## CryptoDB

### Peihan Miao

#### Publications

**Year**

**Venue**

**Title**

2024

PKC

Laconic Branching Programs from the Diffie-Hellman Assumption
Abstract

Laconic cryptography enables secure two-party computation (2PC) on unbalanced inputs with asymptotically-optimal communication in just two rounds of communication. In particular, the receiver (who sends the first-round message) holds a long input and the sender (who sends the second-round message) holds a short input, and the size of their communication to securely compute a function on their joint inputs only grows with the size of the sender's input and is independent of the receiver's input size.
The work on laconic oblivious transfer (OT) [Cho et al. CRYPTO 2017] and laconic private set intersection (PSI) [Alamati et al. TCC 2021] shows how to achieve secure laconic computation for OT and PSI from the Diffie-Hellman assumption.
In this work, we push the limits further and achieve laconic branching programs from the Diffie-Hellman assumption. In particular, the receiver holds a large branching program $P$ and the sender holds a short input $x$. We present a two-round 2PC protocol that allows the receiver to learn $x$ iff $P(x) =1$, and nothing else. The communication only grows with the size of $x$ and the depth of $P$, and does not further depend on the size of $P$.

2023

PKC

Unidirectional Updatable Encryption and Proxy Re-encryption from DDH
Abstract

Updatable Encryption (UE) and Proxy Re-encryption (PRE) allow re-encrypting a ciphertext from one key to another in the symmetric-key and public-key settings, respectively, without decryption. A longstanding open question has been the following: do unidirectional UE and PRE schemes (where ciphertext re-encryption is permitted in only one direction) necessarily require stronger/more structured assumptions as compared to their bidirectional counterparts? Known constructions of UE and PRE seem to exemplify this "gap" -- while bidirectional schemes can be realized as relatively simple extensions of public-key encryption from standard assumptions such as DDH or LWE, unidirectional schemes typically rely on stronger assumptions such as FHE or indistinguishability obfuscation (iO), or highly structured cryptographic tools such as bilinear maps or lattice trapdoors.
In this paper, we bridge this gap by showing the first feasibility results for realizing unidirectional UE and PRE from a new generic primitive that we call Key and Plaintext Homomorphic Encryption (KPHE) -- a public-key encryption scheme that supports additive homomorphisms on its plaintext and key spaces simultaneously. We show that KPHE can be instantiated from DDH. This yields the first constructions of unidirectional UE and PRE from DDH.
Our constructions achieve the strongest notions of post-compromise security in the standard model. Our UE schemes also achieve "backwards-leak directionality" of key updates (a notion we discuss is equivalent, from a security perspective, to that of unidirectionality with no-key updates).
Our results establish (somewhat surprisingly) that unidirectional UE and PRE schemes satisfying such strong security notions do not, in fact, require stronger/more structured cryptographic assumptions as compared to bidirectional schemes.

2023

TCC

On the Round Complexity of Fully Secure Solitary MPC with Honest Majority
Abstract

We study the problem of secure multiparty computation for functionalities where only one
party receives the output, to which we refer as solitary MPC. Recently, Halevi et al. (TCC 2019) studied fully secure (i.e., with guaranteed output delivery) solitary MPC and showed impossibility of such protocols for certain functionalities when there is no honest majority among the parties.
In this work, we study the round complexity of fully secure solitary MPC in the honest majority setting and with computational security. We note that a broadcast channel or public key infrastructure (PKI) setup is necessary for an n-party protocol against malicious adversaries
corrupting up to t parties where n/3 ≤ t < n/2. Therefore, we study the following settings and
ask the question: Can fully secure solitary MPC be achieved in fewer rounds than fully secure
standard MPC in which all parties receive the output?
• When there is a broadcast channel and no PKI:
– We start with a negative answer to the above question. In particular, we show that
the exact round complexity of fully secure solitary MPC is 3, which is the same as
fully secure standard MPC.
– We then study the minimal number of broadcast rounds needed to design round optimal fully secure solitary MPC. We show that both the first and second rounds of broadcast are necessary when $2 \lceil n/5 \rceil \leq t < n/2$, whereas pairwise-private channels suffice in the last round. Notably, this result also applies to fully secure standard MPC in which all parties receive the output.
• When there is a PKI and no broadcast channel, nevertheless, we show more positive results:
– We show an upper bound of 5 rounds for any honest majority. This is superior to the
super-constant lower bound for fully secure standard MPC in the exact same setting.
– We complement this by showing a lower bound of 4 rounds when $3\lceil n/7 \rceil \leq t < n/2$.
– For the special case of t = 1, n = 3, when the output receiving party does not have an input to the function, we show an upper bound of 2 rounds, which is optimal. When the output receiving party has an input to the function, we show a lower bound of 3, which matches an upper bound from prior work.
– For the special case of t = 2, n = 5, we show a lower bound of 3 rounds (an upper bound of 4 follows from prior work).
All our results also assume the existence of a common reference string (CRS) and pairwise private channels. Our upper bounds use a decentralized threshold fully homomorphic encryption (dTFHE) scheme (which can be built from the learning with errors (LWE) assumption) as the
main building block.

