Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It only takes a minute to sign up.
Sign up to join this community
I have an application that stores data on the devices. Currently AES/CBC/PKCS5Padding from javax.crypto.Cipher(see reference) is used to encrypt the stored data.
The padding isn't verified manually. Does that make the process vulnerable to Padding Oracle attacks?
Would the process benefit from using AES/GCM?
The problem with CBC mode is the padding. When there is a padding error, the server must respond to a message back to you so that you can send the message back again or encrypt the message from the beginning.
The padding oracle attack is solely based on this idea. The attacker changes the byte and looks at the response of the server to execute the attack.
As we can see the attacker needs an oracle to execute the padding oracle attack. Data on-rest or encrypted databases has no oracle. Therefore they are not vulnerable to padding oracle attacks.
CBC mode has no integrity and authentication like all basic (archaic) block cipher mode of operations and it is usually used with HMAC. CBC is removed from TLS 1.3. We can see a good reason in a 2019 work.
Scalable Scanning and Automatic Classification of TLS Padding Oracle Vulnerabilities by Merget et. al.
They scanned the top 1M sites and conclude that CBC with MAC-than-Encrypt is hard to implement securely. They find 93 different vulnerabilities. Their
Github page; TLS Padding Oracles
Code page; TLS-Attacker
Video WAC2-2019
AES-GCM which is in TLS 1.3 has an authenticated encryption scheme that provides both integrity and authentication. The internal GCM mode uses CTR mode for encryption that requires no padding. Therefore padding oracle is not applicable.
So in short, in GCM mode you will have
Since you are going to use AES/GCM your next issue will be the IV. Instead of random IV, you can use incremental IV so that you can mitigate IV Reuse catastrophe. For more information on IV recommendations see section 8 of NIST-800-38D
In Java, you can use AES/GCM/NoPadding from javax.crypto.Cipher package.
AES/CBC/PKCS5Padding can create a vulnerability to Padding Oracle attack. The modern, safe option is authenticated encryption, e.g. AES/GCM/NoPadding in modern javax.crypto.Cipher.
$\endgroup$
Yes - any system that uses and mode of operation that requires unpadding could be vulnerable to padding oracle attacks.
But even if the implementation isn't vulnerable to padding oracle attacks: padding oracle attacks are only a subclass on plaintext oracle attacks. If the attacker has information on the plaintext and is able to generate some kind of response that data handling fails in a particular way after decryption, then you'd still have a problem.
This is why authenticated encryption is so important if you have some kind of service (i.e. Oracle) that performs the decryption. That way the authenticity of the message is validated (to originate from somebody who has the established key) before it is decrypted or handled.
This is all moot for data at rest or if the attacker cannot learn anything from the decryption of the message, or if the attacker can only eavesdrop. In that case there is no oracle, and thus this kind of attacks do not apply. It could still make sense to verify integrity for other reasons of course, so defaulting to authenticated encryption is generally a good idea.
