Cryptosystem designers frequently assume that secrets will be manipulated in closed, reliable computing environments. Unfortunately, actual computers and microchips leak information about the operations they process. This paper examines specific methods for analyzing power consumption measurements to find secret keys from tamper resistant devices. We also discuss approaches for building cryptosystems that can operate securely in existing hardware that leaks information.

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Differential Power Analysis

Password-based protocols for authenticated key exchange (AKE) are designed to work despite the use of passwords drawn from a space so small that an adversary might well enumerate, off line, all possible passwords. While several such protocols have been suggested, the underlying theory has been lagging. We begin by defining a model for this problem, one rich enough to deal with password guessing, forward secrecy, server compromise, and loss of session keys. The one model can be used to define various goals. We take AKE (with "implicit" authentication) as the "basic" goal, and we give definitions for it, and for entity-authentication goals as well. Then we prove correctness for the idea at the center of the Encrypted Key-Exchange (EKE) protocol of Bellovin and Merritt: we prove security, in an ideal-cipher model, of the two-flow protocol at the core of EKE.

/pdf/authenticated-key-exchange-secure-against-dictionary-attacks-3e7gjz9v6y.pdf

Authenticated key exchange secure against dictionary attacks

When designing password-authenticated key exchange protocols (as opposed to key exchange protocols authenticated using cryptographically secure keys), one must not allow any information to be leaked that would allow verification of the password (a weak shared key), since an attacker who obtains this information may be able to run an off-line dictionary attack to determine the correct password. We present a new protocol called PAK which is the first Diffie-Hellman-based password-authenticated key exchange protocol to provide a formal proof of security (in the random oracle model) against both passive and active adversaries. In addition to the PAK protocol that provides mutual explicit authentication, we also show a more efficient protocol called PPK that is provably secure in the implicit -authentication model. We then extend PAK to a protocol called PAK-X, in which one side (the client) stores a plaintext version of the password, while the other side (the server) only stores a verifier for the password. We formally prove security of PAK-X, even when the server is compromised. Our formal model for password-authenticated key exchange is new, and may be of independent interest.

/pdf/provably-secure-password-authenticated-key-exchange-using-5dbmuqk5rz.pdf

Provably secure password-authenticated key exchange using Diffie-Hellman

Protocols for authentication and key establishment are the foundation for security of communications. The range and diversity of these protocols is immense, while the properties and vulnerabilities of different protocols can vary greatly.This is the first comprehensive and integrated treatment of these protocols. It allows researchers and practitioners to quickly access a protocol for their needs and become aware of existing protocols which have been broken in the literature.As well as a clear and uniform presentation of the protocols this book includes a description of all the main attack types and classifies most protocols in terms of their properties and resource requirements. It also includes tutorial material suitable for graduate students.

Protocols for Authentication and Key Establishment

This paper presents a new password authentication and key-exchange protocol suitable for authenticating users and exchanging keys over an untrusted network. The new protocol resists dictionary attacks mounted by either passive or active network intruders, allowing, in principle, even weak passphrases to be used safely. It also o ers perfect forward secrecy, which protects past sessions and passwords against future compromises. Finally, user passwords are stored in a form that is not plaintext-equivalent to the password itself, so an attacker who captures the password database cannot use it directly to compromise security and gain immediate access to the host. This new protocol combines techniques of zero-knowledge proofs with asymmetric key exchange protocols and o ers signi cantly improved performance over comparably strong extended methods that resist stolen-veri er attacks such as Augmented EKE or B-SPEKE.

The Secure Remote Password Protocol.

A new simple password exponential key exchange method (SPEKE) is described. It belongs to an exclusive class of methods which provide authentication and key establishment over an insecure channel using only a small password, without risk of offline dictionary attack. SPEKE and the closely-related Diffie-Hellman Encrypted Key Exchange (DH-EKE) are examined in light of both known and new attacks, along with sufficient preventive constraints. Although SPEKE and DH-EKE are similar, the constraints are different. The class of strong password-only methods is compared to other authentication schemes. Benefits, limitations, and tradeoffs between efficiency and security are discussed. These methods are important for several uses, including replacement of obsolete systems, and building hybrid two-factor systems where independent password-only and key-based methods can survive a single event of either key theft or password compromise.

David P. Jablon

Papers

Strong password-only authenticated key exchange