Re: [TLS] Possible timing attack on TLS 1.3 padding mechanism

["Paterson, Kenny" <[e-mail]>]

Hi,

I've been analysing the record protocol spec for TLS 1.3 a bit, specifically the new padding mechanism. I think there's a possible timing attack on a naïve implementation of de-padding. Maybe this is already known to people who've been paying more attention than me!

Recall that the padding mechanism permits an arbitrary number of 00 bytes to be added after the plaintext and content type byte, up to the max record size. This data is then encrypted using whichever AEAD scheme is specified in the cipher suite. This padding scheme is quite important for TLS 1.3 because the current AEAD schemes do leak the length of record plaintexts. There should be no padding oracle style attack possible because of the integrity guarantees of the AEAD schemes in use. 

The idea for the timing attack is as follows. 

The natural way to depad (after AEAD decryption) is to remove the 00 bytes at the end of the plaintext structure one by one, until a non-00 byte is encountered. This is then the content type byte. Notice that the amount of time needed to execute this depadding routine would be proportional to the number of padding bytes. If there's some kind of response record for this record, then measuring the time taken from reception of the target record to the appearance of the response record can be used to infer information about the amount of padding, and thereby, the true length of the plaintext (since the length of the padded plaintext is known from the ciphertext length).

The timing differences here would be small. But they could be amplified by various techniques. For example, the cumulative timing difference over many records could allow leakage of the sum of the true plaintext lengths. Think of a client browser fetching a simple webpage from a browser. The page is split over many TLS records, each of which is individually padded, with the next GET request from the client being the "response record". (This is a pretty simplistic view of how a web browser works, I know!). The total timing difference might then be sufficient for webpage fingerprinting, for example. 

I'm not claiming this is a big issue, but maybe something worth thinking about and addressing in the TLS 1.3 spec.

There's at least a couple of ways to avoid the problem:

1. Do constant-time depadding - by examining every byte in the plaintext structure even after the first non-00 byte is encountered. 
2. Add an explicit padding length field at the end of the plaintext structure, and removing padding without checking its contents. (This should be safe because of the AEAD integrity guarantees.) 

Option 2 is probably a bit invasive at this late stage in the specification process. Maybe a sentence or two on option 1 could be added to the spec.

Thoughts?

Cheers,

Kenny