



Independent Submission                                            D. Kim
Internet-Draft                                               Independent
Intended status: Experimental                              29 April 2026
Expires: 31 October 2026


 Enhanced Collapse Purity Filter Algorithm for Quantum Key Distribution
               draft-kim-cpf-quantum-key-distribution-00

Abstract

   This document specifies an enhanced Collapse Purity Filter (CPF)
   algorithm for Quantum Key Distribution (QKD) systems.  The enhanced
   CPF algorithm improves key generation efficiency by 100% compared to
   conventional CPF implementations while maintaining quantum security
   guarantees.  The algorithm uses adaptive filter verification instead
   of fixed threshold filtering, achieving near-zero Quantum Bit Error
   Rate (QBER) in ideal conditions and accurate eavesdropping detection
   in adversarial scenarios.

   This specification is compatible with BB84 protocol and complies with
   Korean Internet Security Agency (KISA) standards TTAK.KO-12.0281 for
   quantum key distribution protocols.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 31 October 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Enhanced CPF Algorithm Specification  . . . . . . . . . . . .   3
     2.1.  Quantum Circuit Construction  . . . . . . . . . . . . . .   3
     2.2.  Adaptive Filter Verification  . . . . . . . . . . . . . .   4
     2.3.  QBER Analysis and Eavesdropping Detection . . . . . . . .   4
   3.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Key Generation Phase (Alice)  . . . . . . . . . . . . . .   5
     3.2.  Key Verification Phase (Bob)  . . . . . . . . . . . . . .   5
   4.  Performance Analysis  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Efficiency Improvements . . . . . . . . . . . . . . . . .   5
     4.2.  Security Analysis . . . . . . . . . . . . . . . . . . . .   6
   5.  Implementation Considerations . . . . . . . . . . . . . . . .   6
     5.1.  Quantum Hardware Requirements . . . . . . . . . . . . . .   6
     5.2.  Classical Post-Processing . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     6.1.  Quantum Attacks . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Classical Attacks . . . . . . . . . . . . . . . . . . . .   7
     6.3.  Side-Channel Considerations . . . . . . . . . . . . . . .   7
     6.4.  QBER Threshold Selection  . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Example Implementation  . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Quantum Key Distribution (QKD) provides information-theoretic
   security based on the laws of quantum mechanics.  The BB84 protocol
   [BB84] established the foundation for QKD systems.  The Collapse
   Purity Filter (CPF) algorithm is a quantum circuit-based approach for
   generating cryptographic keys using quantum entanglement and
   measurement collapse properties.

   Conventional CPF implementations suffer from a critical inefficiency:
   they accept only filter qubit measurements of '0', rejecting
   approximately 50% of valid quantum states.  This document presents an



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   enhanced CPF algorithm that uses adaptive verification, comparing
   measured filter values against expected values rather than using
   fixed thresholds.

   This work complies with Korean Internet Security Agency (KISA)
   standards [KISA-TTAK] and considers post-quantum cryptography
   guidelines [NIST-PQC].  The reference implementation uses the Qiskit
   framework [QISKIT] for quantum circuit execution.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses the following terms:

   *  CPF: Collapse Purity Filter

   *  QKD: Quantum Key Distribution

   *  QBER: Quantum Bit Error Rate

   *  CNOT: Controlled-NOT quantum gate

   *  Qubit: Quantum bit

2.  Enhanced CPF Algorithm Specification

2.1.  Quantum Circuit Construction

   The enhanced CPF algorithm uses a two-qubit quantum circuit where:

   *  q0: Data qubit (encodes Alice's secret bit)

   *  q1: Filter qubit (verifies quantum state purity)

   Circuit construction procedure:

   1.  Initialize two qubits in |0⟩ state

   2.  If alice_bit = 1, apply X gate to q0

   3.  Apply CNOT(q0, q1) to create entanglement

   4.  If basis_flip = true, apply H gate to q0 (BB84 compatibility)



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   5.  Measure both qubits

   The CNOT gate creates a correlation where q1 becomes a copy of q0 in
   the computational basis.  In ideal conditions, q1 MUST equal the
   initial value of q0 (alice_bit).

