A security API is an Application Program Interface that allows untrusted code to access sensitive resources in a secure way. Examples of security APIs include the interface between the tamper-resistant chip on a smartcard (trusted) and the card reader (untrusted), the interface between a cryptographic Hardware Security Module, or HSM (trusted) and the client machine (untrusted), and the Google maps API (an interface between a server, trusted by Google, and the rest of the Internet). The crucial aspect of a security API is that it is designed to enforce a policy, i.e. no matter what sequence of commands in the interface are called, and no matter what the parameters, certain security properties should continue to hold. This means that if the less trusted code turns out to be malicious (or just faulty), the carefully designed API should prevent compromise of critical data. Designing such an interface is extremely tricky even for experts. A number of security flaws have been found in APIs in use in deployed systems in the last decade. In this tutorial paper, we introduce the subject of security API analy- sis using formal techniques. This approach has recently proved highly successful both in finding new flaws and verifying security properties of improved designs. We will introduce the main techniques, many of which have been adapted from language-based security and security protocol verification, by means of two case studies: cryptographic key management, and Personal Identification Number (PIN) processing in the cash machine network. We will give plenty of exam- ples of API attacks, and highlight the areas where more research is needed.
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