Security & Authentication

Security techniques: Validation and authentication

Security in programming means ensuring that data is correct, safe, and that only authorised users can access the system. Two key techniques are validation and authentication.

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Validation

Types of validation check

Check	Description	Example
Range check	Ensures data falls within a specified range	Age must be between 0 and 120
Type check	Ensures data is the correct data type	Age must be an integer, not text
Length check	Ensures data is the correct number of characters	Password must be at least 8 characters
Presence check	Ensures a field is not left blank	Username must be entered
Format check	Ensures data matches a required pattern	Email must contain @ and a domain
Lookup check	Ensures data matches a value in a predefined list	Country must be in a list of valid countries

Range check example

# Python - Range check
while True:
    age = int(input("Enter your age: "))
    if 0 <= age <= 120:
        print("Valid age entered.")
        break
    else:
        print("Error: Age must be between 0 and 120.")

' VB.NET - Range check
Dim age As Integer
Do
    Console.Write("Enter your age: ")
    age = CInt(Console.ReadLine())
    If age < 0 Or age > 120 Then
        Console.WriteLine("Error: Age must be between 0 and 120.")
    End If
Loop Until age >= 0 And age <= 120
Console.WriteLine("Valid age entered.")

Length check example

# Python - Length check
while True:
    password = input("Enter a password (min 8 characters): ")
    if len(password) >= 8:
        print("Password accepted.")
        break
    else:
        print("Error: Password must be at least 8 characters.")

' VB.NET - Length check
Dim password As String
Do
    Console.Write("Enter a password (min 8 characters): ")
    password = Console.ReadLine()
    If Len(password) < 8 Then
        Console.WriteLine("Error: Password must be at least 8 characters.")
    End If
Loop Until Len(password) >= 8
Console.WriteLine("Password accepted.")

Presence check example

# Python - Presence check
while True:
    username = input("Enter your username: ")
    if username != "":
        print("Username accepted.")
        break
    else:
        print("Error: Username cannot be blank.")

' VB.NET - Presence check
Dim username As String
Do
    Console.Write("Enter your username: ")
    username = Console.ReadLine()
    If username = "" Then
        Console.WriteLine("Error: Username cannot be blank.")
    End If
Loop Until username <> ""
Console.WriteLine("Username accepted.")

Type check example

# Python - Type check
while True:
    user_input = input("Enter a whole number: ")
    if user_input.isdigit():
        number = int(user_input)
        print("Valid number entered:", number)
        break
    else:
        print("Error: You must enter a whole number.")

' VB.NET - Type check
Dim number As Integer
Dim valid As Boolean = False
Do
    Console.Write("Enter a whole number: ")
    valid = Integer.TryParse(Console.ReadLine(), number)
    If Not valid Then
        Console.WriteLine("Error: You must enter a whole number.")
    End If
Loop Until valid
Console.WriteLine("Valid number entered: " & number)

Lookup check example

# Python - Lookup check
valid_colours = ["red", "green", "blue", "yellow"]
while True:
    colour = input("Enter a colour (red/green/blue/yellow): ").lower()
    if colour in valid_colours:
        print("Valid colour selected:", colour)
        break
    else:
        print("Error: Must be one of", valid_colours)

' VB.NET - Lookup check
Dim validColours() As String = {"red", "green", "blue", "yellow"}
Dim colour As String
Do
    Console.Write("Enter a colour (red/green/blue/yellow): ")
    colour = Console.ReadLine().ToLower()
    If Not validColours.Contains(colour) Then
        Console.WriteLine("Error: Must be red, green, blue or yellow.")
    End If
Loop Until validColours.Contains(colour)
Console.WriteLine("Valid colour selected: " & colour)

Combining multiple validation checks

In a real program you would typically combine several checks. For example, a registration form might validate a username with a presence check and a length check, and validate an age with a type check and a range check.

# Python - Combined validation
def get_valid_username():
    while True:
        username = input("Enter username (3-20 characters): ")
        if username == "":
            print("Error: Username cannot be blank.")
        elif len(username) < 3 or len(username) > 20:
            print("Error: Username must be 3-20 characters.")
        else:
            return username

def get_valid_age():
    while True:
        user_input = input("Enter your age: ")
        if not user_input.isdigit():
            print("Error: Age must be a whole number.")
        elif int(user_input) < 0 or int(user_input) > 120:
            print("Error: Age must be between 0 and 120.")
        else:
            return int(user_input)

username = get_valid_username()
age = get_valid_age()
print(f"Welcome {username}, age {age}")

' VB.NET - Combined validation
Function GetValidUsername() As String
    Dim username As String
    Do
        Console.Write("Enter username (3-20 characters): ")
        username = Console.ReadLine()
        If username = "" Then
            Console.WriteLine("Error: Username cannot be blank.")
        ElseIf Len(username) < 3 Or Len(username) > 20 Then
            Console.WriteLine("Error: Username must be 3-20 characters.")
        End If
    Loop Until username <> "" And Len(username) >= 3 And Len(username) <= 20
    Return username
End Function

