Hi, > On 2 Mar 2018, at 08:32, Nikos Mavrogiannopoulos <n...@redhat.com> wrote: > >> On Thu, 2018-03-01 at 21:52 +0000, Paterson, Kenny wrote: >> 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. > > Hi, > It was brought previously to the WG [0], and the bottom line was to push > for any solution to implementations. >
Thanks Nikos - sorry for missing your post from last August. At least I'm now only six months behind the curve :-) > As of the "naïve implementation of de-padding", I wouldn't put like that. > It is a straightforward method of de-padding after reading the draft, and > I believe all implementations out there use that method. Agreed. "Natural" would have been a better choice here. Cheers, Kenny > > regards, > Nikos > > > [0]. > https://www.ietf.org/mail-archive/web/tls/current/msg24365.html > _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