2.2.  Adaptive Filter Verification

   The key innovation of the enhanced CPF algorithm is adaptive filter
   verification.  Instead of accepting only q1='0' measurements, the
   algorithm compares the measured filter value against the expected
   value.

   Verification procedure:

   function verify_quantum_state(measured_filter, expected_filter):
       if measured_filter == expected_filter:
           return ACCEPT  // Pure quantum state
       else:
           return REJECT  // Contaminated by noise or eavesdropping

   Where expected_filter = alice_bit (the initial value encoded in q0).

   This approach achieves 100% acceptance rate in ideal conditions,
   compared to 50% in conventional CPF implementations.

2.3.  QBER Analysis and Eavesdropping Detection

   The Quantum Bit Error Rate (QBER) is calculated as:

   QBER = error_count / (confirmed_bits + error_count)

   QBER thresholds (based on KISA TTAK.KO-12.0281):

   *  QBER ≤ 5%: Safe (continue communication)

   *  5% < QBER ≤ 8%: Warning (increased monitoring)

   *  8% < QBER ≤ 11%: Critical (eavesdropping suspected)

   *  QBER > 11%: Abort (session termination required)

   The enhanced CPF algorithm provides accurate QBER measurements:

   *  No eavesdropping: QBER ≈ 0% (hardware noise only)

   *  20% bit tampering: QBER ≈ 20% (accurate detection)




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   In contrast, conventional CPF shows QBER ≈ 50% even without
   eavesdropping, making attack detection ambiguous.

3.  Protocol Flow

3.1.  Key Generation Phase (Alice)

   1.  Generate random bit: alice_bit ∈ {0, 1}

   2.  Generate random basis: basis_flip ∈ {true, false}

   3.  Construct CPF quantum circuit

   4.  Execute circuit on quantum hardware (50 shots)

   5.  Measure qubits and obtain result: (q1, q0)

   6.  Verify: if q1 == alice_bit, accept q0 as key bit

   7.  Repeat until target key length achieved

   8.  Monitor QBER; abort if QBER > 11%

3.2.  Key Verification Phase (Bob)

   1.  Receive quantum key bits and sample indices from Alice

   2.  Calculate QBER using sample bits (public channel)

   3.  If QBER > 11%, abort session

   4.  Verify HMAC-SHA256 integrity tag

   5.  Apply Privacy Amplification

   6.  Derive encryption key using HKDF-SHA256

   7.  Decrypt ciphertext using AES-256-GCM

4.  Performance Analysis

4.1.  Efficiency Improvements

   Comparison with conventional CPF:







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    +====================+==================+==========+=============+
    | Metric             | Conventional CPF | Enhanced | Improvement |
    |                    |                  | CPF      |             |
    +====================+==================+==========+=============+
    | Bit acceptance     | ~50%             | ~100%    | +100%       |
    | rate               |                  |          |             |
    +--------------------+------------------+----------+-------------+
    | Circuit executions | ~512             | ~256     | -50%        |
    | (256-bit key)      |                  |          |             |
    +--------------------+------------------+----------+-------------+
    | QBER (no           | ~50%             | ~0%      | Accurate    |
    | eavesdropping)     |                  |          |             |
    +--------------------+------------------+----------+-------------+
    | QBER (20%          | ~60%             | ~20%     | Accurate    |
    | tampering)         |                  |          |             |
    +--------------------+------------------+----------+-------------+

                                 Table 1

4.2.  Security Analysis

   The enhanced CPF algorithm maintains the same security guarantees as
   conventional CPF while improving efficiency:

   *  Quantum security: Based on no-cloning theorem and measurement
      collapse

   *  Eavesdropping detection: QBER analysis with 11% threshold

   *  BB84 compatibility: Basis randomization prevents intercept-resend
      attacks

   *  Multi-layer defense: CPF filtering + QBER analysis + HMAC
      verification

5.  Implementation Considerations

5.1.  Quantum Hardware Requirements

   The algorithm can be implemented on any quantum computing platform
   supporting:

   *  Minimum 2 qubits

   *  Single-qubit gates: X, H

   *  Two-qubit gate: CNOT




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   *  Measurement in computational basis

   Tested platforms: IBM Quantum (real hardware and Aer simulator),
   compatible with Qiskit 1.0+.

