X25519c can be attacked by replying several times with invalid data.
This is hard to defend against in the API level without denying service
and using some hard-to-understand semantics.
Masked primitives are gone for now, some countermeasures have been moved
into their respective "regular" impls. I don't think that it's worth it
to care that much about side channels in CC. I haven't seen or managed
to mount any practical attacks myself. The further move away from Cobalt
will probably make them even harder to mount.
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