Encryption (CIE IGCSE Computer Science)

• Many threats exist to system and network security. Examples include:
  • Malware
  • Viruses
  • Spyware
  • Hackers
  • Denial of service attacks
  • Social engineering
  • SQL injection
• Hackers are people who try to gain unlawful or unauthorised access to computers, networks and data by writing programs
• They look for weaknesses in the system and use them to gain access
• Hackers have various motives such as financial gain, a challenge or protests etc
• Hackers sometimes target data in order to steal and use it, or block people from using the data by creating programs called ransomware
• Hackers may also used packet sniffer to intercept and read data transmitted across the internet or a network
• Hackers will often want to use people’s information and therefore it is beneficial to encrypt your data

What is encryption?

• Encryption involves encoding data into a form that is meaningless using an algorithm
  • An example could be turning the phrase “Computer Science” into “YekLKEZizFuFjHNCjHj3Md7qyTiGxLNNwPVFZtJU74I=”
• Once encrypted, data can be decrypted which turns the encrypted data into data that can be understood again
• Encryption doesn’t prevent hackers from hacking but makes the data hard if not impossible to understand unless they have matching decryption tools
• There are two types of encryption: symmetric encryption and asymmetric encryption

• Encryption relies on the use of a key. A key is a binary string of a certain length that when applied to an encryption algorithm can encrypt plaintext information and decrypt ciphertext
  • Plaintext is the name for data before it is encrypted
  • Ciphertext is the name for data after it is encrypted
• Keys can vary in size and act like passwords, enabling people to protect information. A single incorrect digit in the key means the data cannot be decrypted correctly. Strong modern keys can be up to or over 1000 bits long!

Symmetric encryption

• In symmetric encryption both parties are given an identical secret key which can be used to encrypt or decrypt information
• Key distribution problem: If a hacker gains access to the key then they can decrypt intercepted information
• Methods exist to send the secret key to the receiver without sending it electronically:
  • Both parties could verbally share the key in person
  • Both parties may use standard postage mail to share the key (some businesses and banks may do this to ensure someone's identity and authenticity)
  • An algorithm may be used to calculate the key by sharing secret non-key information. An example is shown below

Symmetric Encryption Walkthrough

• • • Both parties A and B choose a number, for example A = 3, B = 2

    • Both parties enter their own respective numbers into the following equations: 7^A MOD 11 or 7^B MOD 11. ^ is another way of writing “to the power of” 

      • 7^3 MOD 11 = 2, 7^2 MOD 11 = 5

    • Both parties swap their respective answers. A receives 5 and B receives 2. These answers replace the initial 7 number and the calculations are performed again

    • Both parties enter their new number into the following equations: 5^3 MOD 11 or 2^2 MOD 11

      • 5^3 MOD 11 = 4, 2^2 MOD 11 = 4

    • The answer should match for both parties and this becomes the encryption and decryption key value

• Once the key is generated, it can be applied to the plaintext in the algorithm that then produces the ciphertext which is sent to the receiver
• The receiver gets a copy of the ciphertext and the key and applies the encryption algorithm. The algorithm then produces the original plaintext for the receiver

Asymmetric encryption

• In asymmetric encryption also known as public key encryption, two keys are used:
  • Public key: a key known to everyone
  • Private key: a key known only to the receiver
• Both keys are needed to encrypt and decrypt information
• Asymmetric encryption works as follows:
  • Person A uses a public key to encrypt their message
  • Person A sends their message over the network or internet
  • Person B decrypts the message using their secret private key 
• Asymmetric encryption works such that only one private key can be used to decrypt the message and it is not sent over the internet like a symmetric key
• Keys can be very large, for example over 1000 bits. To get the correct key a hacker would have to calculate almost every possible combination. To illustrate, a key with only 100 bits would generate 1,267,650,600,228,229,401,496,703,205,376 different combinations

How are encryption keys created?

• Encryption keys can be created manually, randomly or via an algorithm
• Strong encryption keys are created using a hashing algorithm
• A hashing algorithm is a non-reversible mathematical algorithm that converts a given input into an output. Once the output has been generated it is unable to be converted back to the original input
• Encryption keys are created by supplying a message or key to the hashing algorithm which turns it into a string of characters usually shown in hexadecimal
• SHA-2 is an example of a hashing algorithm that creates hashed keys of 244, 256, 384 or 512 bit length
  • If the text string “Computer Science” is run through the SHA-2 algorithm, it would return a 512 bit key in hexadecimal as:
  • “B6e175f5fc647b1a9ce17019594ce55b58e8fd03e3c584ee384121c8b4c7753d”
• The hashed encryption key can then be sent symmetrically or kept secret as part of an asymmetric private key. Both sender and receiver need a copy of the key to decrypt information regardless of using symmetric or asymmetric encryption

Why use hashed encryption keys?

• In symmetric encryption, the key must be sent with the message to the receiver. If a hacker intercepts the key they can read the message
• In asymmetric encryption, the public key is available to everyone and would not be useful to a hacker. The hacker must guess the private key in order to read the message
• Hashing algorithms are many-to-one. This means that many input values, messages or keys can produce the same hash key output
• A hashed encryption key means the hacker must first unhash the key before it is useful
• As hashing algorithms are non-reversible this is extremely difficult
• With SHA-2 for example, a hacker who wants to find the symmetric or asymmetric private key must calculate over 1.3x10^154 combinations; that is 13 with 153 0’s after it. With the computing power available today, this is virtually if not actually impossible