2021

PKC

Multi-Party Threshold Private Set Intersection with Sublinear Communication
📺
Abstract

In multi-party threshold private set intersection (PSI), $n$ parties each with a private set wish to compute the intersection of their sets if the intersection is sufficiently large. Previously, Ghosh and Simkin (CRYPTO 2019) studied this problem for the two-party case and demonstrated interesting lower and upper bounds on the communication complexity. In this work, we investigate the communication complexity of the multi-party setting $(n\geq 2)$. We consider two functionalities for multi-party threshold PSI. In the first, parties learn the intersection if each of their sets and the intersection differ by at most $T$. In the second functionality, parties learn the intersection if the union of all their sets and the intersection differ by at most $T$.
For both functionalities, we show that any protocol must have communication complexity $\Omega(nT)$. We build protocols with a matching upper bound of $O(nT)$ communication complexity for both functionalities assuming threshold FHE. We also construct a computationally more efficient protocol for the second functionality with communication complexity $\widetilde{O}(nT)$ under a weaker assumption of threshold additive homomorphic encryption. As a direct implication, we solve one of the open problems in the work of Ghosh and Simkin (CRYPTO 2019) by designing a two-party protocol with communication cost $\widetilde{O}(T)$ from assumptions weaker than FHE.
As a consequence of our results, we achieve the first "regular" multi-party PSI protocol where the communication complexity only grows with the size of the set difference and does not depend on the size of the input sets.

2021

TCC

Amortizing Rate-1 OT and Applications to PIR and PSI
📺
Abstract

Recent new constructions of rate-1 OT [D\"ottling, Garg, Ishai, Malavolta, Mour, and Ostrovsky, CRYPTO 2019] have brought this primitive under the spotlight and the techniques have led to new feasibility results for private-information retrieval, and homomorphic encryption for branching programs. The receiver communication of this construction consists of a quadratic (in the sender's input size) number of group elements for a single instance of rate-1 OT. Recently [Garg, Hajiabadi, Ostrovsky, TCC 2020] improved the receiver communication to a linear number of group elements for a single string-OT. However, most applications of rate-1 OT require executing it multiple times, resulting in large communication costs for the receiver.
In this work, we introduce a new technique for amortizing the cost of multiple rate-1 OTs. Specifically, based on standard pairing assumptions, we obtain a two-message rate-1 OT protocol for which the amortized cost per string-OT is asymptotically reduced to only four group elements. Our results lead to significant communication improvements in PSI and PIR, special cases of SFE for branching programs.
1. PIR: We obtain a rate-1 PIR scheme with client communication cost of $O(\lambda\cdot\log N)$ group elements for security parameter $\lambda$ and database size $N$. Notably, after a one-time setup (or one PIR instance), any following PIR instance only requires communication cost $O(\log N)$ number of group elements.
2. PSI with unbalanced inputs: We apply our techniques to private set intersection with unbalanced set sizes (where the receiver has a smaller set) and achieve receiver communication of $O((m+\lambda) \log N)$ group elements where $m, N$ are the sizes of the receiver and sender sets, respectively. Similarly, after a one-time setup (or one PSI instance), any following PSI instance only requires communication cost $O(m \cdot \log N)$ number of group elements. All previous sublinear-communication non-FHE based PSI protocols for the above unbalanced setting were also based on rate-1 OT, but incurred at least $O(\lambda^2 m \log N)$ group elements.

2020

CRYPTO

Two-Sided Malicious Security for Private Intersection-Sum with Cardinality
📺
Abstract

Private intersection-sum with cardinality allows two parties, where each party holds a private set and one of the parties additionally holds a private integer value associated with each element in her set, to jointly compute the cardinality of the intersection of the two sets as well as the sum of the associated integer values for all the elements in the intersection, and nothing beyond that.
We present a new construction for private intersection sum with cardinality that provides malicious security with abort and guarantees that both parties receive the output upon successful completion of the protocol. A central building block for our constructions is a primitive called shuffled distributed oblivious PRF (DOPRF), which is a PRF that offers oblivious evaluation using a secret key shared between two parties, and in addition to this allows obliviously permuting the PRF outputs of several parallel oblivious evaluations. We present the first construction for shuffled DOPRF with malicious security. We further present several new sigma proof protocols for relations across Pedersen commitments, ElGamal encryptions, and Camenisch-Shoup encryptions that we use in our main construction, for which we develop new batching techniques to reduce communication.
We implement and evaluate the efficiency of our protocol and show that we can achieve communication cost that is only 4-5x greater than the most efficient semi-honest protocol. When measuring monetary cost of executing the protocol in the cloud, our protocol is 25x more expensive than the semi-honest protocol. Our construction also allows for different parameter regimes that enable trade-offs between communication and computation.

2020

CRYPTO

Private Set Intersection in the Internet Setting From Lightweight Oblivious PRF
📺
Abstract

We present a new protocol for two-party private set intersection (PSI) with semi-honest security in the plain model and one-sided malicious security in the random oracle model. Our protocol achieves a better balance between computation and communication than existing PSI protocols. Specifically, our protocol is the fastest in networks with moderate bandwidth (e.g., 30 - 100 Mbps). Considering the monetary cost (proposed by Pinkas et al. in CRYPTO 2019) to run the protocol on a cloud computing service, our protocol also compares favorably.
Underlying our PSI protocol is a new lightweight multi-point oblivious pesudorandom function (OPRF) protocol based on oblivious transfer (OT) extension. We believe this new protocol may be of independent interest.

2018

CRYPTO

Two-Round Multiparty Secure Computation Minimizing Public Key Operations
📺
Abstract

We show new constructions of semi-honest and malicious two-round multiparty secure computation protocols using only (a fixed)
$$\mathsf {poly}(n,\lambda )$$
poly(n,λ) invocations of a two-round oblivious transfer protocol (which use expensive public-key operations) and
$$\mathsf {poly}(\lambda , |C|)$$
poly(λ,|C|) cheaper one-way function calls, where
$$\lambda $$
λ is the security parameter, n is the number of parties, and C is the circuit being computed. All previously known two-round multiparty secure computation protocols required
$$\mathsf {poly}(\lambda ,|C|)$$
poly(λ,|C|) expensive public-key operations.

#### Program Committees

- Crypto 2024
- TCC 2023
- Eurocrypt 2022
- Crypto 2021
- PKC 2021

#### Coauthors

- Saikrishna Badrinarayanan (2)
- Melissa Chase (2)
- Alessandro Chiesa (1)
- Chongwon Cho (1)
- Nico Döttling (1)
- Sanjam Garg (5)
- Matthew Green (1)
- Divya Gupta (2)
- Mohammad Hajiabadi (2)
- Jialin Li (1)
- Jingcheng Liu (1)
- Ian Miers (1)
- Pratyush Mishra (1)
- Pratyay Mukherjee (1)
- Alice Murphy (1)
- Omkant Pandey (1)
- Sarvar Patel (1)
- Sikhar Patranabis (1)
- Antigoni Polychroniadou (1)
- Srinivasan Raghuraman (1)
- Divya Ravi (1)
- Mariana Raykova (1)
- Peter Rindal (1)
- Karn Seth (1)
- Akshayaram Srinivasan (1)
- Gaven J. Watson (1)
- Moti Yung (1)