2010 | OriginalPaper | Chapter
Cryptographic Extraction and Key Derivation: The HKDF Scheme
Author : Hugo Krawczyk
Published in: Advances in Cryptology – CRYPTO 2010
Publisher: Springer Berlin Heidelberg
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In spite of the central role of
key derivation functions (KDF)
in applied cryptography, there has been little formal work addressing the design and analysis of general multi-purpose KDFs. In practice, most KDFs (including those widely standardized) follow ad-hoc approaches that treat cryptographic hash functions as perfectly random functions. In this paper we close some gaps between theory and practice by contributing to the study and engineering of KDFs in several ways. We provide detailed rationale for the design of KDFs based on the
extract-then-expand
approach; we present the first general and rigorous definition of KDFs and their security that we base on the notion of
computational extractors
; we specify a concrete
fully practical
KDF based on the HMAC construction; and we provide an analysis of this construction based on the extraction and pseudorandom properties of HMAC. The resultant KDF design can support a large variety of KDF applications under suitable assumptions on the underlying hash function; particular attention and effort is devoted to minimizing these assumptions as much as possible for each usage scenario.
Beyond the theoretical interest in modeling KDFs, this work is intended to address two important and timely needs of cryptographic applications: (i) providing a single hash-based KDF design that can be standardized for use in multiple and diverse applications, and (ii) providing a conservative, yet efficient, design that exercises much care in the way it utilizes a cryptographic hash function. (The HMAC-based scheme presented here, named
HKDF
, is being standardized by the IETF.)