4.9.3 The Internet

Question 1

What is encryption?

Answer 1

The process of scrambling data so that it cannot be understood if intercepted.

Question 2

What occurs in symmetric encryption?

Answer 2

• The sender and receiver share the same private key.

* This key is used to both encrypt and decrypt data sent between the two parties.

Question 3

What occurs before data transmission in symmetric encryption?

Answer 3

• The sender and receiver must participate in a key exchange to ensure that they both have a copy of their shared key.

Question 4

What is a flaw in symmetric encryption?

Answer 4

• If the key is shared over a network, the key is vulnerable to interception.

Question 5

What is asymmetric encryption?

Answer 5

• When two devices communicate using asymmetric encryption, four different keys are used.
• Each device has a pair of mathematically related keys, one of which is kept secret ( private key ) and the other shared on the internet ( public key )

Question 6

What happens when a message is encrypted with a public key?

Answer 6

Only the corresponding private key can decrypt it and vice versa.

Question 7

What happens in asymmetric encryption before a message is sent?

Answer 7

• Before a message is sent it is encrypted by the sender using the recipient’s public key.
• This means that the message can only be decrypted by the corresponding private key: the recipient’s private key which only the recipient has access to.
• This means the recipient is the only one who can decrypt the message.

Question 8

What is the purpose of digital signatures?

Answer 8

A digital signature can be used to verify the sender of a message and to verify that the message has not been tampered with during transmission.

Question 9

How is the digest created?

Answer 9

The digest of a message is created by a hashing or checksum algorithm.
The value of the digest depends on the contents of the message and will not be the same if the message is changed.

Question 10

What is the value of the digest dependent on?

Answer 10

The contents of the message.

Question 11

How is the digest encrypted?

Answer 11

The digest is encrypted with the sender’s private key which anyone can decrypt using the sender’s public key.

Question 12

How are the message and appended digest encrypted?

Answer 12

With the recipient’s public key, meaning only the recipient can decrypt the information.

Question 13

What happens when the recipient receives the message via asymmetric encryption?

Answer 13

They first decrypt it using their private key leaving them with the decrypted message and an encrypted digest.

Question 14

What happens when the recipient decrypts the message using their private key?

Answer 14

leaves them with:
• Decrypted message
• Encrypted Digest

Question 15

How is the message verified as having been sent by the sender?

Answer 15

• As the digest is encrypted using the sender’s private key.
• It can only be decrypted using the sender’s public key.
This verifies that the message was really sent by the sender as only they have access to their private key.

Question 16

How is it checked that the message was not tampered with?

Answer 16

• The recipient carries out the same hashing or checksum algorithm on the message and checks whether their results matches the now decrypted digest.
• If everything matches the recipient can be certain that the message was sent by the sender and hasn’t been tampered with or corrupted during transmission.

Question 17

What are the stages to asymmetric encryption?

Answer 17

1. A digest of the message is
   created, perhaps by a
   hashing or checksum
   algorithm. The value of the
   digest depends on the
   content of the message and
   will not be the same if the
   message is changed.
2. This digest is encrypted with the sender’s private key
   (which anyone can decrypt with the sender’s public key)
3. The encrypted digest is appended to the message
4. The message and appended digest are encrypted
   with the recipient’s public key, meaning that only the
   recipient can decrypt the information.

Question 18

What is the purpose of a digital certificate?

Answer 18

• A digital certificate verifies the ownership of a key pair used in asymmetric encryption.
• Check that a fake key pair isn’t being used by an impostor.

Question 19

Who issues digital certificates?

Answer 19

Certificate authorities

Question 20

What do digital certificates contain?

Answer 20

• Serial number
• Owner’s name
• Expiry date
• Owner’s public key
• Certificate Authority’s digital signature