Defensive design

Defensive design is about writing programs that can anticipate problems before they happen – especially incorrect inputs or misuse.
Anticipating misuse: users might enter invalid, unexpected, or malicious input. The program should handle this without crashing.
- Examples of misuse include typing letters into a number input, leaving fields blank, and entering a script or code (e.g. for hacking)
- This can be addressed by using input validation, using authentication (to check who the user is), or displaying error messages and ask for correct input
Authentication: is used to verify a user’s identity, typically using:
- Username + Password
- Sometimes 2FA (two-factor authentication)
Input validation: checks that the user’s input is acceptable before using it in the program. Validation prevents bugs and improves security.

Common types of checks	What it does	Example
Length	Checks input is not too short or long	Password length > = 8
Range	Ensures number is within limits	Age between 0 and 120
Type	Checks if input is the right type	Number not string
Format	Pattern is correct	Email contains @
Presence	Ensures input is not empty	Name is not blank

# Authentication example

username = input(“Enter username:”)

password = input(“Enter password:”)

if username == “admin”

and password == “1234”

print(“Access granted”)

else:

print(“Access denied”)

# Age validation example

age = input(“Enter your age:”)

if age.isdigit():

age = int(age)

if age > = 0 and age < = 120:

print(“Valid age”)

else:

print(“Age out of range”)

else:

print(“Please enter a number”)

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