Crypto Final Actual Exam Questions and CORRECT Answers
6 views 0 purchase
Course
Crypto
Institution
Crypto
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...
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:
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller MGRADES. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $9.49. You're not tied to anything after your purchase.
