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Hybrid PQC + Classical Cryptography

This guide explains Qpher's hybrid cryptographic modes, which combine a post-quantum algorithm with a battle-tested classical algorithm in a single operation. Hybrid mode provides defense-in-depth: your data stays protected even if one algorithm is compromised.

Pro and Enterprise Plans

Hybrid algorithms (X-Wing and Composite-ML-DSA) are available on Pro and Enterprise plans. If you are on a Free, Starter, or Growth plan, your requests will use the PQC-only defaults (Kyber768 and Dilithium3).

Why Hybrid?​

PQC algorithms like Kyber768 and Dilithium3 are rigorously standardized by NIST, but they are relatively new compared to classical algorithms like X25519 and ECDSA that have decades of real-world testing. Hybrid mode ensures that:

  • If a breakthrough in lattice cryptanalysis weakens the PQC component, the classical component maintains security.
  • If a quantum computer breaks the classical component, the PQC component maintains security.
  • Security holds as long as at least one of the two algorithms remains secure.

This is the same approach adopted by AWS, Google Cloud, and Cloudflare in their production deployments.

Supported Hybrid Algorithms​

AlgorithmComponentsOperationStandard
X-WingX25519 + ML-KEM-768 (Kyber768)Encrypt / DecryptIETF draft
Composite-ML-DSAECDSA P-256 + ML-DSA-65 (Dilithium3)Sign / VerifyIETF draft

Prerequisites​

  • A Qpher account on a Pro or Enterprise plan
  • An active API key
  • For encryption: An active X-Wing key pair (generated via the Key Management API with algorithm: "X-Wing")
  • For signatures: An active Composite-ML-DSA key pair (generated with algorithm: "Composite-ML-DSA")

Step 1: Generate a Hybrid Key Pair​

Before using hybrid algorithms, you need to generate the appropriate key pair.

Generate an X-Wing Key Pair
curl -X POST https://api.qpher.ai/api/v1/kms/keys/generate \
-H "Content-Type: application/json" \
-H "x-api-key: qph_your_key_here" \
-d '{"algorithm": "X-Wing"}'

To generate a Composite-ML-DSA key pair, use "algorithm": "Composite-ML-DSA" instead.

Step 2: Encrypt with X-Wing​

To use hybrid encryption, add "algorithm": "X-Wing" to your encrypt request. Everything else stays the same as standard Kyber768 encryption.

POST/api/v1/kem/encryptEncrypt with X-Wing hybrid KEM
Encrypt with X-Wing
curl -X POST https://api.qpher.ai/api/v1/kem/encrypt \
-H "Content-Type: application/json" \
-H "x-api-key: qph_your_key_here" \
-d '{
  "plaintext": "SGVsbG8sIFdvcmxkIQ==",
  "key_version": 1,
  "algorithm": "X-Wing"
}'

Step 3: Decrypt with X-Wing​

When decrypting data that was encrypted with X-Wing, include "algorithm": "X-Wing" in the decrypt request.

Decrypt with X-Wing
curl -X POST https://api.qpher.ai/api/v1/kem/decrypt \
-H "Content-Type: application/json" \
-H "x-api-key: qph_your_key_here" \
-d '{
  "ciphertext": "base64-encoded-xwing-ciphertext...",
  "key_version": 1,
  "algorithm": "X-Wing"
}'

Step 4: Sign with Composite-ML-DSA​

To use hybrid signatures, add "algorithm": "Composite-ML-DSA" to your sign request.

POST/api/v1/signature/signSign with Composite-ML-DSA hybrid signatures
Sign with Composite-ML-DSA
curl -X POST https://api.qpher.ai/api/v1/signature/sign \
-H "Content-Type: application/json" \
-H "x-api-key: qph_your_key_here" \
-d '{
  "message": "SGVsbG8sIFdvcmxkIQ==",
  "key_version": 1,
  "algorithm": "Composite-ML-DSA"
}'

Step 5: Verify with Composite-ML-DSA​

When verifying a Composite-ML-DSA signature, include "algorithm": "Composite-ML-DSA" in the verify request.

Verify with Composite-ML-DSA
curl -X POST https://api.qpher.ai/api/v1/signature/verify \
-H "Content-Type: application/json" \
-H "x-api-key: qph_your_key_here" \
-d '{
  "message": "SGVsbG8sIFdvcmxkIQ==",
  "signature": "base64-encoded-composite-signature...",
  "key_version": 1,
  "algorithm": "Composite-ML-DSA"
}'

PQC-Only vs. Hybrid Comparison​

PQC-Only (default)Hybrid (Pro/Enterprise)
KEM AlgorithmML-KEM-768 (Kyber768)X-Wing: X25519 + ML-KEM-768
Signature AlgorithmML-DSA-65 (Dilithium3)Composite-ML-DSA: ECDSA P-256 + ML-DSA-65
PlansAll plansPro and Enterprise
API ParameterDefault (omit algorithm)"algorithm": "X-Wing" or "Composite-ML-DSA"
Security ModelQuantum-resistant onlyQuantum-resistant + classical
Use CaseStandard PQC protectionDefense-in-depth for high-security requirements
Key Generation"algorithm": "Kyber768" or "Dilithium3""algorithm": "X-Wing" or "Composite-ML-DSA"
When to Use Hybrid

Use hybrid mode when your compliance or security requirements demand defense-in-depth. PQC-only mode is the recommended long-term default for most applications, as PQC algorithms accumulate more real-world deployment history. Hybrid mode adds a safety net during this transition period.

Key Management for Hybrid Keys​

Hybrid keys follow the same lifecycle as PQC-only keys (active, retired, archived) and the same versioning rules:

  • Encrypt/Sign: Requires an active hybrid key
  • Decrypt/Verify: Works with active or retired hybrid keys
  • Rotation: Use POST /api/v1/kms/keys/rotate with the hybrid algorithm name
Separate Key Pairs

Hybrid keys are separate from PQC-only keys. An X-Wing key pair cannot be used for standard Kyber768 operations, and vice versa. You must generate dedicated key pairs for each algorithm you use.

Error Handling​

Hybrid operations return the same error codes as PQC-only operations, plus:

HTTP StatusError CodeCauseResolution
403ERR_FORBIDDEN_001Hybrid algorithms require Pro or Enterprise planUpgrade your plan
404ERR_KEM_003 / ERR_SIG_003No key found for the specified hybrid algorithmGenerate an X-Wing or Composite-ML-DSA key pair first
400ERR_INVALID_001Algorithm mismatch (e.g., X-Wing ciphertext with Kyber768 key)Use the same algorithm for encrypt and decrypt