Multi-Party-TSS (ECDSA-DKLs23)

Silent Shard uses Multiparty computation (MPC) and enables a set of parties that do not trust each other to jointly compute a secret signing key without being constructed in one place and an ECDSA signature over their secret key shards while not sharing them with any of the involved parties, removing single points of trust.

TSS consists of three stages:

• Distributed Key Generation (DKG),
• Distributed Signature Generation, and
• Proactive Security with Key rotation/refresh.

These functions involve cryptographic computing at the participating nodes of the MPC quorum and exchanges of rounds of messages which ultimately lead to the generation of a valid signature at the requested node. These computing nodes can be any device with sufficient computational and memory capability, including but not limited to smartphones, server nodes, and edge devices. The basic philosophy behind Silent Shard remains that no single device holding the private key can be used to generate signatures and move digital assets. The private key is shared among multiple computing nodes so that no party has any information about the key. Then, in order to generate a signature, the threshold number of devices run a secure two-party computation protocol that generates the signature without revealing anything about the parties' key shares to each other. These devices may or may not be associated with the same person or organization and can be any form factor. Thus, one could use this to create a wallet, sharing the private key between one's mobile and one's laptop, between one's mobile and a VM in the cloud, and so on.

Protocol

• Silent Shard is based on DKLs23 threshold signature scheme
• Enabled by well-chosen correlation + simple new consistency check.
• Blackbox use of UC 2-round 2P-MUL. OT-based protocols satisfy UC, but AHE is more complicated.
• No (explicit) ZK proofs during signing or DKG; light protocol and straightforward UC analysis.

Disclaimer

• The code does not handle network communication security.
• The state struct per request has public and private fields.
• Presignatures should be used only once.
• Proper validating of messages per round is needed.

Installation

npm install @silencelaboratories/dkls-wasm-ll-node

Important Data Objects

Party ID

Each participant of DKG or DSG is identified by party id, a small integer range [0..N-1], where N is number of participants of some particular protocol.

Message

A message is an opaque array of bytes with two additional properties: from_id and to_Id. Caller should use from properties to route messages to a receiver after encrypting and authenticating the message in transmit.

// Construct message from array of bytes
new Message(payload: Uint8Array, from: number, to?: number);

KeygenSession

Create a new distributed key generation session.

// N - total number of participants
// T - threshold
function dkg(n: number, t: number): Keyshare[] {
    let parties: KeygenSession[] = [];

    // create KeygenSession for each party
    for (let i = 0; i < n; i++) {
        parties.push(new KeygenSession(n, t, i));
    }

    // execute DKG
    return dkg_inner(parties);
}

function dkg_inner(parties: KeygenSession[]): Keyshare[] {

    // Execution starts by creation of first message
    let msg1: Message[] = parties.map(p => p.createFirstMessage());

    // The following statement emulate message broadcast and receiving
    // by each party messages from all other parties. That is, if N = 3
    // then for party 0 with have to deliver messages from parties 1, 2.
    //
    // method handleMessage() will return a batch of P2P for each other
    // party, and we collect all message in msg2 array.
    //
    // A real code should encrypt each P2P message and use appropriate
    // network transport to communicate the message to a designed party
    // decrypt and pass to next call of handleMessages().
    let msg2: Message[] = parties.flatMap((p, pid) => p.handleMessages(filterMessages(msg1, pid)));

    // after handling batch msg1, all parties calculate final session id,
    // and not we have to calculate commitments for chain_code_sid
    let commitments = parties.map(p => p.calculateChainCodeCommitment());

    // Selected mesages designed for a particular party and handle them.
    // It will generate a batch of P2P messages.
    let msg3: Message[] = parties.flatMap((p, pid) => p.handleMessages(selectMessages(msg2, pid)));

    // handle P2P message and generate last round of broadcast messages.
    let msg4: Message[] = parties.flatMap((p, pid) => p.handleMessages(selectMessages(msg3, pid), commitments));

    // handle the last broadcast messages.
    parties.flatMap((p, pid) => p.handleMessages(filterMessages(msg4, pid)));

    // extract keyshare from session object and consume (deallocate) session object
    return parties.map(p => p.keyshare());
}

function filterMessages(msgs: Message[], party: number): Message[] {
    return msgs.filter((m) => m.from_id != party).map(m => m.clone());
}

function selectMessages(msgs: Message[], party: number): Message[] {
    return msgs.filter((m) => m.to_id == party).map(m => m.clone());
}

KeygenSession object is serializable. Use methods .toBytes() and .fromBytes(). Both Keyshare and KeygenSession need to be properly encrypted and authenticated

Key roation

A key rotation session is very simular to normal key generation.

// Create a key roation session
let session = KeygenSession.initKeyRotation(existingKeyShare);

// then perform key generation as shown above and get newKeyShare.
newKeyShare.finishKeyRotation(existingKeyShare);

// the call above will deallocate existingKeyShare and finish
// key rotation protocol.

SignSession

Create a sign session

// shares is output of dkg(3, 2).

function dsg(shares: Keyshare[], t: number, messageHash: Uint8Array) {
    let parties: SignSession[] = [];

    // for simplicity we always use the first T shares.
    for(let i = 0; i < t; i++) {
        // new SignSession() consumes passed keyshare.
        parties.push(new SignSession(shares[i], "m"));
    }

    let msg1: Message[] = parties.map(p => p.createFirstMessage());

    // broadcast first message to all parties.
    let msg2: Message[] = parties.flatMap((p, pid) => p.handleMessages(filterMessages(msg1, pid)));

    // handle first message and generate first P2P message for all parties.
    let msg3: Message[] = parties.flatMap((p, pid) => p.handleMessages(selectMessages(msg2, pid)));

    // handle batch of P2P message.
    parties.flatMap((p, pid) => p.handleMessages(selectMessages(msg3, pid)));

    // Now each party has a PreSignature. It does not depend on the message to be signed,
    // and caller can generate a batch of pre-signature ahead of time.
    //
    // Take a pre-signature and 32-byte hash of message to sign, produce a last
    // broadcast message.
    //
    // Caller *MUST NOT USE PRE-SIGNATURE MORE THEN ONCE*.
    //
    // *REUSE OF PRE-SIGNATURES LEADS TO PRIVATE KEY FULL EXPOSURE*.
    //
    let msg4: Message[] = parties.map(p => p.lastMessage(messageHash));

    // handle last round of broadcast messages and produce the signature.
    // method .combine() consumes (deallocates) session object.
    let signs = parties.map((p, pid) => p.combine(filterMessages(msg4, pid)));

    return signs;
}

Memory managment

Message object designates a memory buffer in the WASM heap. There is not automatic memory managment and caller is responsible to call .free() at apropriate time.

Methods .handleMessages() consumes passed in messages. This means that caller have to call .free() methods only to deallocate objects as part of error handling.

Error handling

session.handleMessages() may throw an error. It is impossitle to recover from the error. It is impossible to continue execition of a protocol.

In most cases err.message only could help to debug an application.

One special case MUST be handled.

SignSession.handleMessages() could throw an error AbortProtocolAndBanParty. In this case, the error object has property "banParty", the value is in range [0 .. threshold-1]. Zero is valid party ID!