Privacy Fundamentals

Build the mental model

Every previous lesson has been about security. Privacy is a related but genuinely separate discipline.

Privacy concerns how personal data is collected, used, shared, and controlled — including by people and companies fully authorized to have it.

Secure but still a privacy problem

A company can securely store data with strong encryption and access control, while still collecting far more than a user would expect. Security protects data; privacy governs even authorized access.

Personal data covers more than obvious identifiers — location history, account identifiers, photos, financial details, and browsing data all count too.

Sensitivity depends heavily on context: a public username alone is low-sensitivity, but combined with a home address and daily schedule, it can enable real-world harm.

Data minimization: collect and retain only what's needed for a legitimate purpose. It quietly serves privacy, security, and operational simplicity all at once.

Privacy: The discipline of governing how personal data is collected, used, shared, and controlled — including by parties who are fully authorized to access it.

Data Minimization: Collecting and retaining only the data actually needed for a legitimate, stated purpose — nothing more "just in case."

text

SECURITY VS PRIVACY
-------------------
SECURITY                      PRIVACY
-----------------------       -----------------------
Protects data from            Controls what gets
unauthorized access           collected, used, and
                               shared -- even by
                               authorized parties

Both apply to personal data. A system can be secure
AND still over-collect -- that gap is a privacy
problem, not a security bug.

Connect it to a real scenario

The function below turns data minimization into a runnable check: given a data type, a stated purpose, and whether it's actually needed, it returns a clear verdict.

The check itself is simple on purpose: it forces someone to answer, honestly, whether this specific data is actually required for the stated purpose.

Shipping address for delivery

Clearly matches the stated purpose — verdict: OK.

GPS history for a newsletter

Doesn't match the stated purpose at all — verdict: OVER-COLLECTING.

Apply the same question to every field on a signup form or every app permission: name the purpose, then ask honestly if this data is actually required.

Security and privacy overlap but are not identical — a system can be fully secure and still over-collect personal data.
Personal data includes more than obvious identifiers — location history, activity data, and photos are personal data too.
Data minimization means collecting and keeping only what's needed for a stated purpose, nothing more "just in case."

Try the working example

javascript

function checkDataMinimization(practice) {
  const followsMinimization = practice.isNeededForThatPurpose === true;

  return {
    dataType: practice.dataType,
    statedPurpose: practice.statedPurpose,
    followsMinimization,
    verdict: followsMinimization
      ? "OK — collection matches a legitimate, stated purpose."
      : `OVER-COLLECTING — "${practice.dataType}" is not needed for "${practice.statedPurpose}".`,
  };
}

const minimalExample = checkDataMinimization({
  dataType: "shipping address",
  statedPurpose: "deliver a physical order",
  isNeededForThatPurpose: true,
});

const overCollectingExample = checkDataMinimization({
  dataType: "precise GPS location history",
  statedPurpose: "send a monthly email newsletter",
  isNeededForThatPurpose: false,
});

console.log(JSON.stringify(minimalExample, null, 2));
console.log(JSON.stringify(overCollectingExample, null, 2));

You should see

{
  "dataType": "shipping address",
  "statedPurpose": "deliver a physical order",
  "followsMinimization": true,
  "verdict": "OK — collection matches a legitimate, stated purpose."
}
{
  "dataType": "precise GPS location history",
  "statedPurpose": "send a monthly email newsletter",
  "followsMinimization": false,
  "verdict": "OVER-COLLECTING — \"precise GPS location history\" is not needed for \"send a monthly email newsletter\"."
}

5-minute try-it

Extend checkDataMinimization to accept a retentionPeriod field and flag practices that keep data indefinitely (no defined retention period) as a second kind of problem, separate from over-collection.

One important caution

Assuming that because data is stored securely, collecting it in the first place is automatically fine.

Collecting data "just in case it's useful later" instead of tying every field to a specific, stated purpose.

EFF — Privacy — Digital Privacy & Modern Security