Here we will review papers and blogposts, designs and attacks, competitions and conferences - that deal with the real-world applications of cryptography

A recent ePrint submission claims a combination of anonymous credential and reputation system. Here every credential holder is given a review token, which he can anonymously spend. A double spending can be traced and identity revealed. The credentials are based on the bilinear-map signature by Pointcheval-Sanders.

It does not look novel to me. A reputation system with an anonymous one-time review token can be implemented using a privacy-preserving cryptocurrency, like Zcash or Zcoin. Here a token would be a coin, which can not be doublespent as the ledger would prevent this, so this is even better than tracing in this paper. Also what looks weird is that the authors state that idemix does not include range proofs, whereas it does. https://eprint.iacr.org/2018/835

ASIACRYPT 2018 Accepted Papers. Part 1:
1) "New Instantiations of the CRYPTO 2017 Masking Schemes" https://eprint.iacr.org/2018/492.pdf
To protect a multiplication operation from side-channel leakage, we introduce masks. Apparently the proper mask generation is difficult and the authors show how to do it for high number of shares. A lot of matrix playing in the paper, and note the use of Schwartz-Zippel lemma, which occurs frequently now (cf. Groth's SNARKs). #side-channels

2) "Pattern Matching on Encrypted Streams" https://eprint.iacr.org/2017/148.pdf
Not only normal words but patterns like (almost) regular expressions can be searched in an encrypted text. This comes at a cost of large public key and ciphertext growth, but it may worth it. The idea is to encode all words in the plaintext as powers of some generator, and then use pairings to confirm matching. #searchable-encryption

3) "Revisiting Key-alternating Feistel Ciphers for Shorter Keys and Multi-user Security" https://eprint.iacr.org/2018/816.pdf
Just another theoretical result on Feistels.

4) "Practical attacks against the Walnut digital signature scheme" https://eprint.iacr.org/2018/318.pdf
A full break of a post-quantum signature proposal, which relies on non-commutative groups (somewhat similar to code-based signatures). Apparently can be broken easily with memoryless collision search algorithm

5) "Tweakable Block Ciphers Secure Beyond the Birthday Bound in the Ideal Cipher Model" https://eprint.iacr.org/2018/784.pdf
Yet another mode of operations for tweakable ciphers.

6) "On Multiparty Garbling of Arithmetic Circuits" https://eprint.iacr.org/2017/1186.pdf
An improvement in garbled circuits (needed for MPC)

Google has added Ethereum to its data cloud. If they provide signed responses, we can use it for proving Ethereum events in other blockchains https://cloud.google.com/bigquery/

"Privacy Amplification by Iteration" https://ai.google/research/pubs/pub47118 FOCS'2018
Google researchers show that a sequence of privacy-preserving AI algorithms can increase the (differential) privacy with the same utility. It is more a privacy benchmark rather than a guideline how to compose algorithms, but still.

Enigma blockchain project https://blog.enigma.co/
has just announced its testnet, and we decided to take a look at the project.

Apparently the idea is to extend Ethereum with decentralized data storage where data is stored in encrypted form. To process it in a contract, a distributed multiparty computation (!) is performed by a group of nodes. As one would expect, a majority of those nodes can disrupt the procedure, and a bit more of them can break confidentiality.

The assumption that such a majority is never assembled to process valuable data looks unlikely to me. Instead, one should expect that nodes opportunistically join the computing committee to form a critical majority and then decrypt inputs or outputs. Worse than for mining, such a cartel will remain unnoticed until you realize that the data was stolen. So the trust requirements for nodes in this blockchain is very high, not speaking of low performance of available MPC primitives, which will lessen the computing power of the blockchain.

But this is a far future plan anyway, as in the testnet the nodes are equipped with an SGX processor which already has all the necessary keys to encrypt and decrypt. I do not know why it is called testnet and not alpha, but let's see. The whitepaper is here https://enigma.co/enigma_full.pdf

ASIACRYPT 2018 accepted papers part 2
7) "Learning Strikes Again: the Case of the DRS Signature Scheme" https://eprint.iacr.org/2018/294.pdf
Full break of a post-quantum submission. Apparently in many lattice-based signature schemes signatures and messages are algebraically related with this relation being statistically detected and thus secret key deduced. The paper is a very nice collection of geometric tricks that lead to the cryptanalysis.
#lattices

8) "Unbounded Inner Product Functional Encryption from Bilinear Maps" https://eprint.iacr.org/2018/696.pdf
How to compute inner product over encrypted data if you do not know the key. Now it is possible for arbitrary long plaintexts, but no implementation.
#functional_encryption

9) "Practical Fully Secure Unrestricted Inner Product Functional Encryption modulo p" https://eprint.iacr.org/2018/791.pdf
The same topic as number 8
#functional_encryption

