CFB, CTR, OFB, NOFB, NCFB
  • Padding: zero padding, padded with '\0'
  • No Authentication!
  • Deprecated in PHP 7.1


Symmetric encryption

CBC encryption

CBC decryption



  • Hash-based message authentication code (RFC 2140)

    where H is hash function, K is key, K' derivated key, opad is outer padding (0x5c), ipad is inner padding (0x36), ⊕ is XOR, and || is concatenation
  • In PHP:
    hash_hmac ($algo, $data, $key, $raw_output = false)
    hash_hmac_file ($algo, $filename, $key, $raw_output = false)
  • We will use SHA-256 as $algo

Why we need authentication?

  • Encryption only provides confidentiality
  • An attacker can easily alter the encrypted message
  • Padding Oracle Attacks can be applied to retrieve the plaintext without the key!

Padding Oracle Attack

  • Side channel attack which is performed on the padding of a cryptographic message
  • Discovered in 2001 by Serge Vaudenay
  • Works on ECB, CBC, OAEP modes for all the block ciphers
  • In 2010 the attack was applied to several web application frameworks, including JavaServer Faces, Ruby on Rails and ASP.NET.
  • Can be prevented using authentication


function encrypt(string $text, string $key): string {
  $iv      = random_bytes(16); // iv size for aes-256-cbc
  $keys    = hash_pbkdf2('sha256', $key, $iv, 80000, 64, true);
  $encKey  = mb_substr($keys, 0, 32, '8bit');
  $hmacKey = mb_substr($keys, 32, null, '8bit');

  $ciphertext = openssl_encrypt($text, 'aes-256-cbc', $encKey,

  $hmac = hash_hmac('sha256', $iv . $ciphertext, $hmacKey);
  return $hmac . $iv . $ciphertext;


function decrypt(string $text, string $key): string {
  $hmac       = mb_substr($text, 0, 64, '8bit');
  $iv         = mb_substr($text, 64, 16, '8bit');
  $ciphertext = mb_substr($text, 80, null, '8bit');

  $keys    = hash_pbkdf2('sha256', $key, $iv, 80000, 64, true);
  $encKey  = mb_substr($keys, 0, 32, '8bit');
  $hmacKey = mb_substr($keys, 32, null, '8bit');
  $hmacNow = hash_hmac('sha256', $iv . $ciphertext, $hmacKey);
  if (! hash_equals($hmac, $hmacNow)) {
    throw new Exception('Authentication error!');
  return openssl_decrypt($ciphertext, 'aes-256-cbc', $encKey,

Timing attacks?

A timing attack is a side channel attack in which the attacker attempts to compromise a cryptosystem by analyzing the time taken to execute cryptographic algorithms

From Wikipedia

To compare two strings in PHP use
hash_equals($known, $user) to prevent timing attacks

Authenticated Encryption

PHP 7.1 provides AEAD support for OpenSSL:

openssl_encrypt ($data, $method, $key, $options = 0, $iv = "",
                 &$tag = null, $aad = "", $tag_len = 16)
openssl_decrypt ($data, $method, $key, $options = 0, $iv = "",
                 $tag = null, $aad = "")


$data = 'Hello World!';
$key  = random_bytes(32); // 256 bit

// GCM and CCM
$modes = ['aes-256-gcm', 'aes-256-ccm'];
foreach ($modes as $mode) {
  $iv = random_bytes(openssl_cipher_iv_length($mode));
  $ciphertext = openssl_encrypt($data, $mode, $key,
    OPENSSL_RAW_DATA, $iv, $tag
  printf("%s, tag size: %d bytes\n", $mode, mb_strlen($tag, '8bit'));
  $text = openssl_decrypt($ciphertext, $mode, $key,
    OPENSSL_RAW_DATA, $iv, $tag
  printf("Decrypt %s\n\n", $text === $data ? 'Ok' : 'Failed');

Best practices for encryption

  • Use AES as standard algorithm
  • Use CBC or CTR mode
  • Use random IVs, different for each encryption
  • Generate a encryption key from user's password using a KDF function, e.g. PBKDF2
  • ALWAYS use authenticated encryption algorithms
  • Secure random number: random_int() or random_bytes()

Public key encryption


Digital signature

Hybrid cryptosystem

  • Public key algorithms are too slow to encrypt a full text message
  • We need a different approach (Hybrid cryptosystem):
    • generate a random session key;
    • encrypt the session key using a public key algorithm;
    • encrypt the message with a symmetric cipher using the session key;
    • send the ciphertext and the encrypted session key;

Public and private key

// Generate public and private keys
$keys = openssl_pkey_new(array(
  "private_key_bits" => 4096,
  "private_key_type" => OPENSSL_KEYTYPE_RSA,

// Store the private key in a file
$passphrase = 'test';
openssl_pkey_export_to_file($keys, 'private.key', $passphrase);

// Store the public key in a file
$details   = openssl_pkey_get_details($keys);
$publicKey = $details['key'];
file_put_contents('public.key', $publicKey);

Hybrid encryption

$msg = 'This is the secret message';
// Encryption
$key = random_bytes(32);
openssl_public_encrypt($key, $encKey, $publicKey);
$ciphertext = encrypt($msg, $key);
file_put_contents('encrypted.msg', $encKey . $ciphertext);
// Decryption
$ciphertext = file_get_contents('encrypted.msg');
$encKey     = mb_substr($ciphertext, 0, 512, '8bit');
$ciphertext = mb_substr($ciphertext, 512, null, '8bit');
$privateKey = openssl_pkey_get_private(
  $passphrase // to access the private key
openssl_private_decrypt($encKey, $key, $privateKey);
$result = decrypt($ciphertext, $key); // $result === $msg

Digital signature

  • How to provide authentication and non-repudiation?
  • A digital signature is a mathematical scheme for demonstrating the authenticity of a digital message or document
  • Commonly used for software distribution, financial transactions, and in other cases where it is important to detect forgery or tampering

Signature with OpenSSL

// Compute the signature
$priKey = openssl_pkey_get_private(
$data = file_get_contents('/path/to/file_to_sign');
openssl_sign($data, $sign, $priKey, "sha256");
printf("Signature : %s\n", base64_encode($sign));

// Verify the signature
$pubKey = openssl_pkey_get_public(
$result = openssl_verify($data, $sign, $pubKey, "sha256");

echo $result === 1 ? 'Signature ok' : 'Signature not valid';

Crypto library in PHP

References: articles and books

References: blog and websites


Rate this talk at https://joind.in/talk/90174

Contact me: enrico [at] zend.com

Follow me: @ezimuel

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