david wong

Hey! I'm David, a security consultant at Cryptography Services, the crypto team of NCC Group . This is my blog about cryptography and security and other related topics that I find interesting.

Real World Crypto 2017: Day 3

posted January 2017

(The notes for day 2 are here.)

The first talk on Quantum Computers was really interesting, but unfortunately mostly went over my head. Although I'm glad we had a pro who could come and talk to us on the subject. The take away I took from it was to go read the SMBC comics on the same subject.

After that there was a talk about TPMs and DAA for Direct Anonymous Attestation. I should probably read the wikipedia page because I have no idea what that was about.

Helena Handschuh from Cryptography Research talked about DPA Resistance for Real People. There are many techniques we use as DPA countermeasures but it seems like we still don't have the secret sauce to completely prevent that kind of issues, so what we really end up doing is rotating the keys every 3 encryption/decryption operations or so... That's what they do at Rambus, and at least what I've heard other people doing when I was at CHES this year. Mike Hamburg describes the way they rotate keys in his STROBE project a bit. Handschuh also talked about the two ways to certify a DPA-resistant product. There are evaluations like Common Criteria, which is usually the normal route, but now there is also validation. Don't ask me about that.

David Cash then entered the stage and delivered what I believe was the best talk of the conference. He started with a great explanation of ORE vs OPE. OPE (Order Preserving Encryption) allows you to encrypt data in a way that ciphertexts conserve the same lexicographic order, ORE (Order Revealing Encryption) does not, but some function over the ciphertexts end up revealing the order of the plaintexts. So they're kind of the same in the end and the distiction doesn't seem to really matter for his attacks. What matters is the distinction between Ideal ORE and normal ORE (and the obviously, the latter is what is used in the real world).

Ideal ORE only reveals the order while the ORE schemes we use also reveal other things, for example the MSDB (most significant different bits) which is the position of the first non-similar bit between two plaintexts.

Previous research focused on attacking a single column of encrypted data while their new research attacks columns of correlated data. David gives the example of coordinates and shows several illustrations of his attack revealing an encrypted image of the linux penguin, encrypted locations on a map or his go-about saved and encrypted by a GPS. Just by looking at the order of coordinates everything can be visually somewhat restored.

Just by analyzing the MSDB property, a bunch of bits from the plaintexts can be restored as well. It seemed like very bad news for the ORE schemes analyzed.

Finally, two points that seemed really important in this race for the perfect ORE scheme is that first: the security proofs of these constructions are considering any database data as uniformly random, whereas we know that we rarely need to store completely random data :) Especially columns are often correlated with one another. Second, even an (hypothetical) ideal ORE was vulnerable to their research and to previous research (he gave the example of encrypting the whole domain in which case the order would just reveal the plaintexts).

This is a pretty bad stab at ORE scheme in general, showing that it is intuitively limited.

Paul Grubbs followed with an explanation of BoPETS, a term that I believe he recently coined, meaning "Building on Property revealing EncrypTion". He gave a good list of products that I replicated below in a table.

Order Preserving Encryption SAP, Cipherbase, skyhigh, CipherCloud, CryptDB
Searchable Encryption Shadowcrypt, Mylar, kryptonostik, gitzero, ZeroDB, CryptDB
Deterministic Encryption Perspecsys, skyhigh, CipherCloud, CryptDB

They looked at Mylar and saw if they could break it from 3 different attacker setups: a snapshot setup (smash and grab attack), passive (attacker got in, and is now observing what is happening), active (attacker can do pretty much anything I believe).

Mylar uses some encrypted data blob/structure in addition to a "principal graph" containing some metadata, ACL, etc... related to the encrypted data. Grubbs showed how he could recover most of the plaintexts from all the different setups.

Tal Malkin interjected that these attacks would probably not apply to some different PPE systems like IBM OXT. Grubbs said it did not matter. Who's right, I don't know.

As for the active attacker problem, there seem to exist no intuitive solution there. If the attacker can do anything, you're probably screwed.

Raluca Ada Popa (Mylar) followed Grubbs by talking about her product Mylar and rejected all of Grubbs claims, saying that there were out of the scope of Mylar OR were attacking mis-use of the product. IIRC the same thing happened when CryptDB was "broken", CryptDB released a paper arguing that these were false claims.

After Mylar, Popa intend to release two new products with better security: Verena and Opaque.

David Mcgrew mentionned Joy and gave a long talk about detecting PRNG failures. Basically look for public values affected by a PRNG like signatures or the server/client random in TLS.

And that was it. See you next year.

If you have anything to say about my notes, the talks, or other people's notes, please do so in the comments :)

There was a HACS workshop right after RWC, and Tim Taubert wrote some notes on it here.

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