Computer science: Encription

Computer Security

Communications, Networking & Security

Cryptography & Alan Turing

LO: Define cryptography, cipher, encryption, symmetric algorhthm
LO: understand the function of a block cipher
LO: types of encryption: caesar cipher
LO: importance of Alan Turing to computer science.

Well established needs for secure communication

Business transactions

War time communication

Protection of Trade Secrets or Secret Formulas

Requirements of secure communication
1. Secrecy: Only intended receiver understands the message

2. Authentication : Sender and receiver need to confirm each others identity

3. Message Integrity : Ensure that their communication has not been altered, either maliciously or by accident during transmission

Secure Communication: Needs and Requirements

Cryptography is the science of secret, or hidden writing

It has two main Components:

1. Encryption : Practice of hiding messages so that they can not be read by
anyone other than the intended recipient

2. Authentication & Integrity : Ensuring that users of data resources are the persons they
claim to be and that a message has not been surreptitiously
altered

Cryptography: Basics

Cipher is a method for encrypting messages

Encryption algorithms are standardized & published
The key which is an input to the algorithm is secret
Key is a string of numbers or characters
If same key is used for encryption & decryption the algorithm is called symmetric
If different keys are used for encryption & decryption the algorithm is
called asymmetric

Encryption: Cipher

Plain Text

Encryption
Algorithm

Key A

Key B

Cipher Text

Plain Text

Decryption
Algorithm

Multiple Choice

What is the language used by computers?

• HTML code
• CSS code
• JavaScript code
• Binary code

Multiple Choice

What is Binary code made up of?

• True and False
• A and B
• 2 and 3
• 0 and 1

Multiple Choice

What is a bit?

• A bit is a type of insect.
• A bit is a unit of measurement for distance.
• A bit is the smallest unit of data in a computer.
• A bit is a small piece of food.

Multiple Choice

What is a bit made up of?

• Binary digits 0's and 1

• Hexadecimal digit
• Octal digit
• Ternary digit

Multiple Choice

how many bits make one byte?

• 4
• 16
• 8
• 32

Multiple Choice

what is a byte used to represent on a computer?

• a single character
• a line of code
• a whole word
• a single bit

Multiple Choice

What is a character on a computer?

• A character on a computer is a physical device used for input.
• A character on a computer is a small animated figure.
• A character on a computer is a symbol, letter, or number.
• A character on a computer is a type of font.

Algorithms in which the key for encryption and
decryption are the same are Symmetric

Example: Caesar Cipher

Types:
1. Block Ciphers
Encrypt data one block at a time (typically 64 bits, or 128 bits)
Used for a single message
2. Stream Ciphers
Encrypt data one bit or one byte at a time
Used if data is a constant stream of information

Encryption: Symmetric Algorithms

Caesar Cipher is a method in which each letter in the
alphabet is rotated by three letters as shown

Substitution Ciphers: Caesar Cipher

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

Let us try to encrypt the message
–

Assignment: Each student will exchange a secret message with
his/her closest neighbor about some other person in the class
and the neighbor will decipher it.

Draw

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

Assignment: Each student will exchange a secret message with
his/her closest neighbor about some other person in the class
and the neighbor will decipher it.

Substitution Ciphers: Encryption Caesar Cipher

Plain Text

Message:

Attack at Dawn

Cipher Text

Message:

Dwwdfn Dw Gdyq

Cipher:

Caesar Cipher

Algorithm

Key (3)
Decryption

Plain Text

Message:

Attack at Dawn

Cipher Text

Message:

Dwwdfn Dw Gdyq

Cipher:

Caesar Cipher

Algorithm

Key (3)

How many different keys are possible?

Drag and Drop

Requirements of secure communication

​

Placeholder

Only intended receiver understands the message.

​

Placeholder

Sender and receiver need to confirm each others identity.

​

Placeholder

Ensure that their communication has not been altered, either
maliciously or by accident during transmission.

Drag these tiles and drop them in the correct blank above

Secrecy

Authentication

Message Integrity

Confidentiality

Authorization

Data Encryption

Multiple Choice

What is cryptography?

• Cryptography is a type of dance.
• Cryptography is the study of ancient civilizations.
• Cryptography is the art of making cryptic puzzles for entertainment purposes.
• Cryptography is the science of secret, or hidden
  writing

Drag and Drop

There are ​

Placeholder

main components of cryptography

​

Placeholder

Practice of hiding messages so that they can not be read by
anyone other than the intended recipient

​

Placeholder

Ensuring that users of data/resources are the persons they
claim to be and that a message has not been surreptitiously

Drag these tiles and drop them in the correct blank above

Encryption

Authentication & Integrity

two

Decryption

Authorization

three

Multiple Choice

What is a cipher?