5.2.  Classical Post-Processing

   Required classical cryptographic primitives:

   *  Privacy Amplification: Universal hash functions

   *  Key Derivation: HKDF-SHA256 [RFC5869]

   *  Encryption: AES-256-GCM (NIST FIPS 197)

   *  Authentication: HMAC-SHA256 [RFC2104]

6.  Security Considerations

6.1.  Quantum Attacks

   The enhanced CPF algorithm is resistant to known quantum attacks:

   *  Intercept-resend attack: Detected via QBER increase

   *  Photon number splitting: Mitigated by decoy states (future work)

   *  Trojan horse attack: Requires physical security measures

6.2.  Classical Attacks

   Classical security is provided by:

   *  AES-256-GCM: Quantum-resistant symmetric encryption

   *  HMAC-SHA256: Prevents packet tampering

   *  HKDF: Ensures key independence

6.3.  Side-Channel Considerations

   Implementations MUST protect against:

   *  Timing attacks: Use constant-time cryptographic operations

   *  Power analysis: Implement countermeasures in hardware

   *  Electromagnetic leakage: Shield sensitive components



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6.4.  QBER Threshold Selection

   The 11% QBER threshold is based on BB84 theoretical limits.
   Implementations MAY use lower thresholds (e.g., 8%) for higher
   security margins, at the cost of increased false positive rates in
   noisy environments.

7.  IANA Considerations

   This document has no IANA actions.

   Future versions may request registration of:

   *  QKD protocol identifiers

   *  Cryptographic algorithm identifiers for CPF

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [BB84]     Bennett, C.H. and G. Brassard, "Quantum cryptography:
              Public key distribution and coin tossing", Theoretical
              Computer Science Vol. 560, pp. 7-11,
              DOI 10.1016/j.tcs.2014.05.025, 2014,
              <https://doi.org/10.1016/j.tcs.2014.05.025>.




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   [KISA-TTAK]
              Korea Internet and Security Agency (KISA) and
              Telecommunications Technology Association (TTA), "Quantum
              Key Distribution Protocol Standard", TTAK.KO-
              12.0281 Quantum Key Distribution Protocol, December 2020,
              <https://www.tta.or.kr/data/
              weeklyNoticeView.do?news_id=4441>.

   [NIST-PQC] National Institute of Standards and Technology, "Post-
              Quantum Cryptography Standardization", NIST Post-Quantum
              Cryptography Project, 2024,
              <https://csrc.nist.gov/projects/post-quantum-
              cryptography>.

   [QISKIT]   IBM Quantum, "Qiskit: An Open-source Framework for Quantum
              Computing", Available at: https://qiskit.org, 2024,
              <https://qiskit.org>.

Acknowledgments

   The author thanks the IBM Quantum team for providing access to
   quantum hardware and the Qiskit framework.  This work was developed
   in compliance with KISA (Korea Internet & Security Agency) standards
   for quantum cryptography.

Example Implementation

   A reference implementation in Python using Qiskit is available at:

   https://github.com/your-repo/kisa-qkd

   Example quantum circuit construction:



















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   from qiskit import QuantumCircuit

   def build_cpf_circuit(alice_bit, basis_flip):
       qc = QuantumCircuit(2, 2)

       # Encode Alice's bit
       if alice_bit == 1:
           qc.x(0)

       # CPF entanglement
       qc.cx(0, 1)

       # BB84 basis selection
       if basis_flip:
           qc.h(0)

       # Measurement
       qc.measure([0, 1], [0, 1])

       return qc, alice_bit

   Example filter verification:

   def verify_filter(measured_filter, expected_filter):
       if measured_filter == expected_filter:
           return True  # Accept bit
       else:
           return False  # Reject (noise/eavesdropping)

Author's Address

   Dongwook Kim
   Independent
   Korea, Republic of
   Email: vvv861005@gmail.com
















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