risc0-zeroio

Zeroio provides utilities to communicate efficiently with the guest.

This is similar to serde's "Serialize" and "Deserialize", but has different design targets:

• Guest runtime performance is paramount.
• It's a significant performance degradation for the guest to have to read a value that's not aligned on a 32-bit boundary.
• Space efficiency is nice, but less important than guest runtime performance.
• We want to conveniently be able to take cryptographic hashes of a structure.

In response, zeroio does these things differently than serde:

• There is only one on-wire format; zeroio does not try to be as featureful as serde.
• Datatypes available are much more limited
• We don't want to spend any cycles deserializing or copying, so we store the data in a format that's easy to access without copying, similarly to the `rkyv' crate.
• In contrast to rkyv, we use slices instead of pointers to refer to available serialized data. This allows us to avoid unsafe code.
• We store all data buffers as [u32] (as opposed to the more common [u8]).
• There is one canonical format; any serialization of the same data will construct the same structure. To take a cryptographic hash, we simply hash the [u32] slice.
• We null-pad all data buffers up to the size of the hash block (with one word remaining) to avoid copying when computing a hash.

Note that for deserializing, while we guarantee sha(a) == sha(b) implies deserialize_from(a) == deserialize_from(b), we do not guarantee the converse, that deserialize_from(a) == deserialize_from(b) implies a == b.