Function GetValidAge() As Integer
    Dim age As Integer
    Dim valid As Boolean = False
    Do
        Console.Write("Enter your age: ")
        valid = Integer.TryParse(Console.ReadLine(), age)
        If Not valid Then
            Console.WriteLine("Error: Age must be a whole number.")
        ElseIf age < 0 Or age > 120 Then
            Console.WriteLine("Error: Age must be between 0 and 120.")
            valid = False
        End If
    Loop Until valid
    Return age
End Function

Dim username As String = GetValidUsername()
Dim age As Integer = GetValidAge()
Console.WriteLine("Welcome " & username & ", age " & age)

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Authentication

Common authentication methods

Method	How it works	Strengths	Weaknesses
Username & password	User enters credentials matched against stored records	Simple to implement, familiar	Passwords can be guessed, stolen, or shared
Two-factor authentication (2FA)	Requires two different types of proof (e.g. password + SMS code)	Much harder to bypass	Slower, relies on second device
Biometric	Uses unique physical features (fingerprint, face, iris)	Very hard to fake, convenient	Expensive hardware, privacy concerns
Email/SMS verification	A one-time code is sent to a registered device	Confirms ownership of device	Delays, relies on network access

Simple password authentication

# Python - Username and password authentication
stored_username = "admin"
stored_password = "SecurePass123"
max_attempts = 3

for attempt in range(max_attempts):
    username = input("Username: ")
    password = input("Password: ")
    if username == stored_username and password == stored_password:
        print("Access granted. Welcome!")
        break
    else:
        remaining = max_attempts - attempt - 1
        print(f"Incorrect. {remaining} attempts remaining.")
else:
    print("Account locked. Too many failed attempts.")

' VB.NET - Username and password authentication
Dim storedUsername As String = "admin"
Dim storedPassword As String = "SecurePass123"
Dim maxAttempts As Integer = 3
Dim authenticated As Boolean = False

For attempt As Integer = 1 To maxAttempts
    Console.Write("Username: ")
    Dim username As String = Console.ReadLine()
    Console.Write("Password: ")
    Dim password As String = Console.ReadLine()
    If username = storedUsername And password = storedPassword Then
        Console.WriteLine("Access granted. Welcome!")
        authenticated = True
        Exit For
    Else
        Console.WriteLine("Incorrect. " & (maxAttempts - attempt) & " attempts remaining.")
    End If
Next
If Not authenticated Then
    Console.WriteLine("Account locked. Too many failed attempts.")
End If

Password strength requirements

A secure authentication system should enforce strong passwords:

• Minimum length (e.g. 8 characters)
• Mix of character types — uppercase, lowercase, numbers, symbols
• No common words — dictionary words or “password123” should be rejected
• Limited attempts — lock the account after a number of failed attempts

# Python - Password strength checker
def check_password_strength(password):
    if len(password) < 8:
        return "Too short - must be at least 8 characters"
    has_upper = any(c.isupper() for c in password)
    has_lower = any(c.islower() for c in password)
    has_digit = any(c.isdigit() for c in password)
    has_symbol = any(not c.isalnum() for c in password)
    if not (has_upper and has_lower and has_digit and has_symbol):
        return "Must contain uppercase, lowercase, digit and symbol"
    return "Strong password"

password = input("Choose a password: ")
result = check_password_strength(password)
print(result)

' VB.NET - Password strength checker
Function CheckPasswordStrength(password As String) As String
    If Len(password) < 8 Then
        Return "Too short - must be at least 8 characters"
    End If
    Dim hasUpper As Boolean = False
    Dim hasLower As Boolean = False
    Dim hasDigit As Boolean = False
    Dim hasSymbol As Boolean = False
    For Each c As Char In password
        If Char.IsUpper(c) Then hasUpper = True
        If Char.IsLower(c) Then hasLower = True
        If Char.IsDigit(c) Then hasDigit = True
        If Not Char.IsLetterOrDigit(c) Then hasSymbol = True
    Next
    If Not (hasUpper And hasLower And hasDigit And hasSymbol) Then
        Return "Must contain uppercase, lowercase, digit and symbol"
    End If
    Return "Strong password"
End Function

Console.Write("Choose a password: ")
Dim password As String = Console.ReadLine()
Console.WriteLine(CheckPasswordStrength(password))

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Verification vs validation

It’s important to distinguish validation from verification:

	Validation	Verification
Purpose	Checks data is reasonable and within rules	Checks data has been entered or transferred correctly
When	Before data is processed	During or after data entry/transfer
How	Automated checks (range, type, length, etc.)	Human checks (proofreading) or system checks (double entry, checksums)
Example	Checking age is between 0-120	Asking user to type their email twice to confirm