182 CHAPTER 10 Mobile Applications and Devices NOTE A digital signature is a cryptographic transformation that allows the verification of origin authentication, data integrity, and signatory nonrepudiation. 4 For more information on digital signatures, NIST FIPS PUB 186-3 provides a very detailed perspective on how digital signatures work. If you are unfamiliar with digital signatures, or public key infrastructure (PKI) in general, a beginner's guide to PKI can be found at: http://articles.techrepublic.com. com/5100-10878_11-5839988.html. Although code signing can help mitigate the risk involved with downloading applications from the Internet, the risk is not completely eliminated. It is impor- tant to note that code signing does not prevent the original developer from inserting malicious code into their application. Additionally, the user needs to trust that the developer has protected their private encryption key. If a third party has compromised the developer's private key, then that third party would be able to tamper with the application and digitally sign the executable as the original developer. As a real-world example of code signing, iPhones require all applications to be signed by Apple. 5 Thus, developers will need to request a signing certificate from Apple before their application can run on iPhones and be distributed in the App Store. So in this scenario, Apple acts as the CA, who can issue and revoke signing certificates. If an application is later determined to be malware, Apple could revoke the signing certificate for that developer and application to prevent their application from further spreading. Additionally, Apple could refuse to issue further signing certificates to that developer, effectively banning that developer from distributing more malware through the App Store. Apple's code signing technique is susceptible to attacks, though. At the 2010 CanSecWest conference, Halvar Flake compromised an iPhone in the Pwn2Own contest on the first day using an exploit developed by Vincenzo Iozzo and Ralf- Philipp Weinmann. 5 The attack was performed by browsing a malicious Web site on the iPhone, which forced the iPhone to disclose the contents of its SMS database. 5 The exploit circumvents the code signing security control by using return-oriented programming techniques, which manipulate the function call stack to convert valid code into a malicious payload. 5 ATTACKS Mobile applications and devices will be targets in the smart grid. Attackers generally take the path of least resistance to achieve their goal, and mobile applications provide an easily accessible target. Attacks against certain mobile devices, such as cell phones and PMPs, have been limited in the past; however, this will dramatically change as their integration into society and level of connectivity continue to increase.