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Add hpke backend #25

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Add hpke backend #25

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This will be useful for PIV encryption, working together with trussed-dev/trussed-auth#41

This implements the standard HPKE from RFC 9180. This uses a custom implmentation instead of the hpke crate because this crate seals the trait to implement custom ciphers, and we want to use ChaCha8 and not ChaCha20.

The implementation is tested against the RFC test vectors for ChaCha20, and is made generic so that the same code can be used for ChaCha8 in the backend.

For ChaCha8Poly1305 AEAD ID, I used a custom 0xFFFE, which is probably unused. I need to look if there is somewhere someone already using ChaCha8Poly1305 for HPKE and if there is a specified ID.

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src/hpke.rs Outdated
kem_context,
&mut shr,
)
.expect("Length of shr is known to be OK");
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replace expect with error! + unwrap?

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Is it relevant? it's not used in the real firmware since only the SE050 is used.

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It’s not necessary, but if we would ever end up using the implementation e. g. for the Passkey, we will probably not remember to fix this. On the other hand, maybe the expect optimization will have improved by then. ;)

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