Support HPKE DHKEM suite IDs#327
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Add KEM-ID-aware HPKE labeled extract and expand helpers so non-P-256 DHKEM users do not derive values under the P-256 suite ID. Keep the existing labeled HPKE helpers as P-256-compatible wrappers, export public HPKE DHKEM KEM ID constants, and add an RFC 9180 X25519 export-only regression test. Co-authored-by: GPT-5.5 <codex@openai.com>
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Also adds a common `Error` type for the `TryDecapsulate` implementations to use. The `HpkeKemId` trait makes it possible to associate one of the KEM IDs from RFC9180 §7.1 with a given type. The trait is now impl'd for all KEMs in that table we support (the only one we currently don't is X448). The generic `EncapsulationKey::encapsulate(_with_rng)_and_expand` as well as `DecapsulationKey::decapsulate_and_expand` methods provide a convenient way to initialize an `Expander` from the shared key output by the KEM. HPKE mandates an all-zero check for the X25519 output (occurs if the input is all-zero or a low order point) in RFC9810 §7.1.4, which this also adds as the check needs to occur before `Expander` is initialized, so this switches `X25519Kem` to use the fallible `TryDecapsulate`. Closes #262, #268, #327
tarcieri
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Jul 9, 2026
Also adds a common `Error` type for the `TryDecapsulate` implementations to use. The `HpkeKemId` trait makes it possible to associate one of the KEM IDs from RFC9180 §7.1 with a given type. The trait is now impl'd for all KEMs in that table we support (the only one we currently don't is X448). The generic `EncapsulationKey::encapsulate(_with_rng)_and_expand` as well as `DecapsulationKey::decapsulate_and_expand` methods provide a convenient way to initialize an `Expander` from the shared key output by the KEM. HPKE mandates an all-zero check for the X25519 output (occurs if the input is all-zero or a low order point) in RFC9810 §7.1.4, which this also adds as the check needs to occur before `Expander` is initialized, so this switches `X25519Kem` to use the fallible `TryDecapsulate`. Closes #262, #268, #327
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Closing in favor of #360 |
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This fixes HPKE labeled extract/expand support for non-P-256 DHKEMs.
The existing
Expander::new_labeled_hpkeandExpander::expand_labeled_hpkehelpers always use suite IDKEM\x00\x10,which is DHKEM(P-256, HKDF-SHA256). That is correct for P-256, but not
for other DHKEMs exposed by the crate.
This adds KEM-ID-aware helper methods and public KEM ID constants, while
leaving the existing helpers as P-256-compatible wrappers. The regression
test covers the RFC 9180 X25519 export-only vector.
Verification:
This work was completed by Trail of Bits as part of the Patch The Planet project in collaboration with OpenAI. The vulnerability was identified primarily by the Codex coding agent, and manually reviewed before submission.