Encryption (Cambridge (CIE) A Level Computer Science): Revision Note
Exam code: 9618
How encryption works
What is encryption?
Encryption is a method of scrambling data before being transmitted across a network
Encryption helps to protect the contents from unauthorised access by making data meaningless
While encryption is important on both wired and wireless networks, it's even more critical on wireless networks due to the data being transmitted over radio waves, making it easy to intercept
How is wireless data encrypted?
Wireless networks are identified by a 'Service Set Identifier' (SSID) which along with a password is used to create a 'master key'
When devices connect to the same wireless network using the SSID and password they are given a copy of the master key
The master key is used to encrypt data into 'cipher text', before being transmitted
The receiver uses the same master key to decrypt the cipher text back to 'plain text'
To guarantee the security of data, the master key is never transmitted. Without it, any intercepted data is rendered useless
Wireless networks use dedicated protocols like WPA2 specifically designed for Wi-Fi security
How is wired data encrypted?
Wired networks are encrypted in a very similar way to a wireless network, using a master key to encrypt data and the same key to decrypt data
Encryption on a wired network differs slightly as it is often left to individual applications to decide how encryption is used, for example HTTPS
Symmetric & asymmetric encryption
How does symmetric encryption work?
The sender uses a key to encrypt the data before transmission
The receiver uses the same key to decrypt the data
It's usually faster, making it ideal for encrypting large amounts of data
The significant downside is the challenge of securely sharing this key between the sender and receiver
If a bad actor captures the key, they can decrypt all messages intercepted in transmission
Structure of Symmetric Encryption
How does asymmetric encryption work?
Asymmetric encryption uses two keys:
a public key for encryption
and a private key for decryption
Receivers openly share their public key
Senders use this public key to encrypt the data
The receiver's private key is the only key that can decrypt the data and is kept locally on their side
The public and private keys are created at the same time and are designed to work together in this way
It is typically slower than symmetric encryption
It is generally used for more secure and smaller data transactions, e.g. passwords, bank details
Structure of Asymmetric Encryption
Choosing an encryption type
Symmetric encryption is fast but has key-sharing issues; asymmetric is slower but solves these issues.
The choice should be made based on the situation's needs: whether speed or security is more critical.
Encryption Type | Suitable For | Reasons to choose |
|---|---|---|
Symmetric | Large files, databases |
|
Asymmetric | Confidential/secret communications |
|
Quantum cryptography
What is quantum cryptograhy?
Quantum cryptography uses quantum mechanics to securely transmit encryption keys
Its main goal is to enable unbreakable communication by detecting any attempt to intercept or tamper with the key
The most well-known method is:
Quantum Key Distribution (QKD) – uses quantum particles (like photons) to share a secret key between two parties securely
Benefits of quantum cryptography
Benefit | Explanation |
|---|---|
Unbreakable key transmission | Uses quantum physics – measuring a quantum state disturbs it, so eavesdropping is detectable |
Eavesdropper detection | Any interception changes the quantum state of the key, alerting the users |
Perfect forward secrecy | Keys are used once and then discarded, reducing the impact of future key leaks |
Stronger than classical encryption | Not based on mathematical problems like factoring large primes, so it's not vulnerable to advances in computing (e.g. quantum computers) |
Drawbacks of quantum cryptography
Drawback | Explanation |
|---|---|
Expensive and complex | Requires advanced technology, including specialised hardware like photon detectors and fibre-optic channels |
Short distance limits | Works best over short ranges (limited by current fibre-optic and signal loss issues) |
Slow transmission speed | QKD is typically slower than traditional key exchange methods |
Still evolving | Technology is new and not yet widely available or standardised |
Only secures key exchange | Quantum cryptography secures the key, but not the actual data encryption itself – traditional algorithms still needed |
Examiner Tips and Tricks
Quantum cryptography isn't about encrypting the message – it’s about securing the key used in encryption (e.g. with QKD). Always mention this to show clear understanding.
Worked Example
Encryption is used to alter data into a form that makes it meaningless if intercepted.
Describe the purpose of asymmetric key cryptography.
Answer
To provide better security [1 mark]
… by using two different keys / a public key and a private key [1 mark]
One of the keys is used to encrypt the message [1 mark]
… the matching key is used to decrypt the message [1 mark]
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