Chapter 6 Cryptography and Symmetric Key Algorithms

Question 1

What is key space?

Answer 1

Every algorithm has one. It is the range of values that are valid for use as a key for a specific algorithm. Defined by the bit size, which is the number of binary bits in the key.

Question 2

What is Kerchoff’s principle?

Answer 2

A cryptographic system should be secure even if everything about the system, except the key, is public knowledge.

Question 3

What is cryptography, cryptoanalysis, and cryptology?

Answer 3

Cryptography is the art of creating and implementing secret codes and ciphers.

Cryptoanalysis is he study of methods to defeat codes and ciphers.

Cryptology is cryptography and cryptanalysis together.

Question 4

What is a noonce?

Answer 4

A random number that acts as a placeholder in math functions. When the function is executed, the nonce is replaced with a random number. Must be a unique number each time. This is used in the initialization vector, a random bit string that is the same length as the block size.

Question 5

What is Zero Knowledge proof?

Answer 5

Prove your knowledge of a fact to a third party without revealing the fact itself.

Question 6

What is split knowledge?

Answer 6

Information necessary to perform an operation is divided. Key escrow–might need multiple agents to be able to pull a key out of escrow.

Question 7

What is the work function?

Answer 7

Usually represents the time and effort necessary to conduct a brute force attack against an encryption system.

Question 8

What is M of N?

Answer 8

Minimum number of agents out of total number of agents required to perform a task.

Question 9

What are codes vs ciphers?

Answer 9

Codes are cryptographic systems of symbols that represent words or phrases. Example of 10-4 representing “I received your communication and understand it.” Can be secret but does not have to be.

Ciphers are always meant to hide the true meaning of the message. They convert from plaintext to cipher text on a bit, character, or block basis.

Question 10

What are transposition ciphers?

Answer 10

Rearrange the letters of a plaintext message.

Question 11

What are substitution ciphers?

Answer 11

Replace each character or bit of the plaintext message with a different character.

Question 12

What is the Caesar cipher?

Answer 12

Shifted each letter of the alphabet three letters to the right. Aka ROT3.

Question 13

What are the requirements of a one-time pad?

Answer 13

Must be randomly generated.

Must be physically protected against disclosure

Must be used only once.

The key must be at least as long as the message to be encrypted.

Question 14

What is a running key cipher?

Answer 14

AKA a book cipher. The encryption key is as long as the message itself and chosen from a book, newspaper, etc. Advantage is you don’t have to exchange one-time pads.

Question 15

What is the difference between confusion and diffusion?

Answer 15

Confusion occurs when the relationship between the plaintext and the key is so complicated that an attacker can’t continue altering the plaintext and analyzing the results to determine the key.

Diffusion occurs when a change in the plaintext results in multiple changes in the cipher text. Example: first do substitution and then do transposition. The substitution introduces confusion and the transposition adds diffusion.

Question 16

How many keys do you need for symmetric key system?

Answer 16

n(n-1) divided by 2.

Question 17

What is Electric Code Book Mode?

Answer 17

Simplest mode to understand and least secure. Each time it processes a 64-bit block, it will encrypt it with a chosen secret. That means that that if encounters the same block, it will produce the same ciphertext. Currently impractical for anything but small data, such as keys and parameters used to initiate other communications.

Question 18

What is Cipher Block Chaining Mode?

Answer 18

Each block of unencrypted text is XORed with the block of ciphertext before it before it is encrypted. Uses an IV with the first block. The IV must be sent to the recipient; for example, you could send in plain form or used ECB. If one block is corrupted, everything after is is corrupted.

Question 19

What is Ciper Feedback Mode?

Answer 19

CFB. Streaming version of CBC. Instead of breaking a message into blocks, it uses memory buffers of the same size. When the buffer becomes full, it is encrypted and sent to the recipients.

Question 20

What is Output Feedback Mode?

Answer 20

Similar to CFB, but instead of XORing an encrypted version of the previous block, it XORs the plaintext with a seed value. For the first block, it uses an IV; future seed values run an algorithm on the previous seed value. Avoids the same corruption problems.

Question 21

What is Counter Mode?

Answer 21

CTR. Uses a stream cipher, but instead of using the previous value, it ues a simple counter that increments for each operation. Well suited for parallel computing.

Question 22

What is Galois/Counter Mode?

Answer 22

GCM. Takes CTR and adds data authenticity through authentication tags.

Question 23

What is Counter with Cipher Block Chaining Message Authentication Code Mode?

Answer 23

Combines CTR for confidentiality with CBC-Message Authentication Code for authenticity. CCM is only used for block ciphers with a 128-bit block. Requires a nonce that changes with each transmission.

GCM and CCM are known as authenticated models of encryption. Other ones called unauthenticated encryption.

Question 24

Describe DES.

Answer 24

64-bit cipher that had five modes of operation: ECB, CBC, CFB, OFB, and CTR. All of the modes operated on 64 bits of of plaintext at a time. Key was 56 bits. Used 16 rounds of XOR for each encryption operation.

Question 25

Describe Triple DES.

Answer 25

Uses the same algorithm. Several different variants. DES-EDE3 and EEE-3 us three independent keys. E=encryption and D=decryption. EDE3 encrypts the data with K1, decrypts with K2, and encrypts with K3. EEE3 encrypts with all three keys in sequential order. Effective strength is 112 bits. Only EEE3 is considered secure, but only through 2023.

Question 26

Describe the International Data Encryption Algorithm.

Answer 26

IDEA block cipher was developed in response to problems with DES. Still used 64-bit blocks. Begins operation with 128-bit key. It is broken up into 52 16-bit keys. The subkeys use XOR and modulus operations to produce the ciphertext. Same 5 modes as DES. Used in PGP.

Question 27

Describe Blowfish.

Answer 27

Also 64-bit blocks. Variable length keys from 32 bits to 448 bits. Faster than both IDEA and DES. Was released for public use. Often used for SSH.

Question 28

Describe Skipjack.

Answer 28

Approved for USG use. 64-bit blocks. Uses an 80-bit key.. Also supports the escrow of encryption keys. NIST and Treasury hold a portion of the information required to reconstruct a key. Was to be used in Clipper and Capstone chips. Not embraced because people did not trust the USG.

Question 29

Describe Rivest Ciphers.

Answer 29

RC4–developed in 1987. Single round of encryption, variable length keys between 40 and 2048 bits. Used in WEP, WPA, SSL and, TLS. Not considered secure today. Stream cipher.

RC5–Block cipher. Either 32, 64, or 128 bitts at a time. Key size between 0 and 2040. Not considered secure.

RC6–Block cipher. 128-bit blocks, 128/192/256 bit keys. Candidate for AES but not selected.

Question 30

Describe AES.

Answer 30

AES uses 128/192/256 bit keys. AES specifies 128-bit blocks, but Rijndael can handle a block size equal to the key length. 10 rounds for 128-bit keys, 12 rounds for 192 bit, 14 rounds for 256 bit keys.

Question 31

Describe CAST.

Answer 31

CAST 128 uses either 12 or 16 rounds with a key between 40 and 128 bits on 64-bit blocks.

CAST 256 uses 48 rounds of encryption with 128/160/192/224/256 bit keys on 128 bit blocks.

Question 32

Describe Twofish.

Answer 32

128 bit blocks, up to 256 bit keys. Used prewhitening (XOR the plaintext with a separate subkey before the first round of encryption) and postwhitening (similar operation after the 16th round).