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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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