redjubjub/rfcs/0001-messages.md

FROST messages

Proposes a message layout to exchange information between participants of a FROST setup using the jubjub curve.

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
• aggregator
• signer
• nonce
• commitment

Guide-level explanation

We propose a message separated in 2 parts, a header and a payload:

struct Message {
    header: Header,
    payload: Payload,
}

Header will look as follows:

struct Header {
    msg_type: MsgType,
    version: MsgVersion,
    sender: Participant,
    receiver: Participant, 
}

While Payload will be defined as:

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:

#[repr(u8)]
#[non_exhaustive]
enum MsgType {
    DealerBroadcast,
    Commitments,
    SigningPackage,
    SignatureShare,
    FinalSignature,
}

struct MsgVersion(u8);

struct Participant(u8);

Payloads

Each payload defines a new message:

// Dealer must send this message with initial data to each participant involved.
// With this, the participant should be able to build a `SharePackage` and use
//  the `sign()` function.
// `public_key` can be calculated from the `secret_key`.
// from `secret_key`.
struct MsgDealerBroadcast {
    // The secret key as a frost::Scalar.
    secret_key: frost::Scalar,
    // Commitments for the signer as jubjub::AffinePoint.
    commitment: jubjub::AffinePoint,
    // The point and verification bytes needed to generate the group public key
    group_public: (jubjub::AffinePoint, [u8; 32]),
}

// Each signer participant send to the aggregator the 2 points
//  needed for commitment building.
struct MsgCommitments {
    // The hiding Point.
    hiding: jubjub::AffinePoint,
    // The binding Point.
    binding: jubjub::AffinePoint,
}

// The aggergator decide what message is going to be signed and
// send it to each participant with all the commitments collected.
struct MsgSigningPackage {
    // The number of participants.
    participants: u8,
    // The collected unpacked commitments for each signer
    commitments: Vec<(u8, jubjub::AffinePoint, jubjub::AffinePoint)>,
    // The lenght of the message
    message_length: u64,
    // The message to be signed as bytes
    message: &'static [u8],
}

// Each signer send the signatures to the agregator who is going to collect them 
// and generate a final spend signature.
struct MsgSignatureShare {
    // The signature to be shared as a Scalar
    signature: frost::Scalar,
}

// The final signature is broadcasted by the aggegator 
// to any participant.
struct MsgFinalSignature {
    // The r_bytes and s_bytes needed to build the frost::Signature
    final_signature: ([u8; 32], [u8; 32]),
}

Validation

Validation is implemented to each new data type as needed. This will ensure the creation of valid messages before they are send and right after they are received. We create a trait for this as follows:

pub trait Validate {
    fn validate(&self) -> Result<&Self, MsgErr>;
}

And we implement where needed. For example, in the header, sender and receiver can't be the same:

impl Validate for Header {
    fn validate(&self) -> Result<&Self, MsgErr> {
        if self.sender.0 == self.receiver.0 {
            return Err(MsgErr::SameSenderAndReceiver);
        }
        Ok(self)
    }
}

This will require to have validation error messages as:

use thiserror::Error;

#[derive(Clone, Error, Debug)]
pub enum MsgErr {
    #[error("sender and receiver are the same")]
    SameSenderAndReceiver,
}

Then to create a valid Header in the sender side we call:

let header = Validate::validate(&Header {
    ..
}).expect("a valid header");

The receiver side will validate the header using the same method. Instead of panicking the error can be ignored to don't crash and keep waiting for other (potentially valid) messages.

if let Ok(header) = msg.header.validate() {
    ..
}

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.

Byte order

Each byte chunk specified below is in little-endian order unless is specified otherwise.

Header

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

Payloads use data types that we need to specify first. We have 2 primitive types inside the payload messages:

Scalar

Scalar is a an alias for jubjub::Fr and this is a [u64; 4] as documented in https://github.com/zkcrypto/jubjub/blob/main/src/fr.rs#L16

AffinePoint

Much of the math in FROST is done using jubjub::ExtendedPoint. This is a structure with 5 jubjub::Fqs as defined in https://github.com/zkcrypto/jubjub/blob/main/src/lib.rs#L128-L134

Each Fq needed to form a jubjub::ExtendedPoint are Scalars of bls12_381 crate. Scalar here is [u64; 4] as documented in https://github.com/zkcrypto/bls12_381/blob/main/src/scalar.rs#L16

For message exchange jubjub::AffinePoints are a better choice as they are shorter in bytes, they are formed of 2 jubjub::Fq instead of 5: https://github.com/zkcrypto/jubjub/blob/main/src/lib.rs#L70-L73

Conversion from one type to the other is trivial:

https://docs.rs/jubjub/0.6.0/jubjub/struct.AffinePoint.html#impl-From%3CExtendedPoint%3E https://docs.rs/jubjub/0.6.0/jubjub/struct.ExtendedPoint.html#impl-From%3CAffinePoint%3E

Payload

Payload part of the message is variable in size and depends on message type.

MsgDealerBroadcast

Bytes	Field name	Data type
256	secret_key	Scalar
512	commitments	AffinePoint
256+32	group_public	(AffinePoint, [u8; 32])

MsgCommitments

Bytes	Field name	Data type
512	hiding	AffinePoint
512	binding	AffinePoint

MsgSigningPackage

Bytes	Field name	Data type
1	participants	u8
(1+256+256)*partipants	commitments	Vec<(u8, AffinePoint, AffinePoint)>
8	message_length	u64
message_length	message	[u8]

SignatureShare

Bytes	Field name	Data type
256	signature	Scalar

MsgFinalSignature

Bytes	Field name	Data type
64	signature	([u8; 32], [u8; 32])

Testing plan

• Create a happy path unit test similar to https://github.com/ZcashFoundation/redjubjub/blob/frost-messages/tests/frost.rs#L7 and:
  • Make messages on each step.
  • Simulate send/receive.
  • Test round trip serialization/deserialization on each message.
• Create property tests for each message.