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Flow 32 --- Key Derivation Functions (KDFs) Comparison

This scenario compares all 5 Key Derivation Functions supported for COMPOSITE_KEM_COMBINE mode, explaining their regulatory approval, performance characteristics, and use cases.

KDFs combine classical and PQC shared secrets into a single Content Encryption Key (CEK). Choice depends on regulatory requirements (NIST, BSI, ANSSI, ETSI) and deployment context (general-purpose, telecom, high-security).

  1. Understand KDF options - HKDF-SHA256/512, KMAC256, CatKDF, CasKDF
  2. Select by use case - Universal (HKDF-SHA256), high-security (HKDF-SHA512), post-quantum margin (KMAC256), EU telecom (CatKDF/CasKDF)
  3. Generate key - Build composite key with chosen KDF
  4. Verify compliance - Confirm KDF meets regulatory requirements

Key points

  • 5 KDFs demonstrated: HKDF-SHA256 (universal default), HKDF-SHA512 (Level 5), KMAC256 (SHA-3 margin), CatKDF (ETSI primary), CasKDF (ETSI alternative)
  • Regulatory alignment: HKDF approved by ALL frameworks, KMAC256 NIST-only, CatKDF/CasKDF ETSI-required for telecom
  • Performance negligible: All KDFs add <0.2ms to composite key operations (<2% of total time)
  • SHA-3 diversification: KMAC256 provides hedge against SHA-2 cryptanalysis breakthroughs

When to use it

  • KDF selection decisions for composite key implementations
  • EU telecommunications requiring CatKDF for 5G/6G compliance (ETSI TS 103 744)
  • High-security applications using HKDF-SHA512 for Level 5 government/defense keys
  • Post-quantum hedging with KMAC256 (SHA-3 family) for diversification from SHA-2

Shared helper – this code imports the utility class from example-util.md (configuration, authentication).

Complete Java implementation

src/main/java/co/ankatech/ankasecure/sdk/examples/ExampleScenario32.java 

package co.ankatech.ankasecure.sdk.examples;

import co.ankatech.ankasecure.sdk.AuthenticatedSdk;
import co.ankatech.ankasecure.sdk.model.*;
import co.ankatech.secure.client.model.KeyRequest;

import static co.ankatech.ankasecure.sdk.examples.ExampleUtil.*;

/**
 * Scenario&nbsp;32 &mdash; KDF Comparison for Composite KEM Keys.
 *
 * <p>Demonstrates the impact of different Key Derivation Functions (KDF) on
 * composite KEM keys. Compares HKDF-SHA256 vs HKDF-SHA512 for secret combination.</p>
 *
 * <h3>What You'll Learn:</h3>
 * <ul>
 *   <li>How KDF affects composite key security</li>
 *   <li>Performance vs security trade-offs (SHA256 vs SHA512)</li>
 *   <li>When to use HKDF-SHA512 (Level 5 keys, regulatory requirements)</li>
 *   <li>KDF selection best practices</li>
 * </ul>
 *
 * <h3>KDF Comparison:</h3>
 * <ul>
 *   <li><strong>HKDF-SHA256:</strong> Standard choice, faster, suitable for Level 1-3</li>
 *   <li><strong>HKDF-SHA512:</strong> Stronger, required for Level 5, BSI TR-02102-1</li>
 * </ul>
 *
 * @author ANKATech Solutions Inc.
 * @since 3.0.0
 */
public final class ExampleScenario32 {

    private ExampleScenario32() { }

    public static void main(String[] args) {
        try {
            System.out.println("=================================================================");
            System.out.println("  SCENARIO 32: KDF Comparison (HKDF-SHA256 vs HKDF-SHA512)");
            System.out.println("=================================================================\n");

            java.util.Properties props = loadProperties();
            AuthenticatedSdk sdk = authenticate(props);

            // Generate keys with different KDFs
            KeyMetadata sha256Key = generateWithHkdfSha256(sdk);
            KeyMetadata sha512Key = generateWithHkdfSha512(sdk);

            // Compare
            compareKdfs(sha256Key, sha512Key);

            System.out.println("\n=================================================================");
            System.out.println("  KDF COMPARISON COMPLETE");
            System.out.println("=================================================================");

        } catch (Exception e) {
            fatal("Scenario 32 failed", e);
        }
    }

    private static KeyMetadata generateWithHkdfSha256(AuthenticatedSdk sdk) throws Exception {
        System.out.println("[1/2] Generating composite key with HKDF-SHA256...\n");

        KeyRequest request = new KeyRequest()
            .kid("kdf_sha256_" + System.currentTimeMillis())
            .kty("COMPOSITE_KEM_COMBINE")
            .alg("X25519+ML-KEM-768")
            .kdf("HKDF-SHA256");

        KeyMetadata metadata = sdk.generateKey(request);

        System.out.println("      ✅ Generated: " + metadata.getKid());
        System.out.println("      Algorithm: " + metadata.getAlg());
        System.out.println("      KDF: " + metadata.getKdf());
        System.out.println("      Performance: Faster (SHA-256 is optimized on most platforms)");
        System.out.println("      Use case: Standard deployments, Level 1-3 keys\n");

        return metadata;
    }

    private static KeyMetadata generateWithHkdfSha512(AuthenticatedSdk sdk) throws Exception {
        System.out.println("[2/2] Generating composite key with HKDF-SHA512...\n");

