This paper was made together withTechPay AB, by their request. The dissertation has been divided into two separate parts; one of which is fully theoretical, with the focus on the security requirements of a digital cash register, and how to meet these requirements. The second part covers the implementation of anAndroid-oriented Java-library,created with help from the security requirements found in the first part. The purpose of this library is to make it easier to develop a cash register that uses payment terminals from TechPay. This paper also covers different communication protocols, encryption techniques and recommendations from authorities, banks and Android. This paper also contains a small review of how users are validated in applications on the current market that handles money transactions.