10) "Signatures with Flexible Public Key: Introducing Equivalence Classes for Public Keys" https://eprint.iacr.org/2018/191.pdf
How to construct a group of related public and private keys so that the relation is easy to assert but difficult to check. It is somewhat similar to the idea of hierarchical wallets (BIP32) in bitcoin and other currencies, where a new public key is the old one + a constant times the base point. It seems that this paper claims more privacy and less leakage in their approach, but it is not easy to figure out what are the advantages exactly. Maybe an original private key is more difficult to recover if the related one is leaked? A more careful reading is needed. The signatures in the paper are pairing-based so direct application to ECDSA-bound currencies might be problematic. However, better-than-existing ring and group signature schemes are presented, so maybe not for BIP but for Monero-like currencies this would work out.
#group_signature

11) "Compact Multi-Signatures for Smaller Blockchains" https://eprint.iacr.org/2018/483.pdf
Recently Boneh showed that the BLS multisignature is error-prone and needs a protection against rogue keys. Now the authors demonstrate a secure aggregated multisignature based on BLS with clear application to cryptocurrencies (e.g. for multisig addresses in Bitcoin). Note that all public keys must be known before aggregation, so arbitrary subsets are not allowed - so potential difficulty when exact set of signers is unknown. Still, a very useful primitive.
#multisignature #bitcoin

12) "How to Securely Compute with Noisy Leakage in Quasilinear Complexity" https://eprint.iacr.org/2017/929.pdf
Theoretical constructions for leakage-resilient schemes, that withstand side-channel leakage of high rate.
#side_channels

USENIX 2018 Crypto papers
1) "A Single-Decryption EM-Based Attack on OpenSSL’s Constant-Time Blinded RSA" https://www.usenix.org/sites/default/files/conference/protected-files/security18_slides_prvulovic.pdf
The constant-time implementation is shown to be vulnerable to Simple Power Analysis and thus secret exponent derivation just from a few bits. Constant time is not a bulletproof side-channel protection!

2) "DIZK A Distributed Zero Knowledge Proof System" https://www.usenix.org/sites/default/files/conference/protected-files/security18_slides_wu_0.pdf
How to distribute the computation (not MPC!) of a zkSNARK proof. Libsnark can process millions of gates, whereas this system can do billions! Very promising for some ICO :)

3) "Arbitrum: Scalable, private smart contracts" https://www.usenix.org/system/files/conference/usenixsecurity18/sec18-kalodner.pdf
It is not zkSNARKs on Ethereum. Instead, the authors suggest maintaining the current state hash of your chain/contract in a Virtual Machine, which has well-defined rules of state transitioning. Unless some of pre-designated maintainers challenge the new hash value, the VM operates smoothly and the parties agree on the state. In the case of a challenge the VM can be halted or forced to distribute its balance. Somewhat resembles Lightning channels and many other concepts, but seems to be a more powerful and standalone. The authors developed their own assembly and a standard library for VMs. Worth watching

"Bulletproofs: Short Proofs for Confidential Transactions and More"
https://eprint.iacr.org/2017/1066.pdf and IEEE S&P 2018

For quite some time proving computation in zero knowledge needed pairings because when you compute z = x*y and commit to z, x, and y separately it is very difficult to prove that g^z = (g^x)^y without revealing y. With pairings you have e(g^x,g^y)= e(g,g)^(xy) and there are SNARKs that exploit this property. Now Bulletproofs shows that it is apparently possible to prove the same without resorting to pairings. In the meanwhile, a trusted setup is not needed either.

The paper has two main ideas, though the second one is more opaque. The first idea is to reduce a range proof or an arithmetic circuit proof to an inner product argument, concretely proving that for some P you know A,B, c such that P = (X^A)(Y^B)z^c, where A,B are integer vectors, X,Y are group vectors, and c = <A,B> (inner product). Section 3 contains a nice chain of derivations that show how to make such proofs using the proofs for shorter vectors, and finally get the result by recursion. However, the resulting argument is not zero-knowledge.

The more interesting part comes in the last parts of Sections 4 and 5 where for range proofs and circuit proofs the authors show how to get a zero-knowledge proof from the above. After the recursion boils down to single-element vectors, we consider degree-2 polynomials with coefficients from these vectors, and evaluate them on a verifier-given point. Again, since the vectors are short, we can replace the Verifier with Fiat-Shamir and still get the proofs of reasonable size.

After all that we are left with range and circuit proofs of logarithmic (in length) complexity for all parties in both time and memory. Only discrete-log assumptions are needed, and the setup only requires generators without discrete-log relations among them.

The technique is great but I am sure we will see improvements. For circuits it remains to see which hash function delivers the smallest and fast-to-verify proof. Since the circuits operate in a prime field just as SNARKs, such a hash function will probably suit both worlds.