Over the past years, Blockchain (BC) interoperability became a crucial topic of research due to the fast development of new BC platforms for many use-cases, from supply-chain to voting. The need for interoperability can be attributed to the development of such platforms that did not follow any standard, leading to the creation of BC islands that do not exchange data, which hinders innovation. Hence, several BC interoperability solutions were proposed to tackle such problems. One of them is Bifröst, a Notary scheme-based BC interoperability Application Programming Interface (API) that allows the transparent interaction by users or legacy applications with several BCs. An initial prototype of Bifröst was developed, proving its feasibility; however, it did not provide native data encryption or an interaction standard. Thus, this paper presents the design and implementation of an encryption scheme for Bifröst and a JSON format proposal to interact with Bifröst, which also contributes to the state-of-the-art. Evaluations on the encryption scheme show an increase of the data size after the encryption but a constant performance overhead, and discussions on the standard prove that it can be applied to other Notary-based BC interoperability APIs due to its flexibility.