('Dawn Song', 'Midterm 1', '(solution)') Spring 2012

C 2 points how does a mac message authentication code

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Unformatted text preview: ◦ Given the key(s), it is always feasible to invert symmetric encryption, but it is not always feasible to invert a MAC. Answer: “Symmetric encryption has a fixed-size output and a MAC does not.” and “Given the key(s), it is always feasible to invert symmetric encryption, but it is not always feasible to invert a MAC.” (c) (2 points) How does a MAC (Message Authentication Code) differ from a cryptographic hash function? Note: To invert means to compute the input given the output. (circle all that apply) ◦ It doesn’t. Page 11 ◦ A MAC has a pair of keys (public and private) and a cryptographic hash function does not. ◦ A MAC has a single key and a cryptographic hash function does not. ◦ A cryptographic hash function has a pair of keys (public and private) a MAC does not. ◦ A cryptographic hash function has a single key and a MAC does not. ◦ A cryptographic hash function has a fixed-size output and a MAC does not. ◦ A MAC has a fixed-size output and a cryptographic hash function does not. ◦ Given the key(s), it is always feasible to invert a MAC, but it is always not feasible to invert a cryptographic hash function. ◦ Given the key(s), it is always feasible to invert a cryptographic hash function, but it is not always feasible to invert a MAC. Answer: A MAC has a single key and a cryptographic hash function does not. (d) (1 point) If I encrypt a message, then I don’t need to authenticate it to prevent against active attacks. ◦ True ◦ False Answer: False. If the message is not authenticated, the attacker can just replace it with another message or a random garbage message. (e) (2 points) Suppose Alice wants to send a message to Bob so that only Bob can read it and so that Bob gets the correct message. They engage in the following protocol: 1. Bob sends Alice his public key. 2. Alice uses an asymmetric encryption algorithm to encrypt the message with the public key she just received. 3. Alice sends the ciphertext to Bob. 4. Bob decrypts t...
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