Learning Objectives
By the end of this lesson, you will be able to:
- Show understanding of how encryption works including use of public key, private key, plain text, cipher text
- Explain symmetric key cryptography and asymmetric key cryptography
- Describe how keys can be used to send private messages and verified messages
- Show understanding of quantum cryptography including its purpose, benefits and drawbacks
- Explain the purpose and use of SSL/TLS in client-server communication
- Show understanding of digital certificates and how they are acquired and used to produce digital signatures
- Identify situations where use of SSL/TLS would be appropriate
Key Terms
Encryption
The process of converting readable text (plaintext) into an unreadable form (ciphertext) to protect data during transmission.
Plaintext & Ciphertext
Plaintext: Original readable data. Ciphertext: Encrypted unreadable data.
Symmetric Encryption
Uses the same secret key for both encryption and decryption. Faster but has key distribution problems.
Asymmetric Encryption
Uses a pair of keys: public key (shared) and private key (secret). Solves key distribution problem.
Public Key & Private Key
Public Key: Available to everyone for encryption. Private Key: Kept secret by owner for decryption.
Quantum Cryptography
Uses principles of quantum mechanics (photons) to create virtually unbreakable encryption systems.
Digital Signature
A digital code that validates the authenticity and integrity of a digital document or message.
Digital Certificate
An electronic document issued by a Certificate Authority (CA) that verifies the identity of a person or website.
SSL/TLS
Secure Sockets Layer / Transport Layer Security - cryptographic protocols that provide secure communication over networks.
Hash Function
A one-way function that creates a unique fixed-size digest from input data, used in digital signatures.
1. Introduction to Encryption
When data is transmitted over any public network, there is a risk of it being intercepted by hackers (eavesdroppers). Encryption helps to minimise this risk by altering data into a form that is unreadable by anyone except the intended recipient.
How Encryption Protects Data
Security Concerns During Data Transmission
Confidentiality
Only the intended recipient should be able to read the data.
Achieved by: Encryption, Public/Private Keys
Authenticity
Verify who sent the data and that the source is legitimate.
Achieved by: Digital Signatures
Integrity
Data should reach its destination without any changes.
Achieved by: Digital Signature / Message Digest
Non-Repudiation
Neither sender nor recipient can deny their involvement in the transmission.
Achieved by: Digital Certificates
Real-life Example: Online Banking
When you log into your bank account online:
- Confidentiality: Your password and account details are encrypted so hackers can't read them
- Authenticity: The bank's website uses a digital certificate to prove it's really your bank (not a fake site)
- Integrity: When you transfer money, the transaction details can't be altered during transmission
- Non-repudiation: Both you and the bank have proof that the transaction occurred
Activity 1: Security Scenario Analysis
Read each scenario and identify which security concern is most relevant:
Answers: 1. Integrity, 2. Authenticity, 3. Confidentiality, 4. Non-repudiation
Activity 2: Simple Caesar Cipher
Encrypt the word "HELLO" using a Caesar cipher with shift of 3 (A→D, B→E, etc.):
Ciphertext: KHOOR
Check Your Understanding
1. What is the main purpose of encryption in data transmission? [2 marks]
Answer
- [1 mark] To convert readable data (plaintext) into unreadable form (ciphertext)
- [1 mark] To protect data from being understood by unauthorized parties if intercepted during transmission
2. Explain the difference between plaintext and ciphertext. [2 marks]
Answer
- [1 mark] Plaintext is the original, readable form of data before encryption
- [1 mark] Ciphertext is the encrypted, unreadable form of data after encryption
3. What is meant by 'integrity' in data security? [2 marks]
Answer
- [1 mark] Integrity means that data should reach its destination without any changes or alterations
- [1 mark] It ensures that the data received is exactly the same as the data that was sent
4. Give an example of a situation where non-repudiation is important. [2 marks]
Answer
- [2 marks] Online banking transactions - where neither the bank nor the customer should be able to deny that a transaction took place
- [Alternative] Legal contracts signed electronically - where both parties need proof of agreement
5. Why is encryption particularly important for sensitive data? [2 marks]
Answer
- [1 mark] Sensitive data (like passwords, financial information, medical records) could cause harm if accessed by unauthorized people
- [1 mark] Encryption ensures that even if data is intercepted, it cannot be understood without the decryption key
2. Symmetric Encryption
Symmetric encryption uses a single secret key for both encryption and decryption. Both the sender and receiver must have the same key. While fast and efficient, it has a key distribution problem - how to securely share the key without it being intercepted.
