What is the definition of entropy? Briefly describe what salting does and why it is needed. What is a hashing function? What do we mean when we say a hashing function is a one-way function? Should it be easy to generate two different messages that compute to the same hash value? Explain what HMAC (Hashed based message authentication code) is? Briefly discuss what a collision attack is. Name three popular hashing algorithms. Why have MD2, MD4 and MD5 been deprecated? Briefly explain what the block mode of a hashing algorithm does. How long is the output of SHA-1 (in bits)? (SHA-1 has been deprecated) What are all symmetric algorithms based on? Key management involves multiple stages. Name the five stages suggested by the author. In a group of 100 members how many symmetric keys are needed to enable encrypting communications between any pair of members? What is a block cipher? What is a stream cipher? What is the block size of DES? What is a round in the DES algorithm? In the context of DES, what is a weak key? In the context of DES, what is a semi-weak key? When was the 56-bit key DES algorithm broken? How many keys can 3DES use? Briefly describe how the EDE mode of 3DES works. Is AES a block or a stream cipher? How large are the AES blocks (in bits)? What possible key sizes can AES use? What is one advantage of ChaCha20 over AES? Briefly describe what Authenticated Encryption with Associated Data (AEAD) does.