Information Privacy

The following is an excerpt from the book Designing Secure Software: A Guide for Developers by Loren Kohnfelder, Copyright 2022, No Starch Press

In addition to the foundations of information security—C-I-A and the Gold Standard—another fundamental topic I want to introduce is the related field of information privacy. The boundaries between security and privacy are difficult to clearly define, and they are at once closely related and quite different. In this book I would like to focus on the common points of intersection, not to attempt to unify them, but to incorporate both security and privacy into the process of building software.

To respect people’s digital information privacy, we must extend the principle of confidentiality by taking into account additional human factors, including:

  • Customer expectations regarding information collection and use

  • Clear policies regarding appropriate information use and disclosure

  • Legal and regulatory issues relating to handling various classes of information

  • Political, cultural, and psychological aspects of processing personal information

As software becomes more pervasive in modern life, people use it in more intimate ways and include it sensitive areas of their lives, resulting in many complex issues. Past accidents and abuses have raised the visibility of the risks, and as society grapples with the new challenges through political and legal means, handling private information properly has become challenging.

In the context of software security, this means:

  • Considering the customer and stakeholder consequences of all data collection and sharing

  • Flagging all potential issues, and getting expert advice where necessary

  • Establishing and following clear policies and guidelines regarding private information use

  • Translating policy and guidance into software-enforced checks and balances

  • Maintaining accurate records of data acquisition, use, sharing, and deletion

  • Auditing data access authorizations and extraordinary access for compliance

Privacy work tends to be less well defined than the relatively cut-and-dried security work of maintaining proper control of systems and providing appropriate access. Also, we’re still working out privacy expectations and norms as society ventures deeper into a future with more data collection. Given these challenges, you would be wise to consider maximal transparency about data use, including keeping your policies simple enough to be understood by all, and to collect minimal data, especially personally identifiable information.

Collect information for a specific purpose only, and retain it only as long as it’s useful. Unless the design envisions an authorized use, avoid collection in the first place. Frivolously collecting data for use “someday” is risky, and almost never a good idea. When the last authorized use of some data becomes unnecessary, the best protection is secure deletion. For especially sensitive data, or for maximal privacy protection, make that even stronger: delete data when the potential risk of disclosure exceeds the potential value of retaining it. Retaining many years’ worth of emails might occasionally be handy for something, but probably not for any clear business need. Yet internal emails could represent a liability if leaked or otherwise disclosed, such as by power of subpoena. Rather than hang onto all that data indefinitely, “just in case,” the best policy is usually to delete it.

A complete treatment of information privacy is outside the scope of this book, but privacy and security are tightly bound facets of the design of any system that collects data about people—and people interact with almost all digital systems, in one way or another. Strong privacy protection is only possible when security is solid, so these words are an appeal for awareness to consider and incorporate privacy considerations into software by design.

For all its complexity, one best practice for privacy is well known: the necessity of clearly communicating privacy expectations. In contrast to security, a privacy policy potentially affords a lot of leeway as to how much an information service does or does not want to leverage the use of customer data. “We will reuse and sell your data” is one extreme of the privacy spectrum, but “Some days we may not protect your data” is not a viable stance on security. Privacy failures arise when user expectations are out of joint with actual privacy policy, or when there’s a clear policy and it is somehow violated. The former problem stems from not proactively explaining data handling to the user. The latter happens when the policy is unclear, or ignored by responsible staff, or subverted in a security breakdown.