Sergio Penén, Developer in Edinburgh, United Kingdom
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Sergio Penén

Verified Expert  in Engineering

Software Developer

Edinburgh, United Kingdom
Toptal Member Since
July 14, 2022

Sergio considers himself a creative, curious, focused, and self-managed professional. He is always looking for the best way to resolve technical problems or develop new features. Sergio is comfortable working as part of a team and alone. Although most of his work experience is in the payment industry, he is always keen to learn new things and is open to opportunities in other sectors as well.


Telmos Consulting Ltd
Java 8, Apache Tomcat, AWS Elastic Beanstalk...
Java 8, JPA 2, Redis, C++14, Multithreading, Oracle Database, SQL Server 2017...
Fidelity National Information Services
C++98, Bash, KSH, Linux, AIX, ISO 8583, Oracle Database, Sybase, AutoTools...




Preferred Environment

Linux, Spring Boot, Microservices Architecture, Java 11, Redis, ISO 8583, C++14, MySQL, EMV, MasterCard IPM

The most amazing...

...project I've developed is a payment switch cluster system called PayOne, which consists of a real-time message broker developed in C++, Redis, Java, and JPA.

Work Experience

Back-end Developer | CTO

2019 - 2022
Telmos Consulting Ltd
  • Started working in this company in 2019, and my main task was to build the back end, which I did within one year. The back end was fully scalable, PCI DSS compliant, and decentralized.
  • Integrated our system with five acquirer networks using ISO 8583 protocol for four and APACS protocol for the fifth one while working as a back-end software developer. Settlement files were either base-2 or similar to Mastercard IPM.
  • Became a CTO and acquired the ADVT Visa Level 2 and security certification for POS on commercial off-the-shelf (COTS) devices, including PIN on glass.
Technologies: Java 8, Apache Tomcat, AWS Elastic Beanstalk, Amazon Virtual Private Cloud (VPC), Amazon RDS, Hardware Security Modules (HSMs), AWS Key Management Service (KMS), Firebase, DUKPT, HSM, Google Cloud, ISO 8583, Automated Testing, AWS SDK, MongoDB, Spring Boot, JPA 2, JDBC, Android POS, Android Development, Apache Maven, JUnit, Scrum, Kanban, Pinblock Translation, Software Design Patterns, Amazon Web Services (AWS), Java, Operating Systems, Object-oriented Programming (OOP), Software Development, Software Testing, Multithreading, Bash, Git, 3DES, PCI DSS, Transport Layer Security (TLS), JPA, Amazon S3 (AWS S3), AWS SSH Keys, Firmware, Linux, Microservices Architecture, Fintech

Back-end Developer and Payment Systems Specialist | CTO

2010 - 2019
  • Designed and developed a cluster-based credit and debit card transaction processing system with a secure message broker and HA using C++, Java, and JPA for business logic. It is mission-critical, fault-tolerant, flexible, and fully customizable.
  • Performed installations of the PayOne system for several clients, some of which are banks with stringent security rules and service level agreements.
  • Designed and developed a cluster management tool specifically for the PayOne system, capable of managing individual processes and cluster nodes.
Technologies: Java 8, JPA 2, Redis, C++14, Multithreading, Oracle Database, SQL Server 2017, Amazon EC2, Amazon ElastiCache, Amazon RDS, PostgreSQL, Linux, Bash, Git, DUKPT, EMV, RFID, Standard Template Library (STL), Apache Maven, JUnit, Hardware Security Modules (HSMs), AutoTools, CMake, ISO 8583, REST, XML, JSON, Scrum, Kanban, IPC, SQL Stored Procedures, Software Design Patterns, Windows App Development, Qt 5, Amazon Web Services (AWS), Java, Operating Systems, Object-oriented Programming (OOP), Software Development, Software Testing, Microcontrollers, Firmware, Automated Testing, 3DES, Windows, PCI DSS, C++, Transport Layer Security (TLS), JPA, AWS SSH Keys, Microservices Architecture, Fintech

