What does encryption do to data?

It scrambles the data so it is meaningless without the key. Intercepted data cannot be understood.

What is data called after it has been encrypted?

ciphertext. Plaintext goes in, ciphertext comes out.

How many keys does symmetric encryption use?

one, used to encrypt and to decrypt. The same key does both jobs.

In asymmetric encryption, which key decrypts a message that was encrypted with a person's public key?

that person's private key. Only the matching private key works.

What is the main weakness of symmetric encryption?

The key has to be shared, so it could be intercepted. Anyone who gets the key can decrypt everything.

Encryption · Cambridge IGCSE Computer Science 0478 revision

In plain words

Data sent across a network can be intercepted. Encryption cannot stop that, but it makes sure that what is intercepted is meaningless: scrambled text that cannot be understood without the key.

There are two kinds. One uses a single key, like a house key that locks and unlocks. The other uses a pair of keys: one that anyone may use to lock a message, and one, kept secret, that alone can unlock it.

5 things to know

Encryption scrambles data so that it cannot be understood if it is intercepted. It does not stop the data being intercepted.
The original data is called plaintext. An encryption algorithm and a key turn it into ciphertext. The key is needed to turn it back.
Symmetric encryption uses the same key to encrypt and to decrypt. The sender and the receiver must both have it, so the key itself has to be shared, and it could be intercepted.
Asymmetric encryption uses two different keys: a public key, which anyone may have, and a private key, which only its owner has.
A message encrypted with someone's public key can only be decrypted with that person's private key. The private key is never sent, which makes this more secure.

Worked example

Ana wants to send a secret message to Ben using asymmetric encryption. Describe the steps.

Ben has a public key and a private key. He gives Ana his public key. It does not matter who else sees it.
Ana encrypts her message with Ben's public key and sends the ciphertext.
Only Ben's private key can decrypt it. Ben never shares that key, so only he can read the message.

Tips and tricks

Encryption does not prevent interception, hacking or deletion. It only makes the data meaningless to anyone without the key.
In asymmetric encryption, it is the receiver's public key that encrypts and the receiver's private key that decrypts.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Encryption: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What does encryption do to data?
- It stops the data being intercepted.
- It makes the file smaller.
- It scrambles the data so it is meaningless without the key. (the answer)
- It checks the data for errors.
Intercepted data cannot be understood.
What is data called after it has been encrypted?
- plaintext
- ciphertext (the answer)
- payload
- a checksum
Plaintext goes in, ciphertext comes out.
How many keys does symmetric encryption use?
- one, used to encrypt and to decrypt (the answer)
- two, one public and one private
- none
- a new one for every letter
The same key does both jobs.
In asymmetric encryption, which key decrypts a message that was encrypted with a person's public key?
- the same public key
- the sender's private key
- that person's private key (the answer)
- any key
Only the matching private key works.
What is the main weakness of symmetric encryption?
- It is too slow to use.
- The key has to be shared, so it could be intercepted. (the answer)
- It cannot encrypt text.
- It needs no key.
Anyone who gets the key can decrypt everything.

5 questions

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Worksheet

2 questions, 5 marks. Write your answers on paper, then check them.

Encryption

Cambridge IGCSE Computer Science 0478 · 5 marks · papermunch.org

Name ______________________________ Date ______________

Explain the purpose of encrypting data before it is transmitted.[2]
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So that if the data is intercepted it cannot be understood. It appears as meaningless ciphertext without the key.
I got this one
Describe the difference between symmetric and asymmetric encryption.[3]
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Symmetric encryption uses one key both to encrypt and to decrypt, and the key must be shared. Asymmetric encryption uses a public key to encrypt and a different, private key to decrypt, and the private key is never shared.
I got this one

Answers: Encryption

1. So that if the data is intercepted it cannot be understood. It appears as meaningless ciphertext without the key.
2. Symmetric encryption uses one key both to encrypt and to decrypt, and the key must be shared. Asymmetric encryption uses a public key to encrypt and a different, private key to decrypt, and the private key is never shared.