05-03-2013, 10:09 AