Security Engineer

About Bitrefill
Bitrefill is the world's leading crypto lifestyle platform. We enable millions of users globally to pay for gift cards, mobile phone refills, and everyday goods using Bitcoin, Lightning Network, and other digital assets. Our long-term goal is to provide everyone with the tools to use crypto as real "digital money" daily — without needing to register for additional services or log into a web platform.
We are now actively building a new product: the Bitrefill crypto card for paying for goods and services in everyday life.
The Role
We are looking for a Senior DS/ML Engineer to help us make data-driven decisions across fraud detection, Lightning Network liquidity optimization, user behavior modeling, and risk scoring for the crypto card.
You won't just build models — you'll help shape the ML infrastructure of a fast-growing crypto-payments startup.
What You'll Do
Fraud & Risk ML — Develop anomaly detection models, transaction scoring for withdrawals, and real-time predictive rules to flag suspicious activity.
Lightning Network optimization — Build models to optimize payment routing, channel liquidity management, and predict channel health.
Crypto card infrastructure — Create credit risk models and user scoring for card issuance without traditional credit history.
LTV & churn prediction — Forecast user behavior to improve retention and conversion, especially in low-crypto-adoption countries.
Product & growth analytics — Develop ML-driven hypotheses for A/B testing and recommendation systems (e.g., which gift cards to offer).
Data pipeline & MLOps — Help build pipelines for training and deploying models to production (alongside engineers).
Requirements
5+ years of experience in Data Science / Machine Learning, ideally in fintech, payments, crypto, or high-throughput transactional systems.
Strong experience building anti-fraud models, time series forecasting (for transactions), LTV predictions, or classification/clustering models.
Python — solid proficiency (pandas, scikit-learn, xgboost/lightgbm; PyTorch/TensorFlow is a plus).
SQL — advanced level.
Experience with Airflow, dbt, or similar ETL/ML pipeline tools.
Understanding of Lightning Network basics (or strong willingness to dive deep within 1–2 weeks).
English — sufficient to discuss ML metrics and business problems fluently.
Big Pluses
Experience with graph models (Node2Vec, GNNs) for transaction chain detection.
Familiarity with blockchain analytics (on-chain data, addresses, transactions).
Experience in credit scoring without traditional credit history.
MLOps skills (Docker, Kubeflow, MLflow, or at least deploying models via FastAPI).
Personal experience using Bitrefill or crypto as a payment tool.
We Offer
Fully remote work from anywhere in the world (with ~4–6 hours overlap with Stockholm time zone).
Opportunity to build an ML system from scratch in a startup that is truly changing how people use crypto.
Competitive salary + equity (tokens) or profit-sharing.
Minimal bureaucracy — maximum responsibility and freedom.
Work with cutting-edge technologies: Lightning Network and crypto cards.
How to Apply
Send your resume and a link to your GitHub / Kaggle / project where you solved a fraud, time series, or risk modeling problem. Mention if you have experience with Lightning or blockchain analytics — if not, just tell us how quickly you can learn new domains.

Role OverviewKeras Labs is seeking a high-performing Blockchain Engineer to design, build, and scale smart contract systems for a hybrid AMM + perpetual trading platform powered by AI agents.This role also includes building and deploying standard token contracts (ERC20/BEP20) and supporting core on-chain infrastructure in a fast-paced, product-driven environment. Core ResponsibilitiesBuild and maintain smart contracts (Solidity / Rust)Develop and deploy ERC20 / BEP20 (or equivalent EVM) tokensImplement token logic (minting, burning, access control)Build core protocol modules (liquidity, trading, vaults)Integrate price oracles and on-chain data sourcesOptimize contracts for gas efficiency and performanceCollaborate with frontend, product, and research teamsSupport testing, audits, and deployment workflowsMonitor protocol performance and on-chain activity RequirementsStrong proficiency in Solidity (EVM) and/or RustExperience deploying ERC20 / BEP20 or similar token standardsFamiliarity with smart contract testing (Foundry, Hardhat)Understanding of contract security best practicesStrong system design and problem-solving skillsComfortable working in a fast-paced, remote environment Nice to HaveExperience with DeFi protocols (DEX, perps, lending)Understanding of AMM models and perpetual mechanicsExperience building trading systems or financial primitivesExperience with oracles (Chainlink, Pyth, etc.)Exposure to AI agents or automated trading systemsOpen-source contributions in Web3 / DeFi Compensation & BenefitsPerformance Bonus: Annual incentive structureBenefits: Remote-first flexibility, high ownership, impact-driven culture

Title: Lead Web & Mobile Partner for Private Agency
​About the Role:
I am Sadr B, founder of a private development agency. I am currently building a dedicated team of elite developers to handle high-value web and mobile (APK) projects. I am not just looking for a freelancer; I am seeking a long-term partner to grow this agency with me.
​The Partnership Structure:
​Team Spirit: We work as a unified team. You handle the technical execution (Web/App), and I manage agency growth, strategy, and client relations.
​Negotiated Rates: We do not post fixed budgets upfront. Every project is discussed in DM based on the specific client quote and requirements.
​50/50 Milestone Payment: Security is our priority. Once the client pays the initial deposit, 50% of your fee is funded into LaborX Escrow immediately. The final 50% is released upon delivery and client approval.
​Payment: All transactions are handled in Crypto (USDT/Polygon) for speed and transparency.
​Requirements:
​Expertise in Full-Stack development (React, Node.js, Flutter preferred).
​Strong experience in delivering high-quality Android APKs.
​A portfolio demonstrating your ability to handle professional-grade projects.
​How to Apply:
Please DM me with your portfolio and a brief introduction of your experience. Let’s discuss how we can build this agency together.

