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If a TLS 1.3 connection starts failing after you enable post-quantum cryptography (PQC), first establish whether ordinary TLS negotiation still works, then check whether both peers can negotiate the same hybrid key-exchange group. A generic “handshake failure” does not identify the cause: configuration, incompatible implementations, network intermediaries, or message fragmentation can all be responsible.
What changes when you enable post-quantum key exchange?
The TLS 1.3 hybrid groups defined in RFC 10024 combine an ephemeral elliptic-curve Diffie–Hellman exchange (ECDHE) with a post-quantum ML-KEM exchange. The peers need compatible support for the same group and need to negotiate it successfully. Enabling TLS 1.3 or using a library that supports PQC does not by itself prove that a particular hybrid group is enabled or being offered.
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RFC 9954 describes the general TLS 1.3 hybrid key-exchange construction and its confidentiality goal: the shared secret should remain secure as long as at least one component key-exchange mechanism remains unbroken. This is a key-exchange property, not a guarantee that the entire connection—including certificate authentication—is post-quantum secure.
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How should you diagnose the failure?
Work through these checks in order, using a test endpoint where possible. Change one variable at a time so you can tell whether a change fixed the failure or merely changed where it occurs.
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1. Capture the failure and establish a baseline
- Record the client and server software, TLS library and exact versions, relevant build options, configured TLS versions and groups, and the complete connection path.
- Save the exact error or alert and, where policy permits, a handshake trace or packet capture. Do not conclude that PQC caused the failure based only on a generic handshake error.
- Compare against the same endpoint and path using the configuration that worked before PQC was enabled. Note whether ordinary TLS 1.3 succeeds without the hybrid group.
2. Verify TLS 1.3 and hybrid-group negotiation at both ends
Check the negotiated TLS version first. Then inspect the handshake to see whether the client advertises the intended group in supported_groups, whether it sends a compatible key_share, and whether the server selects that group or rejects the offer. A group appearing in a library’s capability list is not proof that the application has enabled it or that the peer will accept it.
Review explicit protocol-version, group-list, and key-share settings in both applications. The July 2026 IETF Internet-Draft Post-Quantum Cryptography Recommendations for TLS-based Applications cautions that upgrading to a library with TLS 1.3 and PQC-extension support may not enable PQC groups by default. Confirm the defaults and configuration behavior for the exact library and version you run rather than relying on a general library-level claim.
3. Check for a peer, encoding, or version mismatch
“PQC-capable” is not precise enough to establish interoperability. Confirm that both endpoints implement the same final group definition and compatible encodings, rather than relying on incompatible experimental draft-era identifiers or formats. Compare implementation versions and build options on both sides.
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NIST’s December 2023 preliminary migration report documented an interoperability failure between s2n-tls and OQS OpenSSL when they followed different versions of a draft. That is a concrete example of version skew causing failure; it does not establish that those experimental versions explain a current deployment’s problem.
If the connection crosses a proxy, TLS-inspection device, load balancer, VPN, or multiple server backends, test the endpoint directly and then add each intermediary or backend back into the path. A working direct connection and a failing routed connection point toward a path-dependent issue, but you still need traces to locate it. The July 2026 application recommendations document is an Internet-Draft, not a final standard; it also calls attention to legacy peers that do not support TLS 1.3 and PQC key-exchange extensions.
4. Investigate handshake size and network behavior
Hybrid public-key shares add data to handshake messages. The July 2026 IETF application draft notes that a larger hybrid key share can contribute to a fragmented ClientHello, which some middleboxes may mishandle; packet loss can also add delay. If failures vary by network or path, compare captures from a controlled path and the failing path. Look for retransmissions, resets, timeouts, and whether the ClientHello is fragmented or dropped.
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Check whether the client is sending redundant key shares or whether its key-share strategy can be adjusted. Test any such change without silently removing a required security mode. RFC 9954 gives broad context that post-quantum public keys and ciphertexts across algorithms can range from hundreds of bytes to over one hundred kilobytes; that range is not a size measurement for each of the groups in RFC 10024.
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On a test endpoint, vary one item per run: TLS-library version, enabled group list, client key-share list, server policy, or network path. Record the negotiated version and group, the point at which the handshake stops, and whether the failure reproduces. If a traditional group succeeds while the hybrid group fails, focus next on group support, encoding, key-share negotiation, or message handling; that result alone does not show that the cryptographic construction is broken.
The July 2026 IETF application draft says clients can send traditional and hybrid shares together to avoid an extra round trip, while warning that a larger ClientHello can create fragmentation and compatibility trade-offs. Treat this as draft guidance, and verify the actual implementation’s behavior and applicable deployment policy before changing the share strategy.
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Which TLS 1.3 hybrid group should you check?
RFC 10024, a Standards Track document published in August 2026, defines three PQ/T hybrid key-agreement groups for TLS 1.3. Each combines ML-KEM with ECDHE. Their intended distinctions are about component mechanisms and deployment requirements, not a universal ranking by speed.
| Group | Components | RFC 10024’s described use context |
|---|---|---|
X25519MLKEM768 |
X25519 ECDHE and ML-KEM-768 | Described as often the most practical choice when using a single hybrid combiner. |
SecP256r1MLKEM768 |
P-256 ECDHE and ML-KEM-768 | For use cases requiring both shared secrets to use FIPS-approved mechanisms. |
SecP384r1MLKEM1024 |
P-384 ECDHE and ML-KEM-1024 | For higher-security environments requiring FIPS-approved mechanisms with an increased security margin. |
These descriptions do not replace deployment policy or implementation checks. Before selecting a group, confirm that the actual client, server, and any relevant intermediary support the same group and interoperate with the versions in production. Do not infer latency or performance differences from the group names alone.
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No. Hybrid key exchange concerns the negotiated shared secret used for session confidentiality; it does not make the certificate signature algorithm or authentication path post-quantum. RFC 9954 explicitly limits its scope to hybrid ephemeral key exchange and excludes post-quantum authentication. RFC 9958 discusses hybrid authentication as a separate property with its own certificate-composition risks. Diagnose and describe key exchange separately from certificate and signature configuration.
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