Janko is available for hire
Hire JankoJanko Krstic
Verified Expert in Engineering
Software Engineer and Web Developer
Belgrade, Serbia
Toptal member since July 27, 2018
Bio
Janko is a software engineer and architect specializing in blockchain and focusing on decentralized finance and self-sovereign identity (SSI). He has years of experience with Bitcoin, Ethereum, Quorum, Fabric, Tendermint, Cosmos blockchains, and Hyperledger Indy and Aries SSI solutions. Janko has recently applied modern distributed systems, cryptography, and blockchain theory to solving SSI-related problems.
Portfolio
Douglas Braxton
Blockchain, Architecture, Full-stack, React, Identity & Access Management (IAM)...
HR&Co
Blockchain, Blockchain Design, Blockchain Architecture, Smart Contracts...
Hoot App Inc.
Blockchain, Smart Contracts, Ethereum Virtual Machine (EVM), Token Design, Web3...
Experience
- Blockchain - 8 years
- Python - 8 years
- Smart Contracts - 7 years
- Hyperledger - 6 years
- Solidity - 6 years
- Go - 6 years
- IPFS - 1 year
- Rust - 1 year
Preferred Environment
Git, Erlang, Rust, Go, Python, Linux, Blockchain
The most amazing...
...project I've worked on was a secure, enterprise-level crypto wallet with tagging features for searching and filtering encrypted records.
Work Experience
Blockchain/Full-stack Architect
2025 - 2026
Douglas Braxton
- Implemented smart contracts layer on Ethereum/Polygon.
- Developed the full platform to manage issuers, verifiers, and holders using TypeScript.
- Implemented the layer that integrates decentralized identifiers (DIDs) and verifiable credentials (VCs) with blockchain.
Technologies: Blockchain, Architecture, Full-stack, React, Identity & Access Management (IAM), HIPAA Compliance, HIPAA Electronic Data Interchange (EDI), Health, Ethereum, Polygon
Senior Blockchain Consultant
2025 - 2025
HR&Co
- Worked directly with the client to understand the problem space.
- Conducted the research phase to pick the correct technical stack for the development phase.
- Created the system architecture for the HR platform using blockchain and SSI technologies.
Technologies: Blockchain, Blockchain Design, Blockchain Architecture, Smart Contracts, API Integration, Cryptocurrency, Artificial Intelligence (AI), Hyperledger
Blockchain Solutions Architect
2025 - 2025
Hoot App Inc.
- Designed the blockchain and token strategy for the project.
- Developed the back-end services to support the platform.
- Built the core parts of the system using TypeScript and NestJS.
Technologies: Blockchain, Smart Contracts, Ethereum Virtual Machine (EVM), Token Design, Web3, Documentation, Technical Leadership, Cryptocurrency, IPFS, Decentralized Systems, Arweave, WalletConnect, Solidity, Rust
Back-end Engineer
2025 - 2025
HOOT APP INC.
- Developed a back end for the new social media platform Hoot.
- Used TypeScript to implement threads and comments feature for the platform.
- Implemented chats and messaging service in the platform.
Technologies: TypeScript, NestJS, React, React Native, Google Cloud
Blockchain Developer
2024 - 2025
X-Margin INC
- Integrated decentralized finance metrics providers into the Credora metrics system.
- Wrote Credora metrics smart contracts for the Ethereum blockchain.
- Conducted research on liquid staking and verified smart contracts.
Technologies: Web3, Blockchain, Smart Contracts, Front-end Development, Back-end Development, React, Python, Data Science, Node.js, TypeScript, Blockchain Development
Cosmos Developer
2024 - 2025
PollPapa Inc.
- Reviewed the blockchain architecture for the PollPapa system.
- Created the blockchain network architecture for the production network.
- Set up the security and performance requirements for the blockchain network.
Technologies: Blockchain, Cosmos SDK, Tendermint, Blockchain Development
Cosmos Blockchain Developer
2023 - 2024
Divine Ray
- Developed the application-specific blockchain using Tendermint and Cosmos SDK.
- Developed blockchain integration service, which interacts with the Cosmos SDK-built blockchain using the Go programming language.
- Integrated the blockchain monitoring service to oversee the blockchain.
Technologies: Blockchain, Cosmos, Go, Cryptocurrency, Blockchain Architecture, Blockchain Development
DLT Engineer
2020 - 2023
ASU Pocket - University Technology Office
- Built a web service that integrated Hyperledger Aries and the university's system, allowing it to issue verifiable credentials to its students.
