Data Encryption

Encryption is the conversion of electronic data into another form, called cipher text, which cannot be easily understood by anyone except authorized parties.
The word encryption comes from the Greek word kryptos, meaning hidden or secret. SSL (Secure Sockets Layer) is standard security technologies for establishing an encrypted links between a server and a client—typically a web server (website) and a browser; or a mail server and a mail client (e.g., Outlook). It allows sensitive information such as credit card numbers, social security numbers, and login credentials to be transmitted securely. To establish this secure connection, the browser and the server need an SSL Certificate.
Symmetric Encryption
Symmetric encryption (or pre-shared key encryption) uses a single key to both encrypt and decrypt data. Both the sender and the receiver need the same key to communicate.

Specimen Paper 2015
Q1) In a simple symmetric encryption system, each letter of the alphabet is substituted with another.
The plain text message:
The quick brown fox jumps over the lazy dog.
becomes the cypher text message:
Zag towns jumpy dmh coilp mngu zag bfke qmx.
(a) (i) Decode this cypher text message.
Agbbm Pmubq

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(ii) Convert these words to cypher text.
Computer Science
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 (b) Both the person who sends the message and the person who receives it need to
know what the substitution key is, and they need to keep this secret. A copy of the
substitution key has been sent using SSL transmission.
Explain why this keeps the copy of the key secret during transmission.
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Q 37) Summer 2014. P12

16 An encryption system gives each letter of the alphabet a value:

A = 1, B = 2, C = 3, ……… , Y = 25, Z = 26.

Each letter is stored in a 12-bit binary register. The letter “S” (19th letter) is stored as:

 

A 4-bit register is used to store the encryption key. This register shows how many places the bits are shifted to the left in the 12-bit register when it is encrypted. So,

means each bit in the 12-bit register is shifted 5 places to the left and the register now becomes:

Therefore, the letter “S” would be transmitted with the 4-bit register and the 12-bit register as follows:

(a) “W” is the 23rd letter of the alphabet.

(i) Show how this letter would be stored in the 12-bit register before encryption:

(ii) The 4-bit register contains the following value:

Show how the letter “W” is now stored in the 12-bit register in encrypted form:

(b) Find which letter of the alphabet has been encrypted here. (Show all your working.)

(c) (i) What is the largest encryption key that can be stored in the 4-bit register?

(ii) Convert this into denary (base 10).

(iii) If this encryption key were used, what problem would it cause?                         [3]

(b) What is meant by encryption?
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(c) (i) A student wrote “I would make backup copies of my data to guard against viruses”. Why is the student’s statement not necessarily true?
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 (ii) The same student also wrote “Encryption would stop a hacker accessing the data
in my computer files”. Why is the student’s statement incorrect?
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Summer 2010

Q4)  A company has set up an Internet website to sell their electrical goods online.
(a) Give two features you would expect to see on the website.
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 (b) Payments for goods can be made by credit/debit cards. Data from the cards is
encrypted.
(i) What is encryption?
(ii) Why is data encrypted? .............................................................................................................................. [2]

 (c) Apart from credit card fraud, people have other fears about buying from the Internet. Describe one of these fears. ............................................................................................................................ [2]

Q5) The student is interested in how simple encryption could be applied to a text message. One of the simplest forms of encryption is a method of ‘substitution’ where each character has a unique substitute character.
The student uses this method with the following character substitutions:

Message character

A

B

C

D

E

F

G

H

I

J

K

L

M

Substitute character

P

L

F

N

O

C

Q

U

D

Z

V

G

I

Message character

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

Substitute character

X

M

W

J

B

K

E

A

H

S

Y

R

T

Assume all messages are made up from the upper-case characters only.
Show the string after the message ATSEVEN is encrypted. ........................................................................................................................................................................................................................................................................ [2]

Q6) The encryption of data is widely used in computing.
(a) One application is online banking.
State two other applications where encryption is used.
Describe the reason for encrypting the data for each application.
Application 1 ................................................................................................................
Reason.......................................................................................................................
Application 2................................................................................................................
Reason ..................................................................................................................... [4]
(b) Explain the terms plain text and cipher text.
Plain text .......................................................................................................................
Cipher text ................................................................................................................[2]
(c) Symmetric encryption uses a single key.
Explain how a message is encrypted and decrypted using symmetric encryption. ........................................................................................................................................................................................................................................................................................... [3]

(d) Authorisation and authentication are processes designed to protect the computer system and data.Give one technique used for each.
Authorisation .................................................................................................................
Authentication ......................................................................................................... [2]

Winter 2014 P13

Data sent across the Internet are frequently encrypted. The following flowchart shows a basic encryption method (Note: the → symbol in the flowchart means “is replaced by”).

(c) Many customers shop online. Apart from encryption, describe three other security features built into many online shopping websites.
1 ................................................................................................................................................
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2 ................................................................................................................................................
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3 ................................................................................................................................................
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