Abstract. Zero-Knowledge Password Policy Checks (ZKPPC), introduced in this work, enable blind registration of client passwords at remote servers, i.e., client passwords are never transmitted to the servers. This eliminates the need for trusting servers to securely process and store client passwords. A ZKPPC protocol, executed as part of the registration procedure, allows clients to further prove compliance of chosen passwords with respect to password policies defined by the servers. The main benefit of ZKPPC-based password registration is that it guarantees that registered passwords never appear in clear on the server side. At the end of the registration phase the server only receives and stores some verification information that can later be used for authentication in a suitable Verifier-based Password Authenticated Key Exchange (VPAKE) protocol. We give general and concrete constructions of ZKPPC protocols and suitable VPAKE protocols for ASCII-based passwords and policies that are commonly used on the web. To this end we introduce a reversible mapping of ASCII characters to integers that can be used to preserve the structure of the password string and a new randomised password hashing scheme for ASCII-based passwords.
Abstract. 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.
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 stateof-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.
Abstract. Smooth projective hash functions have been used as building block for various cryptographic applications, in particular for password-based authentication. In this work we propose the extended concept of distributed smooth projective hash functions where the computation of the hash value is distributed across n parties and show how to instantiate the underlying approach for languages consisting of Cramer-Shoup ciphertexts. As an application of distributed smooth projective hashing we build a new framework for the design of two-server password authenticated key exchange protocols, which we believe can help to "explain" the design of earlier two-server password authenticated key exchange protocols.
Secure protocols for password-based user authentication are well-studied in the cryptographic literature but have failed to see wide-spread adoption on the internet; most proposals to date require extensive modifications to the Transport Layer Security (TLS) protocol, making deployment challenging. Recently, a few modular designs have been proposed in which a cryptographically secure password-based mutual authentication protocol is run inside a confidential (but not necessarily authenticated) channel such as TLS; the password protocol is bound to the established channel to prevent active attacks. Such protocols are useful in practice for a variety of reasons: security no longer relies on users' ability to validate server certificates and can potentially be implemented with no modifications to the secure channel protocol library. We provide a systematic study of such authentication protocols. Building on recent advances in modeling TLS, we give a formal definition of the intended security goal, which we call password-authenticated and confidential channel establishment (PACCE). We show generically that combining a secure Queensland University of Technology, Brisbane, Australia channel protocol, such as TLS, with a password authentication or password-authenticated key exchange protocol, where the two protocols are bound together using the transcript of the secure channel's handshake, the server's certificate, or the server's domain name, results in a secure PACCE protocol. Our prototypes based on TLS are available as a cross-platform client-side Firefox browser extension as well as an Android application and a server-side web application that can easily be installed on servers.
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