- Guided the SSI implementation process of the entire project.
- Built several proof-of-concept applications that showcase the usage of verifiable credentials in education.
Technologies: Blockchain, Distributed Ledgers, Web3.js, Hashgraph, Technical Leadership, Decentralized Systems, Blockchain Architecture
Blockchain and SSI Engineer
2020 - 2023
Pocket (Arizona State Univesity)
- Created a self-sovereign identity (SSI) wallet that stores students' verifiable credentials.
- Wrote a back-end system for issuing verifiable credentials to the institution's users.
- Designed and researched self-sovereign identity libraries to use.
- Architected the platform that issues verifiable credentials to students.
Technologies: Hyperledger, Hyperledger Indy, Blockchain, Blockchain Architecture, Blockchain Development
Blockchain Engineer
2020 - 2020
404 Publishing
- Worked on developing features for the new DAG platform.
- Improved the wallet capabilities of the DAG platform.
- Created the trigger mechanism to allow smarter DAG nodes.
Technologies: JavaScript, Node.js, Directed Acrylic Graphs (DAG), Smart Contracts, Ethereum Smart Contracts, Decentralized Systems, Bots, Chatbots
Blockchain Engineer
2020 - 2020
Forward Edge
- Worked on picking the correct blockchain technology for the client's needs.
- Implemented a prototype on the Tendermint blockchain platform.
- Wrote integration services in Golang, which serve as a blockchain's outer layer.
Technologies: Tendermint, Go, Smart Contracts, Ethereum Smart Contracts, Ethereum Virtual Machine (EVM), Blockchain Development
Blockchain Engineer
2019 - 2020
Pharmwerks
- Worked on designing the prototype for a platform on top of blockchain in the pharmaceutical industry.
- Developed prototype smart contracts on Ethereum, using Solidity.
- Wrote an engine in Python which integrated with Ethereum.
Technologies: Quorum, Ethereum, Python, Smart Contracts, Ethereum Smart Contracts, Decentralized Systems, IPFS
Blockchain Architect
2018 - 2019
Tykn tech
- Architectured the platform.
- Implemented the solution using Hyperledger and Python.
- Coordinated with end-user application team.
Technologies: Hyperledger, Python, Blockchain Architecture, Blockchain Design, Blockchain Development
Senior Blockchain Developer
2018 - 2019
6fusion (via Toptal)
- Wrote software in Go which interacts with a blockchain platform.
- Worked with the Quorum platform.
- Used Docker and kubernetes for deployment.
Technologies: Solidity, Quorum, Ethereum, Go, Smart Contracts, Ethereum Smart Contracts, Artificial Intelligence (AI), Blockchain Architecture, Blockchain Development
Solidity/Blockchain Expert with Go Experience
2018 - 2019
Fractal Blockchain (via Toptal)
- Contributed to software written in Go which interacts with Ethereum nodes (with geth nodes specifically).
- Reviewed and improved smart contracts written in Solidity.
- Reviewed and improved software interacting with the Bitcoin network.
Technologies: Go, Bitcoin, Ethereum, Solidity, Blockchain, Smart Contracts, Ethereum Smart Contracts, Blockchain Development
Software Engineer
2018 - 2019
Evernym
- Developed a secure crypto-wallet intended to be used for Hyperledger Indy.
- Supported the Indy-SDK project.
- Created proprietary software solving SSI-related problems on top of Hyperledger Indy.
- Bulit a system of pluggable crypto-wallet storage back-ends which allows for enterprise-scale wallets.
Technologies: Hyperledger, Rust, Python, Cryptocurrency Wallets
Software Engineer
2016 - 2018
TeleSign
- Developed the Number Discovery Platform using Python and Gevent.
- Extended the PhoneID product written in Python and Bottle.
- Created a URL-shortening service using Python and Flask.
- Built a Number Deactivation product using Python.
Technologies: Flask, Bottle.py, Gevent, Python
Web Developer
2015 - 2016
BEST
- Wrote an internal HR application using JSP.
- Refactored an internal voting system written in a custom in-house framework.
- Supported and extended an internal forum written in JSP.
Technologies: Java, Python
Web Developer
2012 - 2016
Self-employed
- Wrote a web crawler and scraper using Python and Beautiful Soup.
- Developed a custom content manager application back-end using Django.
- Created web microservices using Python.
Technologies: Beautiful Soup, Jakarta Server Pages (JSP), Java, Pyramid, Django, Flask, Python
Experience
Hyperledger Indy Crypto Wallet
https://github.com/hyperledger/indy-sdkDesigned and developed a crypto wallet used in Hyperledger Indy.
