This allows us to allow passing in opaque HsMacKey objects,
rather than untyped byte slices.
Additionally, we now check both MAC and signature unconditionally,
to avoid the large timing side-channel. The small timing
side-channel of combining booleans with `&` is considered safe.
Part of #993.
If we didn't do this, we would need to transfrom
`EncodedLinkSpec`s into a `LinkSpec::Unrecognized`, which is not
semantically right. What's more, every user of this API wants to
consume encoded link specifiers, so encoding them early saves a
little effort.
We now manipulate raw relay cell bodies as (an alias for)
`Box<[u8;509]>` rather than as (an alias for) `[u8;509]`. This
enables us to do much less copying. It will become more important
soon, as we defer parsing relay cell bodies even longer.
Related to #7.
We also use SliceWriter to avoid allocating a Vec<> for every relay
message we want to encode, and instead encode directly into the
cell.
This change lets us use ChannelCodec to encode and decode any
restricted channel message type we want. (Later on, we'll turn the
related Codec class in tor-proto into a more type-restricted version
of this.)
Doing this will make it much easier to implement a macro that
generates restricted instances of the Msg types (for #525).
The Body change is a breaking change. I don't think anybody else
implements Body, but in theory they could.
Nick Mathewson
breezykermo
Ian Jackson