SSS Key Manager

Self-contained SSS key management library

@learncard/sss-key-manager is a self-contained library that implements Shamir Secret Sharing (SSS) key management for LearnCard. It has zero UI dependencies and can be used in any JavaScript/TypeScript environment.

Installation

pnpm add @learncard/sss-key-manager

Overview

This package provides the KeyDerivationStrategy implementation used by the AuthCoordinator. It handles:

• Key splitting and reconstruction using a 2-of-3 Shamir threshold scheme

• Local device share storage in IndexedDB with versioning

• Server communication for storing and retrieving auth shares

• Recovery methods — passkey (WebAuthn PRF), recovery phrase, backup file, email backup

• Cross-device login via QR code with ephemeral ECDH encryption

• Migration from Web3Auth to SSS

Architecture

sss-key-manager/
├── sss.ts                  # Shamir split (2-of-3) and reconstruct primitives
├── sss-strategy.ts         # KeyDerivationStrategy implementation
├── storage.ts              # IndexedDB device share persistence
├── crypto.ts               # AES-GCM encryption, PBKDF2 key derivation
├── passkey.ts              # WebAuthn PRF-based share encryption
├── recovery-phrase.ts      # BIP39-style mnemonic ↔ share encoding
├── qr-crypto.ts            # ECDH ephemeral key exchange
├── qr-login.ts             # Cross-device login flow
├── atomic-operations.ts    # Split-and-verify with rollback
├── api-client.ts           # Typed fetch wrapper for lca-api routes
└── types.ts                # All types, re-exports from @learncard/types

Key Types

ContactMethod

Identifies a user by their primary contact method:

type ContactMethodType = 'email' | 'phone';

interface ContactMethod {
    type: ContactMethodType;
    value: string;
}

SecurityLevel

Describes how well-protected a user's key is:

type SecurityLevel = 'basic' | 'enhanced' | 'advanced';

• basic — device + server share only (no recovery method)

• enhanced — at least one recovery method configured

• advanced — multiple recovery methods configured

RecoveryInput

What the user provides to recover their key:

type RecoveryInput =
    | { method: 'passkey'; credentialId: string }
    | { method: 'phrase'; phrase: string }
    | { method: 'backup'; fileContents: string; password: string }
    | { method: 'email'; emailShare: string };

RecoverySetupInput

What the user provides to set up a new recovery method:

type RecoverySetupInput =
    | { method: 'passkey' }
    | { method: 'phrase' }
    | { method: 'backup'; password: string; did: string }
    | { method: 'email' };

BackupFile

The JSON structure of a downloadable backup file:

interface BackupFile {
    version: 1;
    createdAt: string;
    primaryDid: string;
    shareVersion?: number;
    encryptedShare: {
        ciphertext: string;
        iv: string;
        salt: string;
        kdfParams: {
            algorithm: 'argon2id';
            timeCost: number;
            memoryCost: number;
            parallelism: number;
        };
    };
}

SSSStrategy

The main class that implements KeyDerivationStrategy. It is typically instantiated by the AuthCoordinator, not directly by application code.

Configuration

interface SSSStrategyConfig {
    serverUrl: string;
    enableEmailBackupShare?: boolean;
}

Option	Default	Description
serverUrl	—	Base URL of the lca-api server (e.g., https://api.example.com/api)
enableEmailBackupShare	false	Automatically send a backup share to the user's email during key setup

Key Methods

Method	Purpose
fetchServerKeyStatus(token, providerType)	Check if a key record exists on the server for the authenticated user
setupNewKey(token, providerType, signVp)	Generate a new key, split it, store shares on device and server
reconstructKey(token, providerType)	Reconstruct the key from device share + auth share
recoverKey(token, providerType, input)	Recover the key using a recovery method + auth share
hasLocalKey()	Check if a device share exists in IndexedDB
clearLocalKey()	Remove the device share from IndexedDB
setupRecoveryMethod(params)	Set up a new recovery method (passkey, phrase, backup, email)
getAvailableRecoveryMethods(token, providerType)	List the user's configured recovery methods

API Client

The api-client.ts module provides a typed fetch wrapper for all lca-api /keys/* and /qr-login/* routes. It is used internally by the SSS strategy but can also be used directly:

import { createApiClient } from '@learncard/sss-key-manager';

const client = createApiClient({ serverUrl: 'https://api.example.com/api' });

const status = await client.getAuthShare({
    authToken: token,
    providerType: 'firebase',
    contactMethod: { type: 'email', value: '[email protected]' },
});

Testing

The package includes ~249 unit tests across 13 test files:

pnpm exec nx test sss-key-manager