**Direct Proof of Security of Wegman-Carter Authentication with Partially Known Key**

*Aysajan Abidin and Jan-Åke Larsson*

**Abstract: ** Information-theoretically secure (ITS) authentication is needed in
Quantum Key Distribution (QKD). In this paper, we study security of
an ITS authentication scheme proposed by Wegman\&Carter, in the case
of partially known authentication key. This scheme uses a new
authentication key in each authentication attempt, to select a hash
function from an Almost Strongly Universal$_2$ hash function family.
The partial knowledge of the attacker is measured as the trace
distance between the authentication key distribution and the uniform
distribution; this is the usual measure in QKD. We provide direct
proofs of security of the scheme, when using partially known key,
first in the information-theoretic setting and then in terms of
witness indistinguishability as used in the Universal Composability
(UC) framework. We find that if the authentication procedure has a
failure probability $\epsilon$ and the authentication key has an
$\epsilon'$ trace distance to the uniform, then under ITS, the
adversary's success probability conditioned on an authentic
message-tag pair is only bounded by $\epsilon+|\mT|\epsilon'$, where
$|\mT|$ is the size of the set of tags. Furthermore, the trace
distance between the authentication key distribution and the uniform
increases to $|\mT|\epsilon'$ after having seen an authentic
message-tag pair. Despite this, we are able to prove directly that
the authenticated channel is indistinguishable from an (ideal)
authentic channel (the desired functionality), except with
probability less than $\epsilon+\epsilon'$. This proves that the
scheme is ($\epsilon+\epsilon'$)-UC-secure, without using the
composability theorem.

**Category / Keywords: **secret-key cryptography / Authentication, Strongly Universal hash functions, Partially known key, Trace distance, Universal Composability, Quantum Key Distribution.

**Date: **received 1 Mar 2013

**Contact author: **aysajan at isy liu se

**Available format(s): **PDF | BibTeX Citation

**Version: **20130305:125100 (All versions of this report)

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