Abstract. We present a link between the interactive proof assistant Isabelle/HOL and the Spark/Ada tool suite for the verification of highintegrity software. Using this link, we can tackle verification problems that are beyond reach of the proof tools currently available for Spark. To demonstrate that our methodology is suitable for real-world applications, we show how it can be used to verify an efficient library for big numbers. This library is then used as a basis for an implementation of the RSA public-key encryption algorithm in Spark/Ada. 1