Learning Objectives
- Explain why transmitted data may need encryption.
- Describe plaintext, ciphertext and the role of a key.
- Explain symmetric encryption using one shared key.
- Explain asymmetric encryption using public and private keys.
- Compare the key-management characteristics of the two approaches.
Key Terms
- Encryption
- Converting readable data into an unreadable form using an algorithm and key.
- Plaintext
- The original readable data before encryption.
- Ciphertext
- The scrambled encrypted data.
- Decryption
- Converting ciphertext back into plaintext using the required key.
- Key
- A value used by an encryption or decryption process.
- Symmetric encryption
- Encryption in which the same secret key is used to encrypt and decrypt.
- Asymmetric encryption
- Encryption using a related public-key and private-key pair.
- Public key
- A key that can be distributed openly.
- Private key
- A key kept secret by its owner.
Why Encryption Is Needed
Data transmitted through a network may pass through communication links and devices that are not under the sender’s direct control. An unauthorised person could intercept the transmitted bits. If the data remains readable, confidential information may be exposed.
Encryption converts plaintext into ciphertext before transmission. The ciphertext should not be understandable without the required key. The intended receiver decrypts it to recover the plaintext.
Encryption protects confidentiality. It does not by itself guarantee that data arrived without error, so error-detection methods may still be required. It also does not stop an attacker from deleting or blocking the transmitted ciphertext.
Symmetric Encryption
Symmetric encryption uses the same secret key for encryption and decryption. The sender applies the key to the plaintext, producing ciphertext. The receiver applies the same key to recover the plaintext.
The method can be efficient because both sides use one shared secret. Its central problem is key distribution. The sender and receiver must obtain the key securely. If the key is intercepted while being shared, an unauthorised person can decrypt messages protected by it.
Every authorised participant who uses the shared key must keep it secret. If many separate pairs of users need private communication, managing different shared keys can become difficult. The syllabus requires the principle rather than names of encryption algorithms.
Asymmetric Encryption
Asymmetric encryption uses two related keys: a public key and a private key. The public key can be made available to other people. The private key is kept secret by its owner.
To send confidential data to a receiver, the sender encrypts the data using the receiver’s public key. The resulting ciphertext can be decrypted using the receiver’s corresponding private key. Publishing the public key does not mean publishing the private key.
This arrangement reduces the need to transmit one shared secret key to every sender. The private key remains with its owner. Asymmetric processes are generally more computationally demanding than symmetric processes, but exact performance comparisons and algorithm names are not required by this syllabus.
Comparing And Applying The Methods
Symmetric encryption has one shared secret key. Both sides need that key before secure communication. Asymmetric encryption separates the publicly distributed encryption key from the secret private key used by the owner.
In an examination scenario, identify who owns the key pair. If A wants to send confidential data to B, A uses B’s public key and B uses B’s private key to decrypt. Using A’s public key would not make B the unique holder of the required private key.
Encryption should be described as making intercepted data unreadable without the key, not as making interception impossible. The ciphertext can still be captured; its meaning is protected.
Symmetric And Asymmetric Encryption
| Feature | Symmetric | Asymmetric |
|---|---|---|
| Keys | One shared secret key | Public and private key pair |
| Encryption | Uses the shared key | For confidentiality, uses receiver’s public key |
| Decryption | Uses the same shared key | Uses receiver’s private key |
| Key distribution | Secret key must be shared securely | Public key can be distributed openly |
| Main issue | Shared key exposure | More complex processing and correct key ownership |
Confidential Asymmetric Transmission
| Stage | Action |
|---|---|
| 1 | Receiver creates or owns public and private keys. |
| 2 | Receiver makes the public key available. |
| 3 | Sender encrypts plaintext using receiver’s public key. |
| 4 | Ciphertext is transmitted. |
| 5 | Receiver decrypts using receiver’s private key. |
Worked Examples
Symmetric Key Use
Question: A and B share secret key K. Describe a confidential transmission from A to B.
- A encrypts plaintext using K.
- A sends the ciphertext.
- B applies the same K to decrypt.
Answer: The same shared key K is used at both ends.
Choosing The Public Key
Question: Sara wants to send confidential data to Hamid using asymmetric encryption. Which public key does she use?
- The intended receiver is Hamid.
- The ciphertext must require Hamid’s private key for decryption.
- Therefore the matching public key is Hamid’s.
Answer: Hamid’s public key.
Explaining Interception
Question: An attacker captures encrypted data. Why may the information remain confidential?
- The captured form is ciphertext.
- The attacker does not possess the required secret or private key.
- The original plaintext cannot be readily recovered.
Answer: Encryption protects the meaning even though the transmission can be intercepted.
Examination Guidance
- Use plaintext, ciphertext, encryption, decryption and key accurately.
- For symmetric encryption, state that the same secret key is used at both ends.
- For asymmetric confidentiality, use the receiver’s public key and receiver’s private key.
- Do not say encryption prevents interception.
Common Mistakes
- Saying the public key must be kept secret.
- Using the sender’s public key when explaining confidential transmission to the receiver.
- Claiming encryption detects all transmission errors.
- Confusing compression with encryption.
Knowledge Check
1. What is ciphertext?
2. What key arrangement does symmetric encryption use?
3. Which asymmetric key may be distributed openly?
4. Which key decrypts data encrypted for a receiver using that receiver’s public key?
5. What security property is the main purpose of encryption here?
