T-Chain - Core Threshold Signature Specification

Abstract

LP-7000 specifies the T-Chain (Threshold Signature Chain), Lux Network's specialized blockchain providing Multi-Party Computation (MPC) services including distributed key generation (DKG), threshold signatures, and secure key management. The T-Chain implements CGGMP21 for ECDSA, FROST for Schnorr/BLS, and Ringtail for post-quantum signatures.

Motivation

A dedicated threshold signature chain provides:

1. Trustless Custody: Distribute key control across multiple parties
2. Bridge Security: Enable MPC-secured cross-chain bridges
3. Key Recovery: Support threshold-based key recovery
4. Regulatory Compliance: Meet institutional custody requirements

Specification

Chain Parameters

Parameter	Value
Chain ID	T
VM ID	thresholdvm
VM Name	thresholdvm
Block Time	2 seconds
Consensus	Quasar

Implementation

Go Package: github.com/luxfi/node/vms/thresholdvm

import (
    tvm "github.com/luxfi/node/vms/thresholdvm"
    "github.com/luxfi/node/utils/constants"
)

// VM ID constant
var ThresholdVMID = constants.ThresholdVMID // ids.ID{'t', 'h', 'r', 'e', 's', 'h', 'o', 'l', 'd', 'v', 'm'}

// Create T-Chain VM
factory := &tvm.Factory{}
vm, err := factory.New(logger)

Directory Structure

node/vms/thresholdvm/
├── cggmp/            # CGGMP21 implementation
├── frost/            # FROST signatures
├── ringtail/         # Post-quantum threshold
├── dkg/              # Distributed key generation
├── keygen/           # Key generation ceremonies
├── warp/             # Warp message integration
├── factory.go        # VM factory
├── vm.go             # Main VM implementation
└── *_test.go         # Tests

Supported Protocols

CGGMP21 - Threshold ECDSA

UC-secure non-interactive threshold ECDSA based on IACR 2021/060:

import "github.com/luxfi/node/vms/thresholdvm/cggmp"

// Initialize CGGMP session
session := cggmp.NewSession(threshold, totalParties)

// Distributed Key Generation
keyShare, err := session.DKG(partyID, otherParties)

// Sign message
partialSig, err := session.Sign(keyShare, message)

// Combine signatures
fullSig, err := session.CombineSignatures(partialSigs)

FROST - Threshold Schnorr/BLS

Flexible Round-Optimized Schnorr Threshold signatures:

import "github.com/luxfi/node/vms/thresholdvm/frost"

// Initialize FROST session
session := frost.NewSession(threshold, totalParties)

// Key generation
keyShare, err := session.KeyGen(partyID)

// Sign with BLS
partialSig, err := session.SignBLS(keyShare, message)

// Sign with Schnorr
partialSig, err := session.SignSchnorr(keyShare, message)

Ringtail - Post-Quantum Threshold

Ring-based threshold signatures for quantum resistance:

import "github.com/luxfi/node/vms/thresholdvm/ringtail"

// Initialize Ringtail session
session := ringtail.NewSession(threshold, totalParties)

// Quantum-safe threshold key generation
keyShare, err := session.KeyGen(partyID)

// Sign with post-quantum security
partialSig, err := session.Sign(keyShare, message)

Key Management

Key Share Types

type KeyShare struct {
    ID           ids.ID      `json:"id"`
    PartyID      uint32      `json:"partyId"`
    Threshold    uint32      `json:"threshold"`
    TotalParties uint32      `json:"totalParties"`
    PublicKey    []byte      `json:"publicKey"`
    SecretShare  []byte      `json:"secretShare"`  // Encrypted
    Protocol     Protocol    `json:"protocol"`
}

type Protocol uint8

const (
    ProtocolCGGMP21 Protocol = iota
    ProtocolFROST
    ProtocolRingtail
)

Key Ceremonies

Ceremony	Description
DKG	Generate new threshold key
Resharing	Change threshold or add/remove parties
Refresh	Rotate shares without changing public key

Transaction Types

Type	Description
InitDKG	Initialize distributed key generation
SubmitShare	Submit key share commitment
RevealShare	Reveal key share
RequestSignature	Request threshold signature
SubmitPartialSig	Submit partial signature
RotateKey	Initiate key rotation
ReshareKey	Change threshold parameters

Threshold Parameters

Config	Threshold (t)	Parties (n)	Security
2-of-3	2	3	Basic
3-of-5	3	5	Standard
5-of-9	5	9	High
7-of-11	7	11	Enterprise