Symmetric Encryption Flow
Key Distribution Problem: How to securely share the secret key without it being intercepted?
Example: 10-Digit Symmetric Key
Using key 4 2 9 1 3 6 2 8 5 6 to encrypt "computer science is exciting":
| Key | C | O | M | P | U | T | E | R |
|---|---|---|---|---|---|---|---|---|
| Shift | 4 | 2 | 9 | 1 | 3 | 6 | 2 | 8 |
| Cipher | G | Q | V | Q | X | Z | G | Z |
Each letter is shifted by the corresponding digit in the key. C (+4) = G, O (+2) = Q, etc.
Real-life Example: Wi-Fi Encryption (WPA2)
When you connect to a password-protected Wi-Fi network:
- Your device and router use the same pre-shared key (your Wi-Fi password)
- All data between your device and router is encrypted with this key
- This is symmetric encryption - same key for encryption and decryption
- Problem: If someone steals your Wi-Fi password, they can decrypt all traffic
Activity 1: Key Distribution Problem
Alice wants to send Bob a secret message using symmetric encryption. They've never met before. How can Alice securely give Bob the secret key?
Best solutions: 2. Meet in person (most secure) or 4. Use asymmetric encryption first (practical for online communication).
Email could be intercepted, and posting publicly defeats the purpose of encryption.
Activity 2: Security Level Comparison
Compare the security level of these symmetric encryption scenarios:
AES-256 (bank-grade encryption)
Caesar cipher (shift by 3)
DES (56-bit key, outdated)
Check Your Understanding
1. What is the main characteristic of symmetric encryption? [2 marks]
Answer
- [1 mark] It uses the same secret key for both encryption and decryption
- [1 mark] Both sender and receiver must possess the same key
2. Explain the key distribution problem in symmetric encryption. [3 marks]
Answer
- [1 mark] Both sender and receiver need the same secret key
- [1 mark] The key must be shared securely before communication can begin
- [1 mark] If the key is intercepted during sharing, the encryption is compromised
3. Using the key "3 1 4 1 5 9", encrypt the word "HELLO". [3 marks]
Answer
- [1 mark each] H (+3) = K, E (+1) = F, L (+4) = P, L (+1) = M, O (+5) = T
- [Full answer] Ciphertext: KFPMT
4. Give one advantage and one disadvantage of symmetric encryption. [2 marks]
Answer
- [1 mark] Advantage: Faster than asymmetric encryption (uses simpler algorithms)
- [1 mark] Disadvantage: Key distribution problem - how to securely share the secret key
5. Why might symmetric encryption be suitable for encrypting files on your computer? [2 marks]
Answer
- [1 mark] Only one user needs access (no key distribution problem)
- [1 mark] It's faster than asymmetric encryption for large files
3. Asymmetric Encryption
Asymmetric encryption uses two different but mathematically related keys: a public key (shared with everyone) and a private key (kept secret by the owner). This solves the key distribution problem of symmetric encryption.
Asymmetric Encryption Flow
No key distribution problem: Public keys can be freely shared; only private keys must be kept secret.
