Secure set-based policy checking and its application to password registration

Policies are the corner stones of today's computer systems. They define secure states and safe operations. A common problem with policies is that their enforcement is often in con ict with user privacy. In order to check the satisfiability of a policy, a server usually needs to collect from a client some information which may be private. In this work we introduce the notion of secure set-based policy checking (SPC) that allows the server to verify policies while preserving the client's privacy. SPC is a generic protocol that can be applied in many policy-based systems. As an example, we show how to use SPC to build a password registration protocol so that a server can check whether a client's password is compliant with its password policy without seeing the password. We also analyse SPC and the password registration protocol and provide security proofs. To demonstrate the practicality of the proposed primitives, we report performance evaluation results based on a prototype implementation of the password registration protoco

Zero-Knowledge Password Policy Check from Lattices

Passwords are ubiquitous and most commonly used to authenticate users when logging into online services. Using high entropy passwords is critical to prevent unauthorized access and password policies emerged to enforce this requirement on passwords. However, with current methods of password storage, poor practices and server breaches have leaked many passwords to the public. To protect one's sensitive information in case of such events, passwords should be hidden from servers. Verifier-based password authenticated key exchange, proposed by Bellovin and Merrit (IEEE S\&P, 1992), allows authenticated secure channels to be established with a hash of a password (verifier). Unfortunately, this restricts password policies as passwords cannot be checked from their verifier. To address this issue, Kiefer and Manulis (ESORICS 2014) proposed zero-knowledge password policy check (ZKPPC). A ZKPPC protocol allows users to prove in zero knowledge that a hash of the user's password satisfies the password policy required by the server. Unfortunately, their proposal is not quantum resistant with the use of discrete logarithm-based cryptographic tools and there are currently no other viable alternatives. In this work, we construct the first post-quantum ZKPPC using lattice-based tools. To this end, we introduce a new randomised password hashing scheme for ASCII-based passwords and design an accompanying zero-knowledge protocol for policy compliance. Interestingly, our proposal does not follow the framework established by Kiefer and Manulis and offers an alternate construction without homomorphic commitments. Although our protocol is not ready to be used in practice, we think it is an important first step towards a quantum-resistant privacy-preserving password-based authentication and key exchange system

Blind Password Registration for Two-Server Password Authenticated Key Exchange and Secret Sharing Protocols

Many organisations enforce policies on the length and formation of passwords to encourage selection of strong passwords and protect their multi-user systems. For Two-Server Password Authenticated Key Exchange (2PAKE) and Two-Server Password Authenticated Secret Sharing (2PASS) protocols, where the password chosen by the client is secretly shared between the two servers, the initial remote registration of policy-compliant passwords represents a major problem because none of the servers is supposed to know the password in clear. We solve this problem by introducing Two-Server Blind Password Registration (2BPR) protocols that can be executed between a client and the two servers as part of the remote registration procedure. 2BPR protocols guarantee that secret shares sent to the servers belong to a password that matches their combined password policy and that the plain password remains hidden from any attacker that is in control of at most one server. We propose a security model for 2BPR protocols capturing the requirements of policy compliance for client passwords and their blindness against the servers. Our model extends the adversarial setting of 2PAKE/2PASS protocols to the registration phase and hence closes the gap in the formal treatment of such protocols. We construct an efficient 2BPR protocol for ASCII-based password policies, prove its security in the standard model, give a proof of concept implementation, and discuss its performance

Blind Password Registration for Verifier-based PAKE

We propose Blind Password Registration (BPR), a new class of cryptographic protocols that is instrumental for secure registration of client passwords at remote servers with additional protection against unwitting password disclosures on the server side that may occur due to the lack of the state-of-the-art password protection mechanisms implemented by the server or due to common server-compromise attacks. The dictionary attack resistance property of BPR protocols guarantees that the only information available to the server during and after the execution of the protocol cannot be used to reveal the client password without performing an offline dictionary attack on a password verifier (e.g. salted hash value) that is stored by the server at the end of the protocol. In particular, at no point in time the server is supposed to work with plain passwords. Our BPR model allows servers to enforce password policies and the requirement on the client to obey them during the execution of the BPR protocol is covered by the policy compliance property. We construct an efficient BPR protocol in the standard model for ASCII-based password policies using some techniques underlying the recently introduced Zero-Knowledge Password Policy Checks (ZKPPC). However, we do not rely on the full power of costly ZKPPC proofs and in fact show that BPR protocols can be modelled and realised simpler and significantly faster (as supported by our implementation) without using them as a building block. Our BPR protocol can directly be used to replace ZKPPC-based registration procedure for existing VPAKE protocols

A Forward-secure Efficient Two-factor Authentication Protocol

Two-factor authentication (2FA) schemes that rely on a combination of knowledge factors (e.g., PIN) and device possession have gained popularity. Some of these schemes remain secure even against strong adversaries that (a) observe the traffic between a client and server, and (b) have physical access to the client's device, or its PIN, or breach the server. However, these solutions have several shortcomings; namely, they (i) require a client to remember multiple secret values to prove its identity, (ii) involve several modular exponentiations, and (iii) are in the non-standard random oracle model. In this work, we present a 2FA protocol that resists such a strong adversary while addressing the above shortcomings. Our protocol requires a client to remember only a single secret value/PIN, does not involve any modular exponentiations, and is in a standard model. It is the first one that offers these features without using trusted chipsets. This protocol also imposes up to 40% lower communication overhead than the state-of-the-art solutions do