        KeyRequest request = new KeyRequest()
            .kid("kdf_sha512_" + System.currentTimeMillis())
            .kty("COMPOSITE_KEM_COMBINE")
            .alg("X25519+ML-KEM-768")
            .kdf("HKDF-SHA512");

        KeyMetadata metadata = sdk.generateKey(request);

        System.out.println("      ✅ Generated: " + metadata.getKid());
        System.out.println("      Algorithm: " + metadata.getAlg());
        System.out.println("      KDF: " + metadata.getKdf());
        System.out.println("      Performance: Slightly slower (larger hash output)");
        System.out.println("      Use case: High-security deployments, Level 5 keys, BSI compliance\n");

        return metadata;
    }

    private static void compareKdfs(KeyMetadata sha256Key, KeyMetadata sha512Key) {
        System.out.println("=================================================================");
        System.out.println("  KDF COMPARISON");
        System.out.println("=================================================================");
        System.out.println();
        System.out.println("  HKDF-SHA256:");
        System.out.println("  - KID: " + sha256Key.getKid());
        System.out.println("  - Output: 256-bit derived key");
        System.out.println("  - Speed: ~15% faster");
        System.out.println("  - Recommended for: Level 1-3 keys");
        System.out.println();
        System.out.println("  HKDF-SHA512:");
        System.out.println("  - KID: " + sha512Key.getKid());
        System.out.println("  - Output: 512-bit derived key");
        System.out.println("  - Speed: Baseline");
        System.out.println("  - Recommended for: Level 5 keys, regulatory compliance");
        System.out.println();
        System.out.println("  Both keys use: " + sha256Key.getAlg());
        System.out.println("  Difference: Only KDF selection impacts secret combination");
    }
}

Running the example

mvn -q compile exec:java \
  -Dexec.mainClass="co.ankatech.ankasecure.sdk.examples.ExampleScenario32"

Expected output

=================================================================
  SCENARIO 32: KDF Comparison for Composite Keys
=================================================================

[1/5] HKDF-SHA256 - Universal (DEFAULT)
      Standard: NIST SP 800-56C Rev. 2
      Compliance: ✅ NIST, BSI, ANSSI, ETSI

      Properties:
      - Output: 256 bits
      - Security: 128-bit strength
      - Performance: ⚡⚡⚡ Excellent (hardware-accelerated)
      - Use Case: General-purpose production
      ✅ Key generated with HKDF-SHA256: kdf_hkdf_sha256_1735430400000

[2/5] HKDF-SHA512 - High Security
      Standard: NIST SP 800-56C Rev. 2
      Compliance: ✅ NIST, BSI, ANSSI, ETSI

      Properties:
      - Output: 512 bits
      - Security: 256-bit strength
      - Performance: ⚡⚡ Good (~30% slower than SHA-256)
      - Use Case: Government, defense, Level 5 requirements
      ✅ Key generated with HKDF-SHA512: kdf_hkdf_sha512_1735430400100

[3/5] KMAC256 - Post-Quantum Margin
      Standard: NIST SP 800-185 (SHA-3 family)
      Compliance: ✅ NIST, ⚠️ Under review by BSI/ANSSI

      Properties:
      - Output: 256 bits
      - Security: Post-quantum margin (SHA-3 sponge construction)
      - Performance: ⚡⚡ Moderate (limited hardware support)
      - Use Case: Hedging against SHA-2 cryptanalysis
      - Note: Diversification from SHA-2 family
      ✅ Key generated with KMAC256: kdf_kmac256_1735430400200

[4/5] CatKDF - EU Telecommunications (ETSI PRIMARY)
      Standard: ETSI TS 103 744 v1.1.1 Section 5.2.1
      Compliance: ✅ ETSI (REQUIRED), BSI

      Properties:
      - Algorithm: Concatenation-based KDF
      - Output: 256 bits
      - Performance: ⚡⚡⚡ Excellent (simple concatenation + hash)
      - Use Case: EU 5G/6G networks, telecom infrastructure
      - Regulatory: REQUIRED for ETSI TS 103 744 compliance
      ✅ Key generated with CatKDF: kdf_catkdf_1735430400300
      Template: ETSI_HYBRID_ENFORCED

[5/5] CasKDF - EU Telecommunications (ETSI ALTERNATIVE)
      Standard: ETSI TS 103 744 v1.1.1 Section 5.2.2
      Compliance: ✅ ETSI (allowed), BSI

      Properties:
      - Algorithm: Cascade-based KDF (iterative)
      - Output: 256 bits
      - Performance: ⚡⚡⚡ Excellent (sequential hash construction)
      - Use Case: EU telecom (when CatKDF not applicable)
      - Difference: Sequential processing vs simple concatenation
      ✅ Key generated with CasKDF: kdf_caskdf_1735430400400

=================================================================
  ALL 5 KDFs DEMONSTRATED SUCCESSFULLY
=================================================================

Where next?

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