Some of the features that distinguish this wallet:
1. mechanism for searching through encrypted entities using tags
2. multiple encryption keys using Elliptic Curve Cryptography
3. pluggable storage system supporting enterprise-scale crypto wallets
Some of the features that distinguish this wallet:
1. mechanism for searching through encrypted entities using tags
2. multiple encryption keys using Elliptic Curve Cryptography
3. pluggable storage system supporting enterprise-scale crypto wallets
Identity Solution for Red Cross Refugees on Top of Hyperledger Indy
Part of the team that designed and developed a solution for solving identity problems for the refugees in Red Cross centers. This was a part of Blockchaingers 2018 Hackathon, which was held in Groningen, Netherlands
Multichain Tracker (Blockchain Data Tracker and Aggregator)
This project allowed the user to track through a dashboard different blockchains (and possibly even private sidechains if they exposed correct APIs), and be able to both look at the current data, as well as track data (hashes) across several blockchains.
It integrated with most common blockchains (Bitcoin, Ethereum, etc.) through their APIs, and fed data through web sockets to a front-end dashboard.
It integrated with most common blockchains (Bitcoin, Ethereum, etc.) through their APIs, and fed data through web sockets to a front-end dashboard.
Reward System on Top of Ethereum Blockchain
Developed a part of a platform intended for rewarding loyal users of the platform and provide gamification, using Ethereum.
Pharmachain (Provenance and Supply Chain Blockchain)
Blockchain for tracking provenance and ownership transfers of pharmaceutical supplies. It was intended to function on an international level and enable easier tracking of transfers across international borders, to prevent scam, and track defects and inappropriate batches.
Website and Web App Used in BEST
Redesigned an old website into a current, modern version.
Refactored and extended the internal application.
Refactored and extended the internal application.
Number Discovery Platform
https://www.telesign.com/products/sms-api/One of the key components of the SMS platform used to discover the best routes and providers for delivering an SMS.
Phone ID Service
https://www.telesign.com/products/phone-id/Developed and extended existing features for a Phone ID service. It is a service intended to provide sophisticated information for phone numbers.
Education
2012 - 2016
Bachelor's Degree in Software Engineering
University in Belgrade - Belgrade, Serbia
Skills
Libraries/APIs
Claude API, Beautiful Soup, Gevent, Bottle.py, Node.js, Web3.js, React
Tools
Hyperledger Indy, Git, Stellar SDK
Languages
Go, Python, Solidity, Rust, Erlang, JavaScript, Java, TypeScript, CSS
Platforms
Blockchain Platforms, Ethereum, Blockchain, Hyperledger, Kubernetes, Docker, Hyperledger Fabric, Linux, Hedera Hashgraph, Amazon Web Services (AWS), Decentralized Exchanges (DEXs)
Frameworks
Tendermint, Cosmos SDK, Next.js, Pyramid, Jakarta Server Pages (JSP), Flask, Django, React Native, NestJS, Tailwind CSS
Paradigms
Concurrent Programming, Agile, Functional Programming, HIPAA Compliance
Storage
Relational Databases, Cassandra, PostgreSQL, BigchainDB, MySQL, Google Cloud
Other
Distributed Ledger Technology (DLT), Distributed Ledgers, Decentralized Applications, Distributed Systems, Self-sovereign Identity (SSI), Web3, Decentralized Systems, Blockchain Architecture, Blockchain Development, Blockchain & Cryptocurrency, Crypto, Architecture, System Architecture, Directed Acrylic Graphs (DAG), Applied Cryptography, Ethereum Smart Contracts, Smart Contracts, Quorum, IPFS, Bitcoin, Peer-to-peer Networking, Hyperledger Aries, Cryptocurrency, Non-fungible Tokens (NFT), ÐApp, DApps, Cosmos, APIs, ERC-721, Ethereum Virtual Machine (EVM), Technical Leadership, Cryptocurrency Wallets, Blockchain Design, Front-end, AI Integration, Hashgraph Technology, EOSIO, EOS, Cryptography, Hashgraph, Machine Learning, Artificial Intelligence (AI), Data Science, Front-end Development, Back-end Development, Decentralized Finance (DeFi), Solana, Bots, Chatbots, CEX, Stablecoins, Full-stack, Identity & Access Management (IAM), HIPAA Electronic Data Interchange (EDI), Health, Polygon, Token Design, Documentation, Arweave, WalletConnect, API Integration, Supabase
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