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We demonstrate how adversaries with unbounded computing resources can breakQuantum Key Distribution (QKD) protocols which employ a particular messageauthentication code suggested previously. This authentication code, featuringlow key consumption, is not Information-Theoretically Secure (ITS) since foreach message the eavesdropper has intercepted she is able to send a differentmessage from a set of messages that she can calculate by finding collisions ofa cryptographic hash function. However, when this authentication code wasintroduced it was shown to prevent straightforward Man-In-The-Middle (MITM)attacks against QKD protocols.  In this paper, we prove that the set of messages that collide with any givenmessage under this authentication code contains with high probability a messagethat has small Hamming distance to any other given message. Based on this factwe present extended MITM attacks against different versions of BB84 QKDprotocols using the addressed authentication code; for three protocols wedescribe every single action taken by the adversary. For all protocols theadversary can obtain complete knowledge of the key, and for most protocols hersuccess probability in doing so approaches unity.  Since the attacks work against all authentication methods which allow tocalculate colliding messages, the underlying building blocks of the presentedattacks expose the potential pitfalls arising as a consequence of non-ITSauthentication in QKD-postprocessing. We propose countermeasures, increasingthe eavesdroppers demand for computational power, and also prove necessary andsufficient conditions for upgrading the discussed authentication code to theITS level.

Attacks on quantum key distribution protocols that employ non-ITS authentication