Description:
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Encrypting an empty string in AES-256-GCM should return an empty string and valid tag but instead it returns 32 bytes of data from memory which could leak information.

Test script:
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$aad = random_bytes(32);
$iv = random_bytes(16);
$key = random_bytes(32);

$plaintext = '';
$tag = null;

$ciphertext = openssl_encrypt($plaintext, 'aes-256-gcm', $key, \OPENSSL_RAW_DATA, $iv, $tag, $aad);

// $ciphertext should be an empty string but it is not in PHP 7.1.*
// Instead it returns random data from memory!
printf("ciphertext=%s, tag=%s", bin2hex($ciphertext), bin2hex($tag));


Expected result:
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An empty string must be returned

Actual result:
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A 32-byte string from memory is returned