Example: Tom Sending to Meera
- Tom and Meera both generate their own public-private key pairs
- Meera sends her public key to Tom (can be sent openly)
- Tom encrypts the document using Meera's public key
- Tom sends the encrypted document to Meera
- Meera decrypts it using her private key (only she has this)
Even if someone intercepts the encrypted document and Meera's public key, they cannot decrypt it without her private key.
| Public Key Features | Private Key Features |
|---|---|
| Known to everyone | Only known to the owner |
| Used to encrypt data for the owner | Used to decrypt data encrypted with matching public key |
| Can be distributed via digital certificates | Never shared with anyone |
| Anyone can use it to send encrypted messages | Used to create digital signatures (prove identity) |
Real-life Example: Secure Email (PGP/GPG)
When you send a secure email using PGP:
- You look up the recipient's public key from a key server
- Encrypt the email using their public key
- Send the encrypted email (only the recipient can decrypt it with their private key)
- You can also sign the email with your private key to prove it's from you
Activity 1: Key Pair Matching
Match each action with whether it uses a public key or private key:
Answers:
- Encrypt for Alice: Alice's Public Key
- Decrypt from Bob: Your Private Key
- Sign a document: Your Private Key
- Verify Charlie's signature: Charlie's Public Key
Activity 2: Symmetric vs Asymmetric Comparison
Complete the comparison table:
| Feature | Symmetric | Asymmetric |
|---|---|---|
| Number of keys | 1 | 2 |
| Key distribution | Problematic | Easy (public keys) |
| Speed | Fast | Slow |
| Best for | Large data/files | Key exchange/signing |
Check Your Understanding
1. What are the two keys used in asymmetric encryption and how are they different? [3 marks]
Answer
- [1 mark] Public key: Shared with everyone, used to encrypt messages for the owner
- [1 mark] Private key: Kept secret by the owner, used to decrypt messages
- [1 mark] They are mathematically related but one cannot be derived from the other
2. How does asymmetric encryption solve the key distribution problem? [3 marks]
Answer
- [1 mark] Public keys can be freely shared without security risk
- [1 mark] Only private keys need to be kept secret, and they never need to be shared
- [1 mark] Anyone can encrypt using the public key, but only the private key owner can decrypt
3. If Alice wants to send a private message to Bob using asymmetric encryption, whose key does she use to encrypt? [2 marks]
Answer
- [1 mark] Alice uses Bob's public key to encrypt the message
- [1 mark] Only Bob can decrypt it using his private key
4. What is one advantage and one disadvantage of asymmetric encryption compared to symmetric? [2 marks]
Answer
- [1 mark] Advantage: Solves key distribution problem (public keys can be shared openly)
- [1 mark] Disadvantage: Slower than symmetric encryption due to complex mathematics
5. Why can't someone decrypt a message encrypted with your public key, even if they have your public key? [2 marks]
Answer
- [1 mark] Asymmetric encryption uses mathematically related but different keys for encryption and decryption
- [1 mark] The private key (needed for decryption) cannot be derived from the public key
4. Quantum Cryptography
Quantum cryptography uses principles of quantum mechanics (especially properties of photons) to create virtually unbreakable encryption. Unlike classical cryptography based on mathematical algorithms, quantum cryptography leverages the behavior of quantum particles.
Quantum Key Distribution (QKD)
Four polarizations represent bits: Vertical (↑) = 1, Horizontal (↔) = 0, Diagonal right (↗) = 1, Diagonal left (↖) = 0
Quantum advantage: Any attempt to measure a photon changes its state, immediately revealing eavesdropping.
