The use of biometric-based security protocols is on the steep rise. As biometrics become more popular, we witness more attacks. For example, recent BrutePrint/InfinityGauntlet attacks showed how to brute-force fingerprints stored on an Android phone in about 40 minutes. The attacks are possible because biometrics, like passwords, do not have high entropy. But unlike passwords, brute-force attacks are much more damaging for biometrics, because one cannot easily change biometrics in case of compromise. In this work, we propose a novel provably secure Brute-Force Resistant Biometrics (BFRB) protocol for biometric-based authentication and key reconstruction that protects against brute-force attacks even when the server storing biometric-related data is compromised. Our protocol utilizes a verifiable partially oblivious pseudorandom function, an authenticated encryption scheme, a pseudorandom function, and a hash. We formally define security for a BFRB protocol and reduce the security of our protocol to the security of the building blocks. We implement the protocol and study its performance for the ND-0405 iris dataset.

We revisit the privacy and security analyses of FIDO2, a widely deployed standard for passwordless authentication on the Web. We discuss previous works and conclude that each of them has at least one of the following limitations: (i) impractical trusted setup assumptions, (ii) security models that are inadequate in light of state of the art of practical attacks, (iii) not analyzing FIDO2 as a whole, especially for its privacy guarantees. Our work addresses these gaps and proposes revised security models for privacy and authentication. Equipped with our new models, we analyze FIDO2 modularly and focus on its component protocols, WebAuthn and CTAP2, clarifying their exact security guarantees. In particular, our results, for the first time, establish privacy guarantees for FIDO2 as a whole. Furthermore, we suggest minor modifications that can help FIDO2 provably meet stronger privacy and authentication definitions and withstand known and novel attacks.

Searchable encryption, or more generally, structured encryption, permits search over encrypted data. It is an important cryptographic tool for securing cloud storage. The standard security notion for structured encryption mandates that a protocol leaks nothing about the data or queries, except for some allowed leakage, defined by the leakage function. This is due to the fact that some leakage is unavoidable for efficient schemes.\\ Unfortunately, it was shown by numerous works that even innocuous-looking leakage can often be exploited by attackers to undermine users' privacy and recover their queries and/or data, despite the structured encryption schemes being provably secure. Nevertheless, the standard security remains the go-to notion used to show the 'security' of structured encryption schemes. While it is not likely that researchers will design practical structured encryption schemes with no leakage, it is not satisfactory that very few works study ways to assess leakage.This work proposes a novel framework to quantify leakage. Our methodology is inspired by the quantitative information flow, and we call our method q-leakage analysis. We show how $q$-leakage analysis is related to the standard security. We also demonstrate the usefulness of q-leakage analysis by analyzing the security of two existing schemes with complex leakage functions.

The paper provides the first provable security analysis of the Butterfly Key Mechanism (BKM) protocol from IEEE 1609.2.1 standard. The BKM protocol specifies a novel approach for efficiently requesting multiple certificates for use in vehicle-to-everything (V2X) communication. We define the main security goals of BKM, such as vehicle privacy and communication authenticity. We prove that the BKM protocol, with small modifications, meets those security goals. We also propose a way to significantly improve the protocol's efficiency without sacrificing security.