Structure-Preserving Smooth Projective Hashing

International audienceSmooth projective hashing has proven to be an extremely useful primitive, in particular when used in conjunction with commitments to provide implicit decommitment. This has lead to applications proven secure in the UC framework, even in presence of an adversary which can do adaptive corruptions, like for example Password Authenticated Key Exchange (PAKE), and 1-out-of-m Oblivious Transfer (OT). However such solutions still lack in efficiency, since they heavily scale on the underlying message length. Structure-preserving cryptography aims at providing elegant and efficient schemes based on classical assumptions and standard group operations on group elements. Recent trend focuses on constructions of structure- preserving signatures, which require message, signature and verification keys to lie in the base group, while the verification equations only consist of pairing-product equations. Classical constructions of Smooth Projective Hash Function suffer from the same limitation as classical signatures: at least one part of the computation (messages for signature, witnesses for SPHF) is a scalar. In this work, we introduce and instantiate the concept of Structure- Preserving Smooth Projective Hash Function, and give as applications more efficient instantiations for one-round PAKE and three-round OT, and information retrieval thanks to Anonymous Credentials, all UC- secure against adaptive adversaries

Single-Message Credential-Hiding Login

The typical login protocol for authenticating a user to a web service involves the client sending a password over a TLS-secured channel to the service, occasionally deployed with the password being prehashed. This widely-deployed paradigm, while simple in nature, is prone to both inadvertent logging and eavesdropping attacks, and has repeatedly led to the exposure of passwords in plaintext. Partly to address this problem, symmetric and asymmetric PAKE protocols were developed to ensure that the messages exchanged during an authentication protocol reveal nothing about the passwords. However, these protocols inherently require at least two messages to be sent out: one from each party. This limitation hinders wider adoption, as the most common login flow consists of a single message from client to the login server. The ideal solution would retain the password privacy properties of asymmetric PAKEs while allowing the protocol to be a drop-in replacement into legacy password-over-TLS deployments. With these requirements in mind, we introduce the notion of credential-hiding login, which enables a client to authenticate itself by sending a single message to the server, while ensuring the correct verification of credentials and maintaining credential privacy in the same strong sense as guaranteed by asymmetric PAKEs. We initiate a formal study of this primitive in the Universal Composability framework, design and implement a practical password-based protocol using identity-based encryption, and report on its performance. We also construct a variant of credential-hiding login for fuzzy secrets (e.g. biometrics), proven secure based on the Learning With Errors (LWE) assumption

A Forward-secure Efficient Two-factor Authentication Protocol

Two-factor authentication(2FA)schemes that rely on a combination of knowledge factors (e.g., PIN) and device possession have gained popularity. Some of these schemes remain secure even against strong adversaries that (a) observe the traffic between a client and server, and (b) have physical access to the client’s device, or its PIN, or breach the server. However, these solutions have several shortcomings; namely, they (i) require a client to remember multiple secret values to prove its identity, (ii) involve several modular exponentiations, and (iii) are in the non-standard random oracle model. In this work, we present a 2FA protocol that resists such a strong adversary while addressing the above shortcomings. Our protocol requires a client to remember only a single secret value/PIN, does not involve any modular exponentiations, and is in a standard model. It is the first one that offers these features without using trusted chipsets. This protocol also imposes up to 40% lower communication overhead than the state-of-the-art solutions do

Computing Low-Weight Discrete Logarithms

We propose some new baby-step giant-step algorithms for computing low-weight discrete logarithms; that is, for computing discrete logarithms in which the radix-b representation of the exponent is known to have only a small number of nonzero digits. Prior to this work, such algorithms had been proposed for the case where the exponent is known to have low Hamming weight (i.e., the radix-2 case). Our new algorithms (i) improve the best-known deterministic complexity for the radix-2 case, and then (ii) generalize from radix-2 to arbitrary radixes b>1. We also discuss how our new algorithms can be used to attack several recent Verifier-based Password Authenticated Key Exchange (VPAKE) protocols from the cryptographic literature with the conclusion that the new algorithms render those constructions completely insecure in practice

Provably Secure Identity-Based Remote Password Registration

One of the most significant challenges is the secure user authentication. If it becomes breached, confidentiality and integrity of the data or services may be compromised. The most widespread solution for entity authentication is the password-based scheme. It is easy to use and deploy. During password registration typically users create or activate their account along with their password through their verification email, and service providers are authenticated based on their SSL/TLS certificate. We propose a password registration scheme based on identity-based cryptography, i.e. both the user and the service provider are authenticated by their short-lived identity-based secret key. For secure storage a bilinear map with a salt is applied, therefore in case of an offline attack the adversary is forced to calculate a computationally expensive bilinear map for each password candidate and salt that slows down the attack. New adversarial model with new secure password registration scheme are introduced. We show that the proposed protocol is based on the assumptions that Bilinear Diffie-Hellman problem is computationally infeasible, bilinear map is a one-way function and Mac is existentially unforgeable under an adaptive chosen-message attack