[Paper Review] Comment on "An arbitrated quantum signature protocol based on the chained CNOT operations encryption"

Yiping Luo, Tzonelih Hwang|arXiv (Cornell University)|Dec 2, 2015

Quantum Information and Cryptography11 references3 citations

TL;DR

This paper critiques Li et al.'s 2015 arbitrated quantum signature (AQS) protocol based on chained CNOT operations, demonstrating that an attacker can forge valid signatures without detection. The authors prove the scheme violates unforgeability and non-repudiation—core security properties—rendering it insecure for practical use in quantum digital signatures.

ABSTRACT

In 2015, Li et al. (Quantum Inf Process (2015) 14:2171-2181) proposed an arbitrated quantum signature (AQS) scheme based on the chained controlled-NOT operations encryption. However, this paper points out that in their scheme an attacker can forge a signature without being detected. Therefore, Li et al.'s AQS scheme cannot satisfy the unforgeability and non-repudiation property.

Motivation & Objective

• To analyze the security properties of Li et al.'s arbitrated quantum signature (AQS) protocol based on chained CNOT operations.
• To identify potential vulnerabilities that could allow an attacker to forge signatures undetected.
• To demonstrate that the scheme fails to satisfy the fundamental security requirements of unforgeability and non-repudiation.
• To provide a formal critique of the protocol’s design flaws using quantum attack models.
• To caution researchers and practitioners against deploying the scheme due to its inherent insecurity.

Proposed method

• The authors analyze the structure of Li et al.'s AQS protocol, focusing on the use of chained CNOT operations for encryption.
• They model an active attack scenario where an adversary intercepts and manipulates quantum states during signature generation.
• The attack exploits the lack of proper authentication and verification mechanisms in the protocol’s key exchange and signature verification phases.
• By manipulating entangled states and exploiting symmetry in the CNOT chain, the attacker generates a valid signature without knowledge of the private key.
• The analysis shows that the verification process cannot detect such forged signatures due to flawed state validation.
• The critique relies on quantum information theory principles, particularly the properties of entanglement and unitary operations in CNOT-based circuits.

Experimental results

Research questions

• RQ1 Can an attacker forge a quantum signature in Li et al.'s AQS protocol without being detected?
• RQ2 Does the use of chained CNOT operations in the protocol provide sufficient security against existential forgery?
• RQ3 Is the protocol’s verification mechanism robust enough to detect tampering or impersonation by a malicious party?
• RQ4 To what extent does the scheme satisfy the unforgeability and non-repudiation properties required for digital signatures?
• RQ5 Are there structural flaws in the protocol’s design that allow an adversary to bypass authentication using quantum state manipulation?

Key findings

• An attacker can forge a valid signature in Li et al.'s AQS protocol without being detected by the verifier.
• The scheme fails to satisfy the unforgeability property, as an adversary can generate a signature matching any message without access to the private key.
• The non-repudiation property is also compromised, since the signer cannot be uniquely bound to a signature due to the forgery vulnerability.
• The core flaw lies in the insecure handling of quantum states during the chained CNOT encryption process, enabling state manipulation.
• The verification process does not detect forged signatures due to insufficient checks on the integrity of transmitted quantum states.
• The analysis confirms that the protocol is fundamentally insecure and unsuitable for deployment in real-world quantum signature systems.