Quantum Key Distribution Process
- Sender generates photons with random polarizations representing random bits
- Photons travel through fiber optic cable to receiver
- Receiver uses random beam splitters to measure photons
- Receiver tells sender which beam splitters were used
- Sender reveals which measurements were correct
- Both keep only the correctly measured bits as their shared secret key
- Any eavesdropping attempt changes photon states, revealing intrusion
| Advantages of Quantum Cryptography | Drawbacks of Quantum Cryptography |
|---|---|
| Virtually unbreakable (based on physics, not mathematics) | Requires dedicated fiber optic lines |
| Detects eavesdropping (any measurement changes photons) | Expensive specialist hardware required |
| Allows longer/more secure keys to be exchanged | Limited range (~250 km currently) |
| Future-proof against quantum computers | Polarization can be altered during transmission |
| Guarantees key integrity (cannot be copied undetected) | Can be used by criminals to hide activities |
Real-life Example: Government & Military Communication
Quantum cryptography is primarily used for:
- Securing communications between government agencies
- Protecting military command and control systems
- Securing financial transactions between central banks
- Protecting critical infrastructure (power grids, water systems)
It's not used for everyday communication due to cost and infrastructure requirements.
Activity 1: Eavesdropping Detection
In quantum cryptography, why does eavesdropping change the photons?
Correct answer: Quantum mechanics says measuring a photon changes its state (Heisenberg's uncertainty principle).
This is a fundamental principle of quantum physics that makes quantum cryptography uniquely secure.
Activity 2: Technology Timeline
Place these encryption technologies in order from oldest to newest:
Correct order:
- Caesar Cipher (Ancient Rome)
- DES (1970s)
- RSA (1977)
- Quantum Cryptography (1980s-present)
Check Your Understanding
1. What is the main difference between quantum cryptography and classical cryptography? [2 marks]
Answer
- [1 mark] Classical cryptography relies on mathematical algorithms
- [1 mark] Quantum cryptography uses principles of quantum mechanics (properties of photons)
2. How does quantum cryptography detect eavesdropping? [3 marks]
Answer
- [1 mark] It uses photons to transmit encryption keys
- [1 mark] According to quantum mechanics, measuring a photon changes its state
- [1 mark] Any eavesdropping attempt alters the photons, which can be detected by sender and receiver
3. List two drawbacks of quantum cryptography. [2 marks]
Answer
- [1 mark each] Any two from:
- Requires dedicated fiber optic lines
- Expensive specialist hardware
- Limited range (~250 km)
- Polarization can be altered during transmission
- Can be used by criminals
4. What is Quantum Key Distribution (QKD)? [2 marks]
Answer
- [1 mark] A method of exchanging encryption keys using quantum particles (photons)
- [1 mark] Provides a secure way to share keys that detects any eavesdropping attempts
5. Why is quantum cryptography considered "future-proof"? [2 marks]
Answer
- [1 mark] It's based on laws of physics, not mathematical problems that could be solved by future computers
- [1 mark] It remains secure even against quantum computers that could break classical encryption
5. Digital Signatures & Certificates
Digital signatures validate the authenticity and integrity of digital documents. Digital certificates are electronic documents issued by trusted Certificate Authorities (CAs) that verify the identity of a person or website.
How Digital Signatures Work
Key point: Only the sender's private key could create that signature, and any change to the message changes the hash.
How to Get a Digital Certificate
- Person generates a public-private key pair on their computer
- Person applies to Certificate Authority (CA) with their public key and identity proof
- CA verifies the person's identity (name, address, organization)
- CA creates a digital certificate containing the person's public key and signs it with CA's private key
- CA issues the certificate to the person
- Person can now share their certificate; anyone can verify it using CA's public key
Data Items in a Digital Certificate
- Version number
- Certificate serial number
- Name of issuing CA
- Subject name (owner)
- Subject's public key
- Hashing algorithm used
- Validity period (start/expiry dates)
- Digital signature of CA
- Extensions (optional)
Real-life Example: HTTPS Websites
When you visit a secure website (https://):
- The website sends its digital certificate to your browser
- Your browser checks if the certificate was issued by a trusted CA
- It verifies the certificate's digital signature using the CA's public key
- If valid, your browser shows a padlock icon ✅
- All communication is then encrypted using keys from the certificate
This proves you're really connected to your bank (not a fake site) and ensures encrypted communication.
