Answers to Practice Questions for Exam 1
(Crypto Basics)
Answer 1-Crypto:
a) The three basic requirements that we discussed in class are:
1. Cryptosystem security: Roughly speaking, it must be infeasible for an eavesdropper to
compute xfrom ywithout knowing KAB. Moreover, it must be infeasible to infer KAB
even if the eavesdropper has access to many ciphertexts exchanged by Alice and Bob.
2. Endpoint security: Alice and Bob must store KAB securely (so that no one can steal it)
and use it in a secure computing environment (so that the eavesdropper cannot simply
obtain the plaintext before it is sent or after it is received, without having to “break”
the cryptosystem).
3. Secure key management: Alice and Bob, typically with the assistance of a key-dis-
tribution center, must obtain a unique key KAB and maintain its security until it is
retired.
.
b) Secure key management cannot be done at Internet scale. In particular, e-commerce web-
sites need to communicate securely with potential customers whom they have never “met”
and with whom they have not been able to establish shared keys.
Answer 2-Crypto:
a) Public-key certificates are signed (name, public key) pairs. The problem addressed is
that an impostor or man-in-the-middle, say Manny, could generate a key pair (P KM, SKM)
and then impersonate Alice by publishing the directory entry (Alice, P KM). Subsequently,
if someone sends a ciphertext to Alice that was encrypted using the encryption key PKM,
Manny could intercept it and decrypt it, because it is he, not Alice, who knows the corre-
sponding decryption key SKM. To prevent this, we require that public-key directory entries
be signed by certifying authorities or CAs. Instead of simply generating her key pair and
publishing her public key, Alice takes her public key PKAto a CA, along with proof that
she is indeed Alice. The CA verifies that her identification documents are valid and then
signs the (name, key) pair (Alice, P KA). The entry that is published in the directory is
Alice, P KA, σAlice,C A,
where σAlice,CA is the CA’s signature on (Alice, P KA). Someone who wants to use P KAmust
have a trustworthy copy of the CA’s verification key, but we have reduced the problem of
needing trustworthy copies of PKA, for every user Aof the public-key system, to the much
smaller-scale problem of needing a trustworthy verification key (or keys) for one (or a small
number of) CA(s).