Worklo is building the future of professional services automation — combining our core PSA platform with a blockchain-powered salary distribution system that automates compensation directly from project data, time entries, role assignments, and milestones. This role is for a highly capable engineer who can own both traditional full-stack SaaS development and blockchain infrastructure, designing and scaling an end-to-end system that bridges Worklo’s platform with secure on-chain payroll execution.You’ll be one of the core engineers shaping both Worklo PSA and our blockchain payroll architecture. This is a greenfield, high-ownership opportunity where you’ll make major architectural decisions across smart contracts, backend systems, frontend applications, database security, and blockchain integrations.ResponsibilitiesFull Stack Platform DevelopmentOwn full product features end-to-end: database schema design, backend API development, frontend UI implementation, deployment, and monitoringBuild and maintain scalable applications using Next.js 15 (App Router, React Server Components), TypeScript, Express.js REST APIs, and SupabaseDesign and implement PostgreSQL schemas, optimize queries, and maintain database performanceWrite and maintain Supabase Row Level Security (RLS) policies for secure multi-tenant data isolationBuild polished, reusable UI components using React, shadcn/ui, and Tailwind CSS 4Develop secure REST APIs with proper validation (Zod), authentication, and error handlingIntegrate third-party services including SMTP, file storage, analytics dashboards, and workflow toolsDebug production issues, improve observability, and contribute to architecture decisionsBlockchain System DevelopmentDesign and implement Solidity smart contracts for salary calculation and automated payroll distributionBuild compensation logic covering:Hours workedRole multipliersMilestone bonusesStablecoin payouts (USDC/USDT)Deploy and manage smart contracts on Polygon L2 or similar EVM-compatible networksDevelop backend blockchain integrations that sync Worklo’s Supabase data with on-chain payroll systemsBuild wallet registry systems enabling MetaMask and WalletConnect integrationImplement blockchain event listeners, webhooks, and indexing systems to synchronize off-chain and on-chain statesCreate secure transaction signing, key management, and server-side blockchain interaction layersBuild in-app dashboards for payroll visibility, wallet management, and transaction historyWrite and maintain comprehensive test suites for smart contracts using Hardhat, Foundry, or similar frameworksOptimize contracts for gas efficiency, security, and upgradeabilityRequirements3+ years of professional experience in full-stack software development3+ years of blockchain development with production-deployed smart contractsStrong TypeScript/Node.js expertise across frontend and backend systemsStrong Solidity skills, including:Reentrancy protectionAccess controlUpgradeability patternsGas optimizationDeep experience with React and modern frontend architectureSolid understanding of PostgreSQL, database design, indexing, and optimizationExperience with Supabase, Firebase, or similar backend platformsExperience designing secure REST APIs and authentication systemsFamiliarity with ethers.js, viem, or similar blockchain interaction librariesExperience with Polygon or other EVM-compatible L2 ecosystemsStrong understanding of ERC-20 standards, stablecoin mechanics, and wallet integrationsAbility to work independently in a remote-first, async environmentComfortable making technical decisions with real product ownershipNice to HaveExperience with Next.js App Router and React Server ComponentsHardhat, Foundry, Truffle, or OpenZeppelin expertiseDeFi protocol or on-chain financial system experienceFamiliarity with The Graph or blockchain indexing infrastructureMulti-sig wallet and treasury management experienceExperience building multi-tenant SaaS productsFamiliarity with advanced frontend libraries such as:@xyflow/react@dnd-kitJotaiSWROpen source contributions or demonstrable side projectsWhy Join Worklo?Greenfield opportunity with major technical ownershipBuild cutting-edge infrastructure combining SaaS + blockchainInfluence architecture from database to smart contractsWork on products used by real agencies and consultanciesRemote-first, async team with strong engineering autonomyImmediate product impact with room to shape long-term systemsIf you’re excited about building scalable SaaS platforms while architecting real-world blockchain payroll systems, this is a rare chance to work at the intersection of enterprise software and decentralized finance.

We are looking for a Metaverse Developer to help build interactive virtual environments and immersive digital experiences. The role involves developing 3D worlds, integrating Web3 technologies, and working with modern tools for real-time applications.
You will work with Unity, WebGL, and blockchain technologies to create engaging metaverse features and optimize them for performance across different devices. The developer will collaborate with designers and backend engineers to deliver scalable and stable products.
Requirements include experience with Unity and C#, understanding of 3D development, Web3 or blockchain basics, and familiarity with Git and API integration. Experience with VR/AR, NFTs, or multiplayer systems is a plus.
We offer remote work, flexible hours, crypto payments, and the opportunity to build innovative metaverse products.