Activity 1: Hash Function Demonstration
See how changing one character completely changes the hash:
Activity 2: Signature vs Certificate
Identify whether each statement describes a digital signature or digital certificate:
Answers:
- Certificate: Obtained from a Certificate Authority, Contains the owner's public key
- Signature: Created for each individual message, Uses the sender's private key for creation
Check Your Understanding
1. What is the purpose of a digital signature? [2 marks]
Answer
- [1 mark] To validate the authenticity of a digital document (prove who sent it)
- [1 mark] To ensure the integrity of the document (prove it hasn't been altered)
2. Describe the steps to create a digital signature. [4 marks]
Answer
- [1 mark] Apply a hash function to the message to create a digest
- [1 mark] Encrypt the digest using the sender's private key
- [1 mark] This encrypted digest is the digital signature
- [1 mark] Send the message along with the digital signature
3. What is a Certificate Authority (CA) and what role does it play? [3 marks]
Answer
- [1 mark] A trusted third-party organization that issues digital certificates
- [1 mark] Verifies the identity of individuals or organizations requesting certificates
- [1 mark] Signs certificates with its own private key, allowing anyone to verify them using the CA's public key
4. Why is a digital certificate necessary if we have digital signatures? [2 marks]
Answer
- [1 mark] Digital certificates provide a trusted way to distribute public keys
- [1 mark] Without certificates, someone could create a fake public key pretending to be someone else
5. What happens if a message is altered after being digitally signed? [2 marks]
Answer
- [1 mark] The hash of the altered message will be different from the original hash
- [1 mark] When the recipient verifies the signature, the hashes won't match, revealing the alteration
6. SSL/TLS Protocols
SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are cryptographic protocols that provide secure communication over networks. They ensure encryption, authentication, and data integrity between clients and servers, most commonly seen as HTTPS in web browsers.
SSL/TLS Handshake Process
You see https:// and a padlock icon when this process succeeds
| SSL (Secure Sockets Layer) | TLS (Transport Layer Security) |
|---|---|
| Older protocol (1990s) | Newer, more secure version (successor to SSL) |
| All versions now considered insecure | Current standard for secure web communication |
| Fixed protocol design | Can be extended with new authentication methods |
| Less efficient handshake process | Uses session caching for better performance |
| Combined handshake and record layers | Separates handshake protocol from record protocol |
Purpose of TLS Protocol
- Provide secure communication over networks/internet
- Maintain data integrity (prevent alteration during transmission)
- Provide authentication of servers (and optionally clients)
- Protect against eavesdropping and man-in-the-middle attacks
- Enable encryption of sensitive data (passwords, credit cards)
Real-life Example: Online Shopping
When you buy something online:
- You go to https://www.store.com (not http://)
- Your browser performs TLS handshake with the store's server
- You see a padlock icon ✅ next to the URL
- You enter your credit card details - they're encrypted before sending
- Even if intercepted, the data is unreadable without the session key
- The certificate proves you're really on the store's site (not a phishing site)
| Application | Why SSL/TLS is Used |
|---|---|
| Online banking | Protects account details, transactions |
| Email (IMAP/SMTP over SSL) | Encrypts email content and passwords |
| VPN connections | Secures entire network connection |
| Cloud storage | Protects files during upload/download |
| Social media login | Encrypts passwords and private messages |
| API communications | Secures data exchange between applications |
Activity 1: Browser Security Check
Visit these websites (or imagine visiting) and identify which use SSL/TLS:
Using SSL/TLS: https://onlinebanking.bank.com, https://docs.google.com
http:// URLs don't use SSL/TLS. Localhost often doesn't have certificates installed.