Abstract Secure channel establishment protocols such as Transport Layer Security (TLS) are some of the most important cryptographic protocols, enabling the encryption of Internet traffic. Reducing latency (the number of interactions between parties before encrypted data can be transmitted) in such protocols has become an important design goal to improve user experience. The most important protocols addressing this goal are TLS 1.3, the latest TLS version standardized in 2018 to replace the widely deployed TLS 1.2, and Quick UDP Internet Connections (QUIC), a secure transport protocol from Google that is implemented in the Chrome browser. There have been a number of formal security analyses for TLS 1.3 and QUIC, but their security, when layered with their underlying transport protocols, cannot be easily compared. Our work is the first to thoroughly compare the security and availability properties of these protocols. Toward this goal, we develop novel security models that permit “layered” security analysis. In addition to the standard goals of server authentication and data confidentiality and integrity, we consider the goals of IP spoofing prevention, key exchange packet integrity, secure channel header integrity, and reset authentication, which capture a range of practical threats not usually taken into account by existing security models that focus mainly on the cryptographic cores of the protocols. Equipped with our new models we provide a detailed comparison of three low-latency layered protocols: TLS 1.3 over TCP Fast Open (TFO), QUIC over UDP, and QUIC[TLS] (a new design for QUIC that uses TLS 1.3 key exchange) over UDP. In particular, we show that TFO’s cookie mechanism does provably achieve the security goal of IP spoofing prevention. Additionally, we find several new availability attacks that manipulate the early key exchange packets without being detected by the communicating parties. By including packet-level attacks in our analysis, our results shed light on how the reliability, flow control, and congestion control of the above layered protocols compare, in adversarial settings. We hope that our models will help protocol designers in their future protocol analyses and that our results will help practitioners better understand the advantages and limitations of secure channel establishment protocols.

Abstract We study the problem of privacy-preserving approximate kNN search in an outsourced environment — the client sends the encrypted data to an untrusted server and later can perform secure approximate kNN search and updates. We design a security model and propose a generic construction based on locality-sensitive hashing, symmetric encryption, and an oblivious map. The construction provides very strong security guarantees, not only hiding the information about the data, but also the access, query, and volume patterns. We implement, evaluate efficiency, and compare the performance of two concrete schemes based on an oblivious AVL tree and an oblivious BSkiplist.

We propose the first user authentication and key exchange protocols that can tolerate strong corruptions on the client-side. If a user happens to log in to a server from a terminal that has been fully compromised, then the other past and future user's sessions initiated from honest terminals stay secure. We define the security model for Human Authenticated Key Exchange HAKE) protocols and first propose two generic protocols based on human-compatible (HC) function family, password-authenticated key exchange (PAKE), commitment, and authenticated encryption. We prove our HAKE protocols secure under reasonable assumptions and discuss efficient instantiations. We thereafter propose a variant where the human gets help from a small device such as RSA SecurID. This permits to implement an HC function family with stronger security and thus allows to weaken required assumptions on the PAKE. This leads to the very efficient HAKE which is still secure in case of strong corruptions. We believe that our work will promote further developments in the area of human-oriented cryptography.

QUIC is a secure transport protocol developed by Google and implemented in Chrome in 2013, currently representing one of the most promising solutions to decreasing latency while intending to provide security properties similar with TLS. In this work we shed some light on QUIC's strengths and weaknesses in terms of its provable security and performance guarantees in the presence of attackers. We first introduce a security model for analyzing performance-driven protocols like QUIC and prove that QUIC satisfies our definition under reasonable assumptions on the protocol's building blocks. However, we find that QUIC does not satisfy the traditional notion of forward secrecy that is provided by some modes of TLS, e.g., TLS-DHE. Our analyses also reveal that with simple bit-flipping and replay attacks on some public parameters exchanged during the handshake, an adversary could easily prevent QUIC from achieving minimal latency advantages either by having it fall back to TCP or by causing the client and server to have an inconsistent view of their handshake leading to a failure to complete the connection. We have implemented these attacks and demonstrated that they are practical. Our results suggest that QUIC's security weaknesses are introduced by the very mechanisms used to reduce latency, which highlights the seemingly inherent trade off between minimizing latency and providing "good" security guarantees.

