# Security Against Collective Attacks of a Modified BB84 QKD Protocol with Information only in One Basis

## TL;DR

This paper proves that a modified BB84 quantum key distribution protocol, which sends information only in one basis and tests in both, maintains security against collective attacks, with some trade-offs in testing bits.

## Contribution

It demonstrates the security of a modified BB84 protocol against collective attacks without classical information-theoretical analysis, improving understanding of protocol robustness.

## Key findings

- The modified protocol is as secure as the original BB84 against collective attacks.
- It requires more bits for testing than the original protocol.
- The security proof avoids classical information-theoretical issues with composability.

## Abstract

The Quantum Key Distribution (QKD) protocol BB84 has been proven secure against several important types of attacks: the collective attacks and the joint attacks. Here we analyze the security of a modified BB84 protocol, for which information is sent only in the z basis while testing is done in both the z and the x bases, against collective attacks. The proof follows the framework of a previous paper (Boyer, Gelles, and Mor, 2009), but it avoids the classical information-theoretical analysis that caused problems with composability. We show that this modified BB84 protocol is as secure against collective attacks as the original BB84 protocol, and that it requires more bits for testing.