Activity 2: Security Requirement Matching
Match each scenario with the most important security feature:
Answers:
- Online exam: Integrity (ensuring answers aren't changed)
- Medical results: Confidentiality (private health information)
- Digital contract: Non-repudiation (can't deny signing)
- Bank login: All of the above (need all security features)
Check Your Understanding
1. What is the main purpose of SSL/TLS? [2 marks]
Answer
- [1 mark] To provide secure communication over networks/internet
- [1 mark] To ensure encryption, authentication, and data integrity
2. How can a user tell if a website is using SSL/TLS? [2 marks]
Answer
- [1 mark] The URL begins with https:// instead of http://
- [1 mark] A padlock icon appears in the browser's address bar
3. What is the difference between SSL and TLS? [3 marks]
Answer
- [1 mark] TLS is the newer, more secure successor to SSL
- [1 mark] All SSL versions are now considered insecure and deprecated
- [1 mark] TLS has improvements like session caching and separable protocol layers
4. Why is the digital certificate important in the SSL/TLS handshake? [3 marks]
Answer
- [1 mark] It authenticates the website (proves it's legitimate, not a fake site)
- [1 mark] It contains the website's public key for encryption
- [1 mark] It's issued by a trusted CA, allowing the browser to verify it
5. Give three examples of situations where SSL/TLS should be used. [3 marks]
Answer
- [1 mark each] Any three from:
- Online banking and financial transactions
- E-commerce and online shopping
- Email communication
- Social media logins
- Cloud storage access
- VPN connections
- Corporate intranets/extranets
Key Takeaways
- Encryption converts plaintext to ciphertext to protect data confidentiality during transmission over public networks
- Symmetric encryption uses one shared secret key for both encryption and decryption, making it fast but creating a key distribution problem
- Asymmetric encryption uses public/private key pairs - public keys can be freely shared while private keys remain secret, solving the key distribution problem
- Quantum cryptography uses quantum mechanics (photons) to create virtually unbreakable encryption that detects any eavesdropping attempts
- Digital signatures prove authenticity and integrity - created by encrypting a message hash with the sender's private key
- Digital certificates are issued by trusted Certificate Authorities and verify the identity of individuals or websites while distributing public keys securely
- SSL/TLS protocols provide secure web communication through encryption, authentication, and data integrity protection (seen as https://)
- Security involves four key concerns: Confidentiality (only intended recipient can read), Authenticity (verify sender), Integrity (data unchanged), and Non-repudiation (cannot deny involvement)
- Classical vs quantum cryptography: Classical relies on mathematical algorithms; quantum uses physics principles and is future-proof against quantum computers
- In practice, systems often combine methods: Asymmetric for key exchange and signatures, symmetric for bulk data encryption
- Always look for https:// and padlock icons when transmitting sensitive information online to ensure SSL/TLS protection
Question Bank
1. Explain the difference between symmetric and asymmetric encryption, including one advantage of each. [6 marks]
Marking Scheme & Answer
Symmetric Encryption:
- [1 mark] Uses the same key for encryption and decryption
- [1 mark] Both sender and receiver must have the same secret key
- [1 mark] Advantage: Faster than asymmetric encryption
Asymmetric Encryption:
- [1 mark] Uses a pair of keys: public and private
- [1 mark] Public key encrypts, private key decrypts (or vice versa)
- [1 mark] Advantage: Solves key distribution problem (public keys can be shared openly)
2. Describe how a digital signature is created and verified. [6 marks]
Marking Scheme & Answer
Creation:
- [1 mark] Apply hash function to message to create digest
- [1 mark] Encrypt digest with sender's private key
- [1 mark] This encrypted digest is the digital signature
Verification:
- [1 mark] Decrypt signature with sender's public key to get original digest
- [1 mark] Apply same hash function to received message to create new digest
- [1 mark] Compare two digests - if identical, signature is valid
3. Explain the purpose of quantum cryptography and how it detects eavesdropping. [5 marks]
Marking Scheme & Answer
