Defensive design considerations Anticipating misuse Authentication

The idea

Defensive design means writing programs that stay reliable and secure even when users behave unexpectedly or maliciously. Anticipating misuse involves thinking about every way a user could enter wrong, missing or harmful data, or use the system in an unintended order, and then building safeguards before release. Authentication is the process of confirming that a user is who they claim to be, usually by checking a username with a password, a PIN, a security token or biometric data. Together they reduce crashes, data corruption and unauthorised access.

For example: A login form should limit password attempts, lock an account temporarily after repeated failures and never reveal whether the username or the password was wrong.

What earns the marks

• Defensive design anticipates problems before they occur rather than fixing them after a system fails.
• Anticipating misuse includes considering invalid data, missing data, unexpected sequences of actions and deliberate attempts to break the system.
• Authentication verifies a user's identity, commonly through something the user knows, has or is.
• A typical authentication method combines a username with a password and may add a second factor such as a code sent to a phone.
• Good defensive design limits the number of login attempts and locks or delays access after repeated failures.
• Error messages should be helpful to legitimate users but should not reveal sensitive system details to an attacker.

Common mistakes and how to put them right

• Confusing authentication with validation.

  Put it right: Authentication checks identity, while validation checks that data is sensible and in the correct format.

• Believing that a strong password alone is enough.

  Put it right: Authentication also needs secure storage, attempt limits and safe error messages.

• Assuming defensive design only matters for large systems.

  Put it right: Even a small quiz program should reject unexpected input and protect any stored user data.

Exam tips

• Link each defensive measure to a specific misuse you are preventing, such as limiting attempts to stop repeated password guessing.
• Use correct terminology: state that authentication verifies identity and give a concrete method rather than writing only 'make it secure'.
• When asked to improve a program, name the check, say where it is placed and explain what happens when the check fails.