Crypto Study 2

Question 1

1. What does ECB stand for?
2. How does ECB work
3. Name security flaw(s),
4. What algorithm is it part of?

Answer 1

1. Electronic Codebook
2. Encrypts 64 bit blocks with the same secret key
3. Subject to eavesdropping
4. DES

Question 2

1. How does CBC work?

2. Name a security flaw or issue.

3. What algorithm is it part of?

4. What does CBC stand for?

Answer 2

1. Each block of unencrypted text is XORed with the block of ciphertext immediatley preceding it before its encrypted with DES. CBC also implements a IV and XORs the first block of the message producing a unique output. IV must be sent to recipient in plaintext,
2. If one block is corrupted errors propogate making decryption impossible
3. DES
4. Cipher Block Chaining Mode

Question 3

1. How does CFB work

2. Name security flaw(s),

3. What algorith is it part of?

4. What does CFB stand for?

Answer 3

1. Streaming version of CBC. XORs and uses IV. Uses memory buffers the same block size instead of breaking the message into blocks. As buffer becomes full it gets encrypted. Uses real time data (done in the buffer) and uses IV and chaining.
2. Errors propogate
3. DES
4. Cipher Feedback Mode

Question 4

1. How does OFB work?

2. Name security flaw(s) or advantages,

3. What algorith is it part of,?

4. What does OFB stand for?

Answer 4

1. XORs plaintext with a seed value. For first block IV is used to create seed value and then running DES. Future seed values are derived by running DES on previous seed values.
2. No chaining function- errors do not propogate to affect decryption
3. DES
4. Output Feedback Mode

Question 5

1. How does CTR work

2. Name security flaw(s).

3. What algorith is it part of,?

4. What does CTR stand for?

Answer 5

1. Stream cipher similiar to CFB. Similar to CFB but does not use a seed value, uses a counter that increments for each operation.
2. Uses chaining so may propogate errors
3. DES
4. Counter Mode

Question 6

1. What is IDEA?,
2. How does it work?
3. What modes is it capable of operating in?
4. What secure e-mail package is based off of IDEA?

Answer 6

1. International Data Encryption Algorithm
2. Operates on 64 bit block of text, begins with a 128 bit key, broken into fifty -two 16 bit subkeys which act on input text with XOR and Modulus operations to produce encrypted/decrypted versions of the input message
3. Same modes as DES: ECB, CBC, CFB, OFB, CTR
4. PGP- Pretty Good Privacy

Question 7

1. Is BLOWFISH Symmetric or Assymetric?

2. What is the block size when encrypting?

3. How many bits is the key length?

4. What are its advantages?

Answer 7

1. It is a SYMMETRIC cryptographic alogrithm
2. Works on 64 bit BLOCKS of text
3. Allows variable key lengths from 32 to 448 bits
4. Faster than IDEA and DES

Question 8

1. Is Skipjack Symmetric or Asymmetric?
2. How does it work?
3. What does it support that is unique?

Answer 8

1. A symmetric encryption algorithm
2. Works on 64 bit-blocks of text. Uses an 80 bit key and supports the four modes of operation of DES (CBC, OFB, ECB, CTR)
3. Supports escrow of encryption keys- NIST and Dept. of Treasury have the keys

Question 9

1. What is RC5?
2. Block cipher or stream cipher?
3. What are the key sizes it supports?
4. What are the security implications of the algorithm?

Answer 9

1. Rivest Cipher 5, a symmetric algorithm
2. It is a variable block cipher 32, 64, or 128 bits
3. Uses key sizes 0 to 2,040 bits
4. 64 bit key took 4 years to crack a single message

Question 10

1. ​What is AES?
2. Name the three key sizes and the round of encryption associated with each?

Answer 10

1. Advanced Encryption Standard/Rijndael (Symmetric Encyption)
2. 128 bit key=10 rounds of encryption

192 bit key= 12 rounds of encryption

256 bit key= 14 rounds of encryption

Question 11

1. What is TWOFISH?
2. How does it work
3. What two special techniques does it apply to its encryption??

Answer 11

1. It’s a block AES cipher
2. It operates on 128 bit blocks of data and uses keys up to 256 bits
3. Does PREWHITENING- XORing the plaintext witha subkey before first round of encryption, THEN does POSTWHITENING which a similar operationn but done aftet the 16 th round of encryption

Question 12

In regards to SYMMETRIC KEY management, what are three methods to accomplish?

Answer 12

1. Offline distribution- Has to be physically provided

2. Public Key Encryption- Used to set up the initial communication link- then the parties switch to Private Key encryption which is faster.

3. Diffie-Hellman- use when there is no offline or public key encryption is available- random large integers selected and calculations performed- each party uses their integers to produce a result and exchange to verify they got the same results

Question 13

What are the key lengths of:

RSA

DSA

Elliptic Curve

Answer 13

RSA= 1,024

DSA=1,024

Elliptic Curve=160 bits

Question 14

In Publick Key Cryptography, Bob wants to send a message to Sue, what does

he use to encrypt the message and how does Sue Decrypt the message?

Answer 14

Bob uses Sue’s Public Key to encrypt the message. Sue uses her

Private Key to decrypt the message.

Question 15

1. What type of encryption algorithm is RSA?
2. What does the system depend on for the strength of its security?
3. What will be the largest value?

Answer 15

1. RSA is a Public Key cryptosystem
2. It depends upon the difficulty of factoring large prime numbers
3. n will always be the largest value if the choice is between p and q

Question 16

1. What type of algorithm is Merkele-Hellman
2. What theory did it rely on for its security?
3. Is the algorithm considered secure today?

Answer 16

1. It is an Asymmetric Public Key algorithm
2. Super-increasing sets rather than factoring large prime numbers
3. It is no longer considered secure

Question 17

How long are the key lengths for:

1. RSA

2. DSA

3. Elliptic curve

Answer 17

1. RSA= 1,024 bits
2. DSA= 1024 bits
3. Elliptic curve =160 bits

Question 18

1. What type of algorithm is EL Gamal
2. What is a major problem with EL Gamal?
3. What is unique and considered an advantage with EL Gamal?

Answer 18

1. El Gammal is an Assymetric key exchange algorithm like Diffie Hellman
2. It doubles the length of any message it encrypts which creates issue with transmission over narrow bandwitdh communication circuits
3. It was released to the public domain, it was free to use by anyone

Question 19

1. What is Elliptic Curve?

2. What is the Eliiptic Curve equation?

3. How many keys in Elliptic Curve are equal to 1024 RSA?

Answer 19

1. Elliptic Curve is an assymetric public key algotrithm encryption algorithm
2. . y=x3 + ax + b and Q =xp
3. 1024 bit RSA keys are equivalent to 160 bit- Elliptic Curve key.

Question 20

Name the 4 versions of TRIPLE DES (3 DES) and the number of encryption (and or decryption) operations.

Answer 20

1. DES-EEE3 - 3 Encryption Operations
2. DES-EDE3- 2 encryption operations with decryption in the middle
3. DES-EEE2- 3 Encryption operations
4. DES-EDE2 - 2 encryption operations with a decryption operation in the middle