- [1 mark] Purpose: To create virtually unbreakable encryption using quantum mechanics
- [1 mark] Uses photons with specific polarizations to represent bits
- [1 mark] Based on Heisenberg's uncertainty principle: measuring a quantum particle changes its state
- [1 mark] Any eavesdropping attempt requires measuring the photons
- [1 mark] This measurement alters the photon states, which can be detected by sender and receiver
4. What is a digital certificate and why is it necessary in public key distribution? [4 marks]
Marking Scheme & Answer
- [1 mark] A digital certificate is an electronic document issued by a Certificate Authority (CA)
- [1 mark] It contains the owner's public key and identity information
- [1 mark] It's digitally signed by the CA to verify its authenticity
- [1 mark] Necessary because without certificates, someone could create a fake public key pretending to be someone else
- [Additional] Provides a trusted way to distribute public keys and verify identities
5. Describe the SSL/TLS handshake process between a web browser and server. [6 marks]
Marking Scheme & Answer
- [1 mark] Client (browser) sends "Client Hello" to server
- [1 mark] Server responds with "Server Hello" and sends its digital certificate
- [1 mark] Browser verifies certificate with Certificate Authority
- [1 mark] Browser generates a session key (symmetric key)
- [1 mark] Browser encrypts session key with server's public key (from certificate) and sends it
- [1 mark] Server decrypts session key with its private key, secure session established
6. Compare classical cryptography with quantum cryptography. [5 marks]
Marking Scheme & Answer
Classical Cryptography:
- Based on mathematical algorithms
- Security depends on computational difficulty
- Can be broken by powerful computers/quantum computers
- Uses traditional communication channels
- Doesn't detect eavesdropping
Quantum Cryptography:
- Based on quantum mechanics (physics)
- Security based on laws of physics
- Virtually unbreakable, even by quantum computers
- Requires dedicated fiber optic lines
- Detects any eavesdropping attempts
7. Explain the four security concerns in data transmission with examples. [8 marks]
Marking Scheme & Answer
Confidentiality:
- [1 mark] Only intended recipient can read the data
- [1 mark] Example: Encrypted email that only recipient can decrypt
Authenticity:
- [1 mark] Verify who sent the data
- [1 mark] Example: Digital signature proving email came from claimed sender
Integrity:
- [1 mark] Data reaches destination unchanged
- [1 mark] Example: Bank transfer amount cannot be altered in transit
Non-repudiation:
- [1 mark] Neither party can deny involvement
- [1 mark] Example: Digitally signed contract that both parties must acknowledge
8. Why is asymmetric encryption slower than symmetric encryption, and how do systems overcome this? [4 marks]
Marking Scheme & Answer
- [1 mark] Asymmetric encryption uses complex mathematical operations (like prime factorization)
- [1 mark] Symmetric encryption uses simpler operations (like bit shifting or XOR)
- [1 mark] Systems overcome this by using hybrid approach
- [1 mark] Asymmetric encryption establishes a session key, then symmetric encryption encrypts the actual data
- [Additional] Example: SSL/TLS uses asymmetric for handshake, symmetric for data transfer
9. What are the main drawbacks of quantum cryptography that limit its widespread use? [4 marks]
Marking Scheme & Answer
- [1 mark] Requires dedicated fiber optic lines (cannot use existing internet infrastructure)
- [1 mark] Expensive specialist hardware needed
- [1 mark] Limited range (currently about 250 km maximum)
- [1 mark] Photon polarization can be altered during transmission, causing errors
- [Additional] Primarily used for military/government due to cost, not everyday communication
10. Describe a real-world scenario where SSL/TLS would be essential and explain what would happen if it weren't used. [5 marks]
Marking Scheme & Answer
- [1 mark] Scenario: Online banking login and transactions
- [1 mark] SSL/TLS provides: Encryption of account details/passwords, Authentication of bank website, Data integrity for transaction amounts
- [1 mark] Without SSL/TLS: Data sent in plaintext over internet
- [1 mark] Consequences: Hackers could intercept passwords and account details
- [1 mark] Additional risk: Users could be directed to fake banking sites (phishing)
- [Additional] Real example: Man-in-the-middle attacks could steal funds or identity information