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.

Algorand: Scaling Byzantine Agreements for Cryptocurrencies
https://people.csail.mit.edu/nickolai/papers/gilad-algorand-eprint.pdf

There were a few Algorand papers; the first one by two authors describes quite a different version of Algorand with (seemingly) stronger claims and simpler algorithm.

The most important novelty of Algorand is its consensus, as the rest looks similar to Bitcoin. The consensus is Proof-of-Stake, where the probability to create a new block is proportional to party's wealth. As usual, not every public key participates in the block selection; moreover, those who do can step aside in the middle. There are alternatives: Ouroboros, SnowWhite, and Casper (some will be reviewed as well).

The main problem the PoS algorithms face is how to select a leader who proposes blocks so that the result can not be influenced by an adversary who joins at the last moment. There exist distributed key generation protocols and their derivatives, where m parties participate and exchange messages. If m/2+1 out of m parties are honest, all of them can reconstruct a collectively generated key. However, there are still some technical difficulties to overcome, which the Algorand designers claim to solve as follows.

First, Algorand selects a committee using a so-called Verifiable Random Function (VRF), such that each user can check and prove in zero-knowledge if he is in the committee and what his priority is, but he can not check that for other users (that's crucial). This is called sortition, but its straightforward implementation is still vulnerable to adversarial guessing. Algorand has to refresh the seed for VRF and use it only after a certain delay. Note that the sortition provides only a weak guarantee that the selected committee has enough honest nodes. For example, when honest users possess 80% of wealth, as many as 2000 members of committee are needed to ensure the majority for the next step, and this happens with probability 2^(-30) - which is not so low for me.

At the next step the committee decides on a block in the BFT-like fashion. In the normal situation the most prioritized committee member suggests a block, and everyone agrees, but when the leader is not available or there are conflicting ones, or even worse the network malfunctions, then the procedure gets much more involved. For example there is an option when an empty block is suggested. There are again some heuristically chosen parameters to prevent that from happening, but we do not know the behavior under a real attack. Once finalized, the block can not be reverted but it is a common BFT property.

The authors' benchmarking shows that Algorand can handle up to 50K users (the number of different IPs is much smaller though) with latency of under a minute and block size of a few MBytes. This is bigger than in the wild Ethereum but is comparable to the permissioned (Proof-of-Authority) Ethereum.

To summarize, the algorithm has some novelties and interesting performance, but it remains to see how it behaves under active attacks.

ASIACRYPT 2018 Accepted Papers Part 3:

13) "Hidden Shift Quantum Cryptanalysis and Implications" https://eprint.iacr.org/2018/432.pdf
When the quantum encryption oracle is available, the authors show how to break several symmetric schemes that use the modular addition (previous attacks exploited XORs). The complexity is practical, so never encrypt on a quantum computer.

14) "Secure Computation with Low Communication from Cross-checking" https://eprint.iacr.org/2018/216.pdf
Very efficient multiparty-computation protocols for generic circuits and a few (4 or 6) players: up to 1 bit per circuit gate. However, needs bigger preprocessing data.

15) "Programming the Demirci-Selcuk Meet-in-the-Middle Attack with Constraints" https://eprint.iacr.org/2018/813.pdf
The DS-MITM attack is a very powerful cryptanalysis method for AES-like ciphers, but the parameters (constraints) are non-trivial to find. This papers makes it automatic for some ciphers.

16) "Parameter-Hiding Order Revealing Encryption" https://eprint.iacr.org/2018/698.pdf
When you encrypt a database of integers and wants that the relative order of elements can be deduced from ciphertexts - it is ORE. However, for non-random data and practical-performance implementations of ORE - much more information can be obtained. This paper defines a parameter that can be guaranteely concealed even for such ORE

17) "Computing supersingular isogenies on Kummer surfaces" https://eprint.iacr.org/2018/850.pdf
Supersingular isogenies are candidates for quantum-secure public crypto with small keys and slow processing. This paper tries to make them faster

18) "Robustly Reusable Fuzzy Extractor from Standard Assumptions" https://eprint.iacr.org/2018/818.pdf
To compute a key from fingerprint, one can choose a codeword of a linear code as the key and difference to this codeword as a witness. Then applying a witness to a noisy reading of the same finger would bring to the vicinity of the same codeword. It is an example of a fuzzy extractor. Original designs could not derive many unrelated keys from the same source, and this paper provides a mechanism for that.