Warp Message Integration

import "github.com/luxfi/node/vms/thresholdvm/warp"

// Create Warp message with threshold signature
warpMsg, err := warp.CreateMessage(
    sourceChainID,
    destChainID,
    payload,
)

// Sign with threshold key
signedMsg, err := tvm.ThresholdSignWarp(warpMsg, keyID)

// Verify threshold-signed Warp message
valid, err := tvm.VerifyThresholdWarp(signedMsg)

Bridge Integration

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   B-Chain   │────▶│   T-Chain   │────▶│  External   │
│  (Bridge)   │     │ (Threshold) │     │   Chain     │
└─────────────┘     └─────────────┘     └─────────────┘
       │                   │                   │
       │   Lock Request    │                   │
       │──────────────────▶│                   │
       │                   │                   │
       │                   │   Threshold Sign  │
       │                   │──────────────────▶│
       │                   │                   │
       │   Signature OK    │                   │
       │◀──────────────────│                   │

API Endpoints

RPC Methods

Method	Description
threshold.initDKG	Start key generation ceremony
threshold.getKeyInfo	Get threshold key information
threshold.requestSign	Request threshold signature
threshold.getSignature	Get completed signature
threshold.reshare	Initiate key resharing

REST Endpoints

POST /ext/bc/T/threshold/dkg/init
GET  /ext/bc/T/threshold/keys/{keyId}
POST /ext/bc/T/threshold/sign
GET  /ext/bc/T/threshold/signature/{sigId}
POST /ext/bc/T/threshold/reshare

Configuration

{
  "thresholdvm": {
    "defaultThreshold": 2,
    "defaultParties": 3,
    "defaultProtocol": "CGGMP21",
    "keyRotationInterval": "24h",
    "signatureTimeout": "30s",
    "maxPendingSignatures": 1000,
    "enableRingtail": true
  }
}

Security Properties

1. Unforgeability: t parties required to sign
2. Key Secrecy: < t parties learn nothing about key
3. Robustness: Signing succeeds with any t honest parties
4. Proactive Security: Regular share refresh

Performance

Operation	Time	Notes
DKG (3-of-5)	500ms	One-time setup
CGGMP21 Sign	100ms	Per signature
FROST Sign	50ms	Per signature
Ringtail Sign	200ms	Post-quantum
Warp Sign	150ms	Including serialization

Rationale

Design decisions for T-Chain:

1. Multiple Protocols: Different use cases need different security/performance tradeoffs
2. Warp Integration: Native cross-chain messaging support
3. Proactive Security: Regular refresh prevents compromise accumulation
4. Quantum Bridge: Ringtail provides migration path to PQ security

Backwards Compatibility

LP-7000 supersedes LP-0083. Both old and new numbers resolve to this document.

Test Cases

See github.com/luxfi/node/vms/thresholdvm/*_test.go:

func TestThresholdVMStats(t *testing.T)
func TestEncryptedWarpThroughThreshold(t *testing.T)
func TestRingtailProtocolForWarp(t *testing.T)
func TestWarpMessageHashForSigning(t *testing.T)
func TestThresholdConfigDefaults(t *testing.T)
func TestKeyShareInterface(t *testing.T)
func TestDKGCeremony(t *testing.T)
func TestCGGMP21Signing(t *testing.T)
func TestFROSTSigning(t *testing.T)

Reference Implementation

Repository: github.com/luxfi/node Package: vms/thresholdvm Dependencies:

• vms/thresholdvm/cggmp
• vms/thresholdvm/frost
• vms/thresholdvm/ringtail
• vms/thresholdvm/dkg

Security Considerations

1. Share Storage: Key shares must be encrypted at rest
2. Communication Security: All MPC communication over TLS 1.3
3. Party Authentication: Strong identity verification for ceremonies
4. Timeout Handling: Proper cleanup of incomplete ceremonies
5. Audit Logging: Full audit trail of all signing operations

Related LPs

LP	Title	Relationship
LP-0083	T-Chain Specification	Superseded by this LP
LP-7100	CGGMP21 ECDSA	Sub-specification
LP-7200	FROST Schnorr/BLS	Sub-specification
LP-7300	Ringtail PQ	Sub-specification
LP-7400	DKG Ceremonies	Sub-specification
LP-6000	B-Chain	Uses T-Chain for bridge custody

Copyright

Copyright and related rights waived via CC0.