This paper provides the provable-security treatment of path vector routing protocols. We first design a security definition for routing path vector protocols by studying, generalizing, and formalizing numerous known threats. Our model incorporates three major security goals. It is quite strong, yet simple to use. We prove by reduction that S-BGP satisfies two out of the security model's three goals, assuming the underlying signature scheme is secure. Under the same assumption, we next show how the protocol can be modified to meet all three security goals simultaneously. Finally, we study security of partial PKI deployment of path vector protocols when not all nodes have public keys. We investigate the possibilities of relaxing the PKI requirement and relying on the non-cryptographic physical security of the protocol in order to achieve possibly weaker, but still well-defined, notions of security. We also present the necessary and sufficient conditions to achieve full security in the partial PKI deployment scenario. We believe our conclusions will prove useful for protocol developers, standards bodies and government agencies.

Kerberos is a widely deployed network authentication protocol currently being considered for standardisation. Many works have analysed its security, identifying flaws and often suggesting fixes, thus promoting the protocol's evolution. Several recent results present successful, formal methods-based verifications of a significant portion of the current version, v.5 and some even imply security in the computational setting. For these results to hold, encryption in Kerberos should satisfy strong cryptographic security notions. However, prior to the authors' work, none of the encryption schemes currently deployed as part of Kerberos, nor their proposed revisions, were known to provably satisfy such notions. The authors take a close look at Kerberos' encryption, and they confirm that most of the options in the current version provably provide privacy and authenticity, although some require slight modifications which they suggest. The authors' results complement the formal methods-based analysis of Kerberos that justifies its current design.

OAEP is one of the few standardized and widely deployed public-key encryption schemes. It was designed by Bellare and Rogaway as a scheme based on a trapdoor permutation such as RSA. RSA-OAEP is standardized in RSA's PKCS #1 v2.1 and is part of several standards. OAEP was shown to be IND-CCA secure assuming the underlying trapdoor permutation is partial one-way, and RSA-OAEP was proven to be IND-CCA under the standard RSA assumption, both in the random oracle model. However, the latter reduction is not tight, meaning that the guaranteed level of security is not very high for a practical parameter choice. We observe that the situation is even worse because both analyses were done in the single-query setting, i.e., where an adversary gets a single challenge ciphertext. This does not take into account the fact that in reality an adversary can observe multiple ciphertexts of related messages. The results about the multiquery setting imply that the guaranteed concrete security can degrade by a factor of$q$, which is the number of challenge ciphertexts an adversary can get. We propose a very simple modification of the OAEP encryption, which asks that the trapdoor permutation instance is only applied to a part of the OAEP transform. We show that IND-CCA security of this scheme is tightly related to the hardness of one-wayness of the trapdoor permutation in the random oracle model. This implies tight security for RSA-OAEP under the RSA assumption. We also show that security does not degrade as the number of ciphertexts an adversary can see increases. Moreover, OAEP can be used to encrypt long messages without using hybrid encryption. We believe that this modification is easy to implement, and the benefits it provides deserves the attention of standard bodies.

Identity-based encryption (IBE) is an exciting alternative to public-key encryption, as IBE eliminates the need for a Public Key Infrastructure (PKI). Any setting, PKI- or identity-based, must provide a means to revoke users from the system. Efficient revocation is a well-studied problem in the traditional PKI setting. However in the setting of IBE, there has been little work on studying the revocation mechanisms. The most practical solution requires the senders to also use time periods when encrypting, and all the receivers (regardless of whether their keys have been compromised or not) to update their private keys regularly by contacting the trusted authority. We note that this solution does not scale well – as the number of users increases, the work on key updates becomes a bottleneck. We propose an IBE scheme that significantly improves key-update efficiency on the side of the trusted party (from linear to logarithmic in the number of users), while staying efficient for the users. Our scheme builds on the ideas of the Fuzzy IBE primitive and binary tree data structure, and is provably secure.

