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**2004/162**

**Title:**

On the Key-Uncertainty of Quantum Ciphers and the Computational Security of One-way Quantum Transmission

**Authors:**
Ivan Damgaard, Thomas Pedersen, Louis Salvail

**Abstract:**

We consider the scenario where Alice wants to send a secret(classical) n-bit message to Bob using a classical key, and where

only one-way transmission from Alice to Bob is possible. In this

case, quantum communication cannot help to obtain perfect secrecy

with key length smaller then n. We study the question of whether

there might still be fundamental differences between the case where

quantum as opposed to classical communication is used. In this

direction, we show that there exist ciphers with perfect security producing quantum ciphertext where, even if an adversary knows the

plaintext and applies an optimal measurement on the ciphertext, his

Shannon uncertainty about the key used is almost maximal. This is in

contrast to the classical case where the adversary always learns n

bits of information on the key in a known plaintext attack. We also

show that there is a limit to how different the classical and

quantum cases can be: the most probable key, given matching plain-

and ciphertexts, has the same probability in both the quantum and

the classical cases. We suggest an application of our results in

the case where only a short secret key is available and the message

is much longer.

**ePrint:**
https://eprint.iacr.org/2004/162

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