Task 2. a) List and explain the meaning of four security aims desired in communication networks. b) Attacks on communication systems can be classified as passive and active. Describe the difference between passive and active attacks. c) Name and describe one active attack that can occur in computer networks. d) Name and describe one passive atlack that can occur in compuler networks. Task 3. Consider the following teat about security and cryplograptry. Identify the missing words/phrases. Select the correct wordiphrase from the following list. Some words/phrases might be used muliple times, while others might not be used at all. attack, authentication; block; brute force; cipher, ciphertext; cryptanalysis, cryptography; decrypt; decryption; hash, encrypt; encryption; plaintext; private key; public key; signature; stream; substitution; symmetric, fransposition A (1) is a particular algorithm used for encryplion. When (2) is encrypled with a key. (3) is produced as oulput. There are two general types of ciphers that differ in the number of keys used: symmetric ciphers and (4) ciphers. (5) ciphers are usually faster than (6) _ ciphers. Another classilication of ciphers is: -(7) ciphers and (8)_ciphers. (9) _ ciphers use the operations (10) and _(11)___(12)__ ciphers are usually faster than __(13)__ciphers. An allacker's aim. is usually to find the plainteat ar key. It is assumed the attacker knows the (14) and (15) - In some cases, the attacker may also know pairs of previous (plaintext, cipherlext). There are two general methods of attack: _(16)__ and Blockchain makes use of the benefits of (18) In a blockchain the curtent (19) of the current block depends on the hash of the previous _ (20) Answer: (1)= ? (2) = ? (3) = ? (4)= ? (20)=