• A cipher is a type of dance
• A cipher is a type of bird
• A cipher is a type of dessert
• A cipher is a method of encoding information to keep it secure and hidden from unauthorized users.

Categorize

Options (4)

Encrypt data one block at a time (typically 64 bits, or 128 bits)

Used for a single message

Encrypt data one bit or one byte at a time

Used if data is a constant stream of information

Match the following types of Caesar Ciphers to their description

Block Ciphers

Stream Ciphers

Drag and Drop

If same key is used for encryption & decryption the algorithm is called ​

Placeholder

If different keys are used for encryption & decryption the algorithm is

Placeholder

Drag these tiles and drop them in the correct blank above

symmetric

asymmetric

Who Was Alan Turing?

Alan Turing was a British mathematician. During the Second World War,
he worked as a codebreaker, cracking German codes created by Enigma
machines. His work was pivotal in the Allies’ victory.

Alan Turing was born on 23rd June
1912. He had an older brother called
John. Their father worked for the
British civil service in India.

Turing’s parents wanted their sons to
be raised in Britain so the boys
stayed with family friends while
their parents were in India.

Alan Turing’s Education

When Turing was 13, he went to a boarding school called Sherborne

School. At the time, the school did not place much emphasis on

mathematics and science so Alan felt quite discouraged.

While at the school, his close friend
Christopher Morcom died of a
disease called tuberculosis. Turing’s
grief inspired him to dedicate
himself to scientific discoveries.

He later won a scholarship to
study mathematics at King’s
College, Cambridge. He obtained a
degree in mathematics with
distinction.

The Universal Machine

In 1936, Turing created the idea of a special machine that could follow

simple codes. He called this the ‘Universal Machine’.

These machines are now known as a ‘Universal Turing Machine’ and

they formed a lot of the ideas behind computing.

Second World War

Cryptanalysis is the study of encrypted messages. During the Second

World War, Germany sent encrypted messages about its military

strategies. To win the war, it was vital that the

Allies were able to decode these messages.

A team of workers at a place called Bletchley Park were set the task of

decoding these messages.

Encrypted – where information
is changed into a code so it can
only be understood by the
recipient.

Second World War

Not long after the First World War, a
German man named Arthur
Scherbius invented a machine called
the Enigma.

The Enigma was a machine that
looked like a typewriter. As you
typed in your message, a different
letter lit up. The message was
converted into a code using these
illuminated letters. The person
receiving the message also had an
Enigma. When they typed in the
coded message, the letters from the
original message lit up and it could
be read. This meant that orders could
be passed secretly.

Bletchley Park

Bletchley Park is a stately home in Buckinghamshire. During the Second
World War, it was the home of the Government Code and Cipher School
(GC&CS).

Turing started work at Bletchley Park. Soon after his arrival, he had
devised a way of cracking the German’s code. He invented something
called a ‘Bombe’, which tried out lots of different solutions for breaking a
code before finding the correct one.

Although the ‘Bombe’ was successful,
Bletchley Park didn’t have enough
machines or the workers to read all
the German messages. Turing wrote
to the prime minister, Winston
Churchill. Before long, Bletchley Park
had more workers and
more ‘Bombes’.

The Impact of Turing’s Work

Breaking the German code meant the
Allies knew when and where attacks
were planned.

Without Turing’s discovery, military
experts believe that the Second World
War could have carried on for
another two years. Consequently,
millions of lives were saved.

Turing’s Work

Turing was awarded the OBE (Order of the British Empire) in 1946 for his
wartime work.

He died on 7th June 1954.

On 15th July 2019, the Bank of England announced that Alan Turing’s
image will be featured on the new £50 note, which is due to enter
circulation in 2021.

Multiple Choice

What year was Alan Turing born?

• 1905
• 1912
• 1920
• 1930

Multiple Choice

Why was Alan Turing a key figure during World War 2?

• He designed the first computer
• He led the Manhattan Project
• He was a spy for the Soviet Union
• He played a crucial role in breaking the German Enigma code.

Multiple Choice

What was the Bombe that Alan Turing created.

• A form of dance popular in the 1920s
• An electromechanical device used to decrypt German Enigma machine-encrypted messages.
• A type of musical instrument
• A cooking device used to prepare meals

Multiple Choice

What was Alan Turing Awarded in 1946?

• George Cross
• Victoria Cross
• Knight Bachelor
• Order of the British Empire (OBE)