Crypto Final Actual Exam Questions and CORRECT Answers

Crypto Final Actual Exam Questions and
CORRECT Answers
Cipher and symmetric key encryption:

Can we prove security of block ciphers already? - CORRECT ANSWER✔✔- no


Cipher and symmetric key encryption:

Define shift cipher - CORRECT ANSWER✔✔- A shift cipher can be defined like that:
Enc(m) = m + k mod 26, and Dec(c) = c - k mod 26, here k is the secret key


Cipher and symmetric key encryption:
Show that the shift cipher is trivial to break using known-plaintext attack - CORRECT
ANSWER✔✔- the adversary can observe a pair of plaintext and ciphertext at least, denoted
by (α,β), so it can derive the secret key k = β − α mod 26


Cipher and symmetric key encryption:
Show that the shift cipher is trivial to break using chosen-plaintext attack - CORRECT
ANSWER✔✔- the adversary can choose a plaintext (a), then get the corresponding
ciphertext (β), which will be easily to derive the secret key k= β − a mod 26.


Cipher and symmetric key encryption:
Show that the shift cipher is trivial to break using chose-ciphertext attack - CORRECT
ANSWER✔✔- the adversary not only can choose a plaintext and get the its ciphertext
(access Enc oracle), but also can choose ciphertext (a) and get its plaintext α(access Dec
oracle). It also can derive the secret keys easily: k= a-α mod 26.


Cipher and symmetric key encryption:
What is the cause of that frequency analysis can help break ciphers? - CORRECT
ANSWER✔✔- Because in the English text, individual letters follow some specific
probability distributions


Cipher and symmetric key encryption:

Define the syntax of SKES. - CORRECT ANSWER✔✔- skes consists of three algorithms:

, • KeyGen - a randomized algorithm which takes a security parameter k and returns K ∈ S by
its respective distribution.
• Encrypt - a randomized or deterministic algorithm which takes K and a plaintext m and
returns c ∪ {⊥}.
• Decrypt - a deterministic algorithm which takes K and c and returns m ∪ {⊥}.


Cipher and symmetric key encryption:

Define the semantics (i.e., IND-CPA) of SKES. - CORRECT ANSWER✔✔- SKES is IND-
CPA if and only if the adversary cannot distinguish a single- bit of an unknown ciphertext
given a polynomial number of adaptively chosen plain- text/ciphertext pairs with a non-
negligible probability.


Cipher and symmetric key encryption:

Define the semantics (i.e., IND-CCA) of SKES. - CORRECT ANSWER✔✔- SKES is IND-
CCA if and only if the adversary cannot distinguish a single-bit of an unknown ciphertext
given an oracle to decrypt a polynomial number of ciphertext with a non-neglibile probability


Cipher and symmetric key encryption:
Explain why the simple classic substitution cipher we studied in the lecture does not meet the
security definitions of IND-CPA and IND-CCA - CORRECT ANSWER✔✔- IND-CPA and
IND-CCA require that the encryption algorithm is probabilistic. However, in the substitution
cipher, the encryption algorithm is deterministic.


Cipher and symmetric key encryption:
Eve has tricked Alice into decrypting a bunch of ciphertexts that Alice encrypted last month
but forgot about. What type of attack is Eve employing? - CORRECT ANSWER✔✔-
ciphertext only attack


Cipher and symmetric key encryption:

Explain EBC mode of encyption - CORRECT ANSWER✔✔- ECB (Electronic Code Book)
encrypts each successive k-bits of a plaintext independently, then transmits and decrypts the
blocks independently. This is a method by which we use block ciphers to encrypt arbitrary-
long messages.


Cipher and symmetric key encryption: