dhkem: add HpkeKemId trait; make X25519Kem fallible#360
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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
This was referenced Jul 9, 2026
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Also adds a common
Errortype for theTryDecapsulateimplementations to use.The
HpkeKemIdtrait 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_expandas well asDecapsulationKey::decapsulate_and_expandmethods provide a convenient way to initialize anExpanderfrom 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
Expanderis initialized, so this switchesX25519Kemto use the fallibleTryDecapsulate.Closes #262, #268, #327