12 Cryptography

Question 1

What is a encryption?

Answer 1

• measure to secure messages like e-mail, SMS against unauthorized reading

Question 2

What is the caesar method?

1.
2.
3.

Answer 2

• shift cipher (alphabet is shifter by a given number of characters)
• method: simple shift
• key: 2 (shifts to the right)

Question 3

What is the Vignere method?

Answer 3

• vignere cipher
• a word is choosen as the key, e.g. ONE
• letters of keyword determine about how any positions a letter of the clear text is shifted
• O=14, N=13, E=4 (A=0, B=1, …)

Question 4

What is the enigma?

Answer 4

• very complex encryption machine
• used in WW2
• a decryption machine called Turing-Bomb was built up in a unit in the UK

Question 5

How does the enigma machine work?

Answer 5

• numberphile = YTHMYIURFGW
• letters can turn up twice
• each time you get completely different code
• initial position of rotors and plugboard is the important component

Question 6

What is the Kerckhoff´s Principle?

Answer 6

• the security of an encryption must depend on the secrecy of the key
• it must not depend on the used encryption method

“Even when the attacker knows which cipher is used, he must not be able to decipher the secret text.”

Question 7

What does kerckhoff´s principle mean for computer science?

Answer 7

• encryption methods can easily be standartised and be implemented by all involved parties. the secrurity thus depends on the secrecy of the keys!

Question 8

What are three different encryption methods?

Answer 8

• caesar
• vignere
• enigma machine

Question 9

What are modern encryption standarts?

Answer 9

• DES: Data encryption standard, 56 bit key, used in ATMs and in encryption of speech (smartphones), due to moore´s law not really secure anymore
• AES: advamced encrpytion standard, keys having length of 128 and 256 bit

Question 10

What is the symmetric encryption?

Answer 10

• classic approach, sender and receiver use the same key (e.g. vignere, DES, Enigma)
• clear text sender to clear text receiver
• common secret key
• encrypt to decrypt

Question 11

What is an asyymetric encryption?

Answer 11

• sender and receiver use different krys
• one to encrypt and one to decrypt
• public key of sender
• vs private key of receiver
• pair of keys created, public one forwarded to certification agency, private stays with owner
• someone gets public key by owner from certificatin agency, he can now encrypt his message to owner
• someone is not(!) able to decrypt thia

Question 12

How does the public key encryption work?

Answer 12

• even if attacker knows oublic key, he is not able to decrypt the message as the privte key is required
• best knwon method is RSA

Question 13

What is special about asymmetric encryption?

Answer 13

• always need a PAIR of keys that belong to each other
• RSA based on prime factrosisation

Question 14

What is DES and AES?

Answer 14

• both are encryption methods
  DES: Data Encryption Standard; 56 bit key; not secure anymore due to Moore´s Law
  AES: Advanced Encryption Standard; 128,192,256 bit; more secure and efficient