/**
* Decrypts a ciphertext (legacy -- before version tagging)
*
* $ciphertext is the ciphertext to decrypt.
* $key is the key that the ciphertext was encrypted with.
* You MUST catch exceptions thrown by this function. Read the docs.
*
* @param string $ciphertext
* @param string $key
* @return string
* @throws Ex\CannotPerformOperationException
* @throws Ex\CryptoTestFailedException
* @throws Ex\InvalidCiphertextException
*/
public static function legacyDecrypt($ciphertext, $key)
{
RuntimeTests::runtimeTest();
$config = self::getVersionConfigFromHeader(Core::LEGACY_VERSION, Core::LEGACY_VERSION);
// Extract the HMAC from the front of the ciphertext.
if (Core::ourStrlen($ciphertext) <= $config->macByteSize()) {
throw new Ex\InvalidCiphertextException("Ciphertext is too short.");
}
$hmac = Core::ourSubstr($ciphertext, 0, $config->macByteSize());
if ($hmac === false) {
throw new Ex\CannotPerformOperationException();
}
$ciphertext = Core::ourSubstr($ciphertext, $config->macByteSize());
if ($ciphertext === false) {
throw new Ex\CannotPerformOperationException();
}
// Regenerate the same authentication sub-key.
$akey = Core::HKDF($config->hashFunctionName(), $key, $config->keyByteSize(), $config->authenticationInfoString(), null, $config);
if (self::verifyHMAC($hmac, $ciphertext, $akey, $config)) {
// Regenerate the same encryption sub-key.
$ekey = Core::HKDF($config->hashFunctionName(), $key, $config->keyByteSize(), $config->encryptionInfoString(), null, $config);
// Extract the initialization vector from the ciphertext.
Core::EnsureFunctionExists("openssl_cipher_iv_length");
$ivsize = \openssl_cipher_iv_length($config->cipherMethod());
if ($ivsize === false || $ivsize <= 0) {
throw new Ex\CannotPerformOperationException("Could not get the IV length from OpenSSL");
}
if (Core::ourStrlen($ciphertext) <= $ivsize) {
throw new Ex\InvalidCiphertextException("Ciphertext is too short.");
}
$iv = Core::ourSubstr($ciphertext, 0, $ivsize);
if ($iv === false) {
throw new Ex\CannotPerformOperationException();
}
$ciphertext = Core::ourSubstr($ciphertext, $ivsize);
if ($ciphertext === false) {
throw new Ex\CannotPerformOperationException();
}
$plaintext = self::plainDecrypt($ciphertext, $ekey, $iv, $config);
return $plaintext;
} else {
/*
* We throw an exception instead of returning false because we want
* a script that doesn't handle this condition to CRASH, instead
* of thinking the ciphertext decrypted to the value false.
*/
throw new Ex\InvalidCiphertextException("Integrity check failed.");
}
}