Problem with constructing MAC. The messages X to be authenticated consists of z independent blocks, so that:X = x1||x2|| . . . ||xz, where every xi consists of |xi| = 8 bits. The input blocks are consecutively put into the compression function ci = h(ci−1, xi) = ci−1⊕xiAt the end, the MAC value: MACk(X) = cz+k mod 28 is calculated, where k is a 64-bit long shared key. Describe how exactly the (effective part of the) key k can be calculated with only one known message X.

To recover the effective part of the key k, exploit the fact that the MAC value MACk(X) is only a…

Answered: Problem with constructing MAC. The…

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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Problem with constructing MAC. The messages X to be authenticated consists of z independent blocks, so that:

X = x₁||x₂|| . . . ||x_z, where every x_i consists of |x_i| = 8 bits. The input blocks are consecutively put into the compression function

c_i = h(c_i₋₁, x_i) = c_i₋₁⊕x_i

At the end, the MAC value: MAC_k(X) = c_z+k mod 2⁸is calculated, where k is a 64-bit long shared key. Describe how exactly the (effective part of the) key k can be calculated with only one known message X.

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