Payment System Specialist

2000 - 2010
Fidelity National Information Services
  • Implemented Mastercard Maestro in collaboration with other team members. It was a downsizing project, and our system successfully became Brazil's main credit and debit card transaction processing system, reaching high transaction numbers.
  • Implemented the clearing system for Mastercard credit transactions for Brazil in IPM format files in collaboration with the team. This meant that we completed a process that had been in production for many years.
  • Participated in the initial design and development of the Visa online transaction processing implementation.
Technologies: C++98, Bash, KSH, Linux, AIX, ISO 8583, Oracle Database, Sybase, AutoTools, Java 8, Apache Tomcat, IBM WebSphere, MasterCard IPM, Base II, CSH, SQL Stored Procedures, Pinblock Translation, 3DES, Software Design Patterns, Operating Systems, Object-oriented Programming (OOP), Software Development, Software Testing, Multithreading, IPC, PCI DSS, C++, Transport Layer Security (TLS), Hardware Security Modules (HSMs), Fintech

Software Developer

1995 - 1999
  • Developed the Windows application for the ATCS tool designed to automate testing using a proprietary language. The tool communicated with testing devices using a proprietary protocol RS-232.
  • Maintained proprietary tools to support the development environment.
  • Developed proprietary tools to convert WAV files into a proprietary format for the switch exchange system.
Technologies: Windows, C++98, Software Design Patterns, Firmware, RS232, Operating Systems, Object-oriented Programming (OOP), Software Development, Software Testing, Microcontrollers, Assembly Language, Automated Testing, Bash, C++

Acquirer Payment System Simulator (Dissertation project)

Currently working on a Card Payment System simulator as a dissertation project to explore the benefits of CI/CD along with zero-downtime techniques on core payment systems. Idealized, designed, and implemented the whole project using CI/CD pipelines from AWS.

Maxa payments backend

A disruptive payments app that brings security to eCommerce transactions by allowing the buyers to tap their credit card using their own cellphone as a payments terminal when buying online. Within less than a year, I implemented the core back-end using Java/JDBC and Spring Boot/JPA for some modules. Additionally, I designed and implemented the secure key injection process to the phone as well as the pinblock generation at the terminal using the DUKPT algorithm as well as pinblock translation at the server using Thales HSM. The first version of the back end was already integrated with more than five European acquirer networks using different versions of ISO 8583 protocol.

PayOne System

PayOne is a cluster-based payment system processor for credit and debit cards, which has been designed and developed in collaboration with his business partner at Quadrisoft in Brazil. The system was designed to be secure, PCI DSS compliant, fault-tolerant at the server and process levels, highly available, and scalable.
2020 - 2022

Master of Science Degree in Computer Science and International Business

University of Cumbria - Online

1995 - 1998

Engineer's Degree Coursework in Systems Engineering

National University of La Matanza - Buenos Aires, Argentina

1988 - 1994

High School Diploma in Electronics technician

Instituto San José (A-355) - Buenos Aires, Argentina


Standard Template Library (STL), JPA 2, JDBC


Amazon Virtual Private Cloud (VPC), Git, Apache Tomcat, Amazon ElastiCache, Apache Maven, AutoTools, CMake, AWS Key Management Service (KMS), AWS SDK, AWS CodeCommit, AWS CloudFormation, AWS CLI


Java 8, Bash, Java, XML, C++98, KSH, CSH, C++, Java 11, C++14, Assembly Language, C++11, Stored Procedure


Linux, Amazon EC2, Windows, AWS Elastic Beanstalk, Oracle Database, Software Design Patterns, AIX, Amazon Web Services (AWS), Firebase, IBM WebSphere


Object-oriented Programming (OOP), Software Testing, Automated Testing, Scrum, Kanban, REST, Windows App Development, Microservices Architecture


Databases, PostgreSQL, JSON, SQL Stored Procedures, Redis, Sybase, MySQL, MongoDB, SQL Server 2017, Amazon S3 (AWS S3), Spring Data JPA, Google Cloud


JUnit, Spring Boot, Qt 5, JPA


ISO 8583, MasterCard IPM, Operating Systems, Software Development, Amazon RDS, DUKPT, Multithreading, IPC, Pinblock Translation, 3DES, Transport Layer Security (TLS), Fintech, Firmware, Hardware Security Modules (HSMs), RFID, Base II, AWS SSH Keys, EMV, Internet of Things (IoT), CI/CD Pipelines, Digital Marketing, Microcontrollers, Digital Circuit Design, Android POS, Android Development, RS232, PCI DSS, MySQL DBA, HSM

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