Proposes a message layout to exchange information between participants of a FROST setup.
## Motivation
Currently FROST library is complete for 2 round signatures with a dealer/aggregator setup.
This proposal is only considering that specific features, additions and upgrades will need to be made when DKG is implemented.
Assuming all participants have a FROST library available we need to define message structures in a way that data can be exchanged between participants. The proposal is a collection of data types so each side can do all the actions needed for a real life situation.
## Definitions
-`dealer`
-`aggergator`
-`signer`
-`nonce`
-`commitment`
-
## Guide-level explanation
We propose a message separated in 2 parts, a header and a payload:
```rust
struct Message {
header: Header,
payload: Payload,
}
```
`Header` will look as follows:
```rust
struct Header {
msg_type: MsgType,
version: MsgVersion,
sender: Participant,
receiver: Participant,
}
```
While `Payload` will be defined as:
```rust
enum Payload {
DealerBroadcast(MsgDealerBroadcast),
Commitments(MsgCommitments),
SigningPackage(MsgSigningPackage),
SignatureShare(MsgSignatureShare),
FinalSignature(MsgFinalSignature),
}
```
All the messages and new types will be defined in a new file `src/frost/messages.rs`
## Reference-level explanation
Here we explore in detail the header types and all the message payloads.
### Header
Fields of the header define new types. Proposed implementation for them is as follows:
// The final signature is broadcasted by the aggegator
// to any participant.
struct MsgFinalSignature {
final_signature: Signature<SpendAuth>,
}
```
## Serialization/Deserialization
Each message struct needs to serialize to bytes representation before it is sent through the wire and must deserialize to the same struct (round trip) on the receiver side. We use `serde` and macro derivations (`Serialize` and `Deserialize`) to automatically implement where possible.
This will require deriving serde in several types defined in `frost.rs`.
Manual implementation of serialization/deserialization will be located at a new mod `src/frost/serialize.rs`.
FROST inherit types from `jubjub` such as `Scalar`, `ExtendedPoint`, `AffinePoint`, etc. We need to decide how serialization of these types that are defined in external crates will be done (maybe with wrappers?).
Validation is implemented to each new data type as needed. This will ensure the creation of valid messages before they are sent and right after they are received. We create a trait for this as follows:
The `Header` part of the message is 4 bytes total:
Bytes | Field name | Data type
------|------------|-----------
1 | msg_type | u8
1 | version | u8
1 | sender | u8
1 | receiver | u8
## Primitive types
`Payload`s use data types that we need to specify first. We have 3 primitive types inside the payload messages:
`Scalar`
`Scalar` is a better name for `jubjub::Fr` and this is a `[u64; 4]` as documented in https://github.com/zkcrypto/jubjub/blob/main/src/fr.rs#L16
`Commitment`
`Commitment` is a wrapper of `jubjub::ExtendedPoint` and this is a structure with 5 `jubjub::Fq`s as defined in https://github.com/zkcrypto/jubjub/blob/main/src/lib.rs#L128-L134
Each `Fq` needed to form a `jubjub::ExtendedPoint` are `Scalar`s of `bls12_381` crate. Scalar here is `[u64; 4]` as documented in https://github.com/zkcrypto/bls12_381/blob/main/src/scalar.rs#L16
`ExtendedPoint`
`ExtendedPoint` was detailed above, it is 5 `[u64; 4]`. The total size of an `ExtendedPoint` is 1280 bytes.
## Payload
Payload part of the message is variable in size and depends on message type.