ZK-STARKs
"Scalable, transparent, and post-quantum secure computational integrity" https://eprint.iacr.org/2018/046.pdf

STARKs are one of the most interesting concepts of year 2018. I do not say "interesting papers" because the paper is unfortunately not so interesting but is quite involving. It remains to see when an easier formal description appears in a follow-up paper, as currently "popular" explanations (such as Buterin's https://vitalik.ca/general/2017/11/09/starks_part_1.html ) are largely based on Ben-Sasson's video lectures
https://www.youtube.com/watch?v=9VuZvdxFZQo
https://www.youtube.com/watch?v=L7tZeO8ihcQ
which I too recommend for watching.

STARKs serve for zero-knowledge proof of program execution. Like SNARKs are based on the idea of representing a program code as a circuit with algebraic operations in nodes. A statement about a program execution can be written as a statement on polynomials. It is apparently easy to design a short interactive proof about statements like "this list is a set of values of a low degree function F" with a small assumption that the prover selects another low-degree polynomial G as part of the proof. The proof requires the prover to compute F on a much larger set and can be used in zero-knowledge if only queries to these extra values are allowed.

The difficulty lies in making the prover to use a low-degree G. In SNARKs this is done by using quadratic polynomials with trapdoors so that the statement XY=Z can be asserted using pairings over user-provided X and pre-committed Y as g^Y. In STARKs the prover instead creates a proof of low-degree, which is a major contribution by itself. During the proof the prover commits to the values of F and G via Merkle trees and provides selective openings. The authors' widely-advertised FRI algorithm of proximity proof works in these conditions. The proof size depends logarithmically on the circuit size, and thus the verifier's work, but for hash function circuits this all is within 1 MB size and 1 second time.

It is interesting to note that due to nature of proximity proof algorithm, the computation is done in not so large binary fields (e.g. 2^32), which contrasts to big prime fields of SNARKs. Consecutively, for zero-knowledge proofs of hash computations, the best hash functions are those native to these fields (or subfields). The AES-based hash function from the paper is insecure, but others with bigger S-boxes are worth looking at.

ASIACRYPT 2018 Accepted Papers Part 4

19) "Adaptively Simulation-Secure Attribute-Hiding Predicate Encryption"
not available yet

20) "A Framework for Achieving KDM-CCA Secure Public-Key Encryption" https://eprint.iacr.org/2018/845.pdf
How to secure a cryptosystem if an adversary can ask for encryption of secret keys.

21) "On the Statistical Leak of the GGH13 Multilinear Map and some Variants" https://eprint.iacr.org/2017/482.pdf
The multilinear maps were a long standig problem seemingly solved in 2013. Since then, a number of attacks have been proposed making the original proposal insecure. It is still in use as there are few alternatives. Many attacks result from statistical attacks on lattices, as lattices are crucial part of the MM design. This paper tries to quantify the statistical leak in many settings.

22) "Understanding and Constructing AKE via Double-key Key Encapsulation Mechanism" https://eprint.iacr.org/2018/817.pdf
AKE (authenticated key exchange) is a group of secure channel protocols where for each session a new (ephemeral) key pair is generated using the long-term key (which is common in secure messengers such as Signal, for example). This paper provides a formal treatment of this approach.

23) "Block Cipher Invariants as Eigenvectors of Correlation Matrices" https://eprint.iacr.org/2018/763.pdf
When round constants or their combination with round keys have low entropy, a state invariant can undergo quite many rounds. The paper shows how to express such invariants via correlation matrices. This does not lead to any significant new attack on well known ciphers, but is a good reminder to choose random constants when possible.

24) "Quantum Lattice Enumeration and Tweaking Discrete Pruning" https://eprint.iacr.org/2018/546.pdf
Lattice-based cryptography seemingly resist quantum computers which makes it promising for post-quantum NIST competition. However, quantum lattice attacks get better even though they remain exponential. This paper establishes new security parameters for some post-quantum candidates based on new attacks.

ACM CCS 2018, Part 1.

This major security conference takes place in 2 weeks, so it is good time to review real-crypto-related papers presented there in case you can not attend. There are a few interesting sections, and today we'll look at "Crypto attacks".

1) "Practical state recovery attacks against legacy RNG implementations" https://duhkattack.com/paper.pdf
The ANSI X9.17 key generation algorithm used a block cipher in a way that state or key compromise allow both earlier and future outputs. Interestingly, the secret key for the cipher can often be retrieved from binaries or source code of popular applications.

2) "Prime and Prejudice: Primality Testing Under Adversarial Conditions" https://eprint.iacr.org/2018/749.pdf
Many applications and protocols need prime numbers, and if an adversary manages to slip composite numbers in, the security level drops. Apparently many libraries use Miller-Rabin test as a check, which can be fooled rather easily. The paper shows how easily, where it can be exploited, and which test should be used instead. By the way, I know another usecase: in Zerocoin an issued coin is a commitment to some secret values, which must be prime to be included in an RSA accumulator. If such a coin is not a prime, a forgery inclusion proof can be produced.

3) "Release the Kraken: New KRACKs in the 802.11 Standard" https://papers.mathyvanhoef.com/ccs2018.pdf
How to affect a new key installation in the WiFi protocol.

4) "Pump up the Volume: Practical Database Reconstruction from Volume Leakage on Range Queries"
Not yet available, but the title is self-explaining.

ZKSNARK sidechain for Ethereum https://ethresear.ch/t/roll-up-roll-back-snark-side-chain-17000-tps/3675

A group of authors has suggested a sidechain, which is simultaneously anchored to the Ethereum blockchain (resides at some contract address) and whose updates are proved in zero knowledge to be correct Ethereum Virtual Machine executions. Their current suggestion is to use SHA-256 (later Pedersen commitment) as a hash function for the state Merkle tree. A designated operator converts all sidechain transactions to SNARKs. The calculation is 2K constraints for signatures + 30K constraints for the Merkle opening proof. Using DIZK (Usenix'18, see above) for distributed proof generation, they aim for 17K sidechain transactions per single snark proof, but the transactions are currency transfer only (no contracts!).

I also note that for 17K transactions in a single snark you would not need a full Merkle opening for all of them, as these can be compressed. Also Pedersen hash is not 1K constraints but more. For contract calls each transaction would take significantly more time to generate a SNARK. Still, would be nice to see. And of course, replace Pedersen with a suitable version of MIMC.

ACM CCS 18 Crypto Part 2

5) "Improved Non-Interactive Zero Knowledge with Applications to Post-Quantum Signatures" https://eprint.iacr.org/2018/475.pdf
ZK proofs of knowledge and thus signatures can be derived from multiparty computation protocols by proving the view correctness of some subset of parties. The benefit of this approach is that only symmetric primitives are needed such as ciphers and hash functions, and the disadvantage is signatures being linear in circuit size. This paper presents some performance improvements in this direction. Note again, like for SNARK and STARKs, that dedicated (low-AND-gate) ciphers may lead to drastic performance improvements.

6) "Symbolic Proofs for Lattice-Based Cryptography" https://eprint.iacr.org/2018/765.pdf
How to prove the security of lattice-based protocols symbolically.

7) "Lattice-Based zk-SNARKs from Square Span Programs" https://eprint.iacr.org/2018/275.pdf
Authors show how to create SNARKs using lattices (in contrast to quantum-vulnerable elliptic curve SNARKs), but the performance is quite slow (and measurements are not fully provided).

8) "Lattice-Based Group Signatures and Zero-Knowledge Proofs of Automorphism Stability" https://eprint.iacr.org/2018/779.pdf
Another lattice-based cryptosystem. Public, private keys and signatures are all hundreds of KBytes. Seems fine in the post-quantum world:)

ASIACRYPT 2018 Part 5

25) "Tighter Security Proofs for GPV-IBE in the Quantum Random Oracle Model" https://eprint.iacr.org/2018/451.pdf

As Learning With Errors (LWE) primitives seem to withstand quantum computers, it is natural to see if the security proofs that involve LWE also are quantum-proof (for example that a hash function can be computed on a quantum machine). This paper creates a proof for one identity-based encryption scheme based on LWE.

26) "Attribute-Based Signatures for Unbounded Languages from Standard Assumptions" https://eprint.iacr.org/2018/842.pdf
How to create fine grained authentication for policies expressed in powerful languages.

27) "Free IF: How to Omit Inactive Branches and Implement S-Universal Garbled Circuit (Almost) for Free" https://eprint.iacr.org/2018/789.pdf
Optimizing secure function evaluation based on garbled circuits.

28) "Constructing Ideal Secret Sharing Schemes based on Chinese Remainder Theorem?" https://eprint.iacr.org/2018/837.pdf
Even though Shamir's secret sharing has perfect rate, it can not handle arbitrary weights. This paper is the first example of weight-capable secret sharing scheme based on CRT.

29 ) "Improved Inner-product Encryption with Adaptive Security and Full Attribute-hiding" https://eprint.iacr.org/2018/833.pdf
Improving attribute-based encryption. In ABE each plaintext has some boolean attributes, and multiple secret keys exist for decryption. Each secret key can decrypt only a subset of ciphertexts, concretely those whose attributes turn a boolean function F (specific to this key) - to 1.

30) "Simple and More Efficient PRFs with Tight Security from LWE and Matrix-DDH" https://eprint.iacr.org/2018/826.pdf
Pseudo-random functions from LWE: not so fast, not so small, but interesting in the view of post-quantum use of LWE.

Mirror curves

Bulletproofs is a great technique but by default it works with circuits over a prime field. Therefore if the algorithm to prove about works points on elliptic curves over some F_p and order N, it might be nontrivial and/or expensive to express point arithmetic over some another field F_p'. Wonderfully, since a group of curve points as multiples of a base point is isomorphic to prime field F_N (when N is prime), one can extend the bulletproof proofs to curve point statements by replacing integers with scalars.

So if you have two curves (called mirror): A with prime order N and field F_P, and B with order P and field F_N' (maybe N'=N but not necessary). Then you can assert about, for example, signatures on curve A with Bulletproofs using curve B natively. Interestingly, for the Bitcoin curve Secp256k1 there exists a curve with P and N merely swapped (for this to be legitimate the numbers should be within min(2sqr_root(N,P)) due to the Hasse theorem). Existence of such mirror curve for other curves and the security implications of using both curves simultaneously is the subject of future research.

https://twitter.com/pwuille/status/993572063389605889
https://github.com/BlockchainCommons/secp256k1/issues/1
https://mathoverflow.net/questions/249982/elliptic-curve-related-equivalence-between-fields-of-different-characteristic

How Zcash works

In (simplified) Bitcoin, a transaction consists of a list of unspent outputs (=sender public key + hash of transaction he spends), a list of receiver addresses (=public keys), total amount, and which receiver gets how much. All this is public. In Zcash, all this is hidden: you can think that there are only hashes of all values + proof of correctness. Transactions are ordered by the consensus, and are appended to the addition-only tree T as a leaf, where the root of the tree is assigned with the value of the hash of all leaves.

The proof of correctness evidently includes:
- the proof that for every sender public key the sender knows both his private key, amount and all receiver keys.
- the proof that the amounts for all receivers add up to the total amount.
- the proof that every unspent output is part of the tree T (or some its earlier version).

Since all the values to prove about are hashed, we need a protocol that proves the correctness of computation without revealing inputs and some outputs. In this case only the root value of T and the transaction itself are revealed. The protocol is ZK-SNARK: the proof is short and fast to verify, but its generation is linear (or more) in the computation length. Since the last proof is about a chain of hash function invocations in the tree, it is the most expensive (took 40 seconds before the Sapling upgrade).

One thing we have missed: an unspent output can not be spent twice. To prevent that, we add to every receiver address a spending secret, and open it for every unspent output. The ledger then checks uniqueness. The proof then includes:
- the proof that the each revealed spending secret corresponds to one unspent output.

To prevent front-running, the spending secret is a function of the receiver address, and it is also proved. That's it.

ASIACRYPT 2018 Part 6
31) "Optimal Linear Multiparty Conditional Disclosure of Secrets Protocols" https://eprint.iacr.org/2018/441.pdf
Many MPC and attribute-based encryption schemes require a Conditional Disclosure of Secrets (CDS) protocol, where a party learns a secret known to the group only if the messages from the group satisfy some predicate. This paper explores messages that are linear functions of the secret value.

32) "Measuring, simulating and exploiting the head concavity phenomenon in BKZ" https://eprint.iacr.org/2018/856.pdf
Investigating the lattice cryptanalysis method BKZ with few results.

33) "LWE Without Modular Reduction and Improved Side-Channel Attacks Against BLISS" https://eprint.iacr.org/2018/822.pdf
BLISS is a lattice-based signature algorithm, which was attacked recently with side-channels. The authors demonstrate that the private key can be recovered using a special, easy instance of the LWE problem. However, they need 20000 traces.

34) "Short Digital Signatures and ID-KEMs via Truncation Collision Resistance" https://eprint.iacr.org/2017/061.pdf
Using collision resistance assumption for truncated hash functions to prove the security of various primitives in the standard model rather than with random oracles.

35) "Simple and Efficient Two-Server ORAM" https://eprint.iacr.org/2018/005.pdf
ORAM is the oblivious RAM protocol, when client stores memory at the server, but hides the access pattern. While most (if not all) methods require the server to compute over the entire data at least once, they compete in communication complexity. This paper shows how to work with two servers and logarithmic communication, improving the previous work by a large constant factor.

36 "Statistical Ineffective Fault Attacks on Masked AES with Fault Countermeasures" https://eprint.iacr.org/2018/357.pdf
Fault attacks are mitigated by redundant computations and other methods; the attackers respond with ineffective fault attacks where faults are analyzed regarding whether they caused an error despite redundancy. This paper shows new features of such attacks (if you can run them).