We construct two new multiparty digital signature schemes that allow multiple signers to sequentially and non-interactively produce a compact, fixed-length signature. First, we introduce a new primitive that we call ordered multisignature (OMS) scheme, which allows signers to attest to a common message as well as the order in which they signed. Our OMS construction substantially improves computational efficiency and scalability over any existing scheme with suitable functionality. Second, we design a new identity-based sequential aggregate signature scheme, where signers can attest to different messages and signature verification does not require knowledge of traditional public keys. The latter property permits savings on bandwidth and storage as compared to public-key solutions. In contrast to the only prior scheme to provide this functionality, ours offers improved security that does not rely on synchronized clocks or a trusted first signer. We provide formal security definitions and support the proposed schemes with security proofs under appropriate computational assumptions. We focus on applications of our schemes to secure network routing, but we believe that they will find other applications as well.

We construct new multiparty signature schemes that allow multiple signers to sequentially produce a compact, fixed-length signature simultaneously attesting to the message(s) they want to sign. First, we introduce a new primitive that we call ordered multisignatures (OMS), which allow signers to attest to a common message as well as the order in which they signed. Our OMS construction substantially improves computational efficiency over any existing scheme with comparable functionality. Second, we design a new identity-based sequential aggregate signature scheme, where signers can attest to different messages and signature verification does not require knowledge of traditional public keys. The latter property permits savings on bandwidth and storage as compared to public-key solutions. In contrast to the only prior scheme to provide this functionality, ours offers improved security that does not rely on synchronized clocks or a trusted first signer. Security proofs according to the corresponding security definitions and under appropriate computational assumptions are provided for all the proposed schemes. We give several applications of our schemes to secure network routing, and we believe that they will find many other applications as well.

Kerberos is a widely-deployed network authentication protocol that is being considered for standardization. Many works have analyzed its security, identifying flaws and often suggesting fixes, thus helping the protocol's evolution. Several recent results present successful formal-methods-based verification of a significant portion of the current version 5, and some even imply security in the computational setting. For these results to hold, encryption in Kerberos should satisfy strong cryptographic security notions. However, neither currently deployed as part of Kerberos encryption schemes nor their proposed revisions are known to provably satisfy such notions. We take a close look at Kerberos' encryption and confirm that most of the options in the current version provably provide privacy and authenticity, some with slight modification that we suggest. Our results complement the formal-methods-based analysis of Kerberos that justifies its current design.

This paper proposes several new schemes which allow a sender to send encrypted messages to multiple recipients more efficiently (in terms of bandwidth and computation) than by using a standard encryption scheme. Most of the proposed schemes explore a new natural technique called randomness reuse. In order to analyze security of our constructions, we introduce a new notion of multirecipient encryption schemes (MRESs) and provide definitions of security for them. We finally show a way to avoid ad hoc analyses by providing a general test that can be applied to a standard encryption scheme to determine whether the associated randomness reusing MRES is secure. The results and applications cover both asymmetric and symmetric encryption.

Encrypting data in unprotected memory has gained much interest lately for digital rights protection and security reasons. Counter mode is a well-known encryption scheme. It is a symmetric-key encryption scheme based on any block cipher, e.g. AES. The scheme's encryption algorithm uses a block cipher, a secret key and a counter (or a sequence number) to generate an encryption pad which is XORed with the data stored in memory. Like other memory encryption schemes, this method suffers from the inherent latency of decrypting encrypted data when loading them into the on-chip cache. In this paper, we present a novel technique to hide the latency overhead of decrypting counter mode encrypted memory by predicting the sequence number and pre-computing the encryption pad that we call one-time-pad or OTP. In contrast to the prior techniques of sequence number caching, our mechanism solves the latency issue by using idle decryption engine cycles to speculatively predict and pre-compute OTPs before the corresponding sequence number is loaded. This technique incurs very little area overhead. In addition, a novel adaptive OTP prediction technique is also presented to further improve our regular OTP prediction and precomputation mechanism. This adaptive scheme is not only able to predict encryption pads associated with static and infrequently updated cache lines but also those frequently updated ones as well. Experimental results using SPEC2000 benchmark show an 82% prediction rate. Moreover, we also explore several optimization techniques for improving the prediction accuracy. Two specific techniques, two-level prediction and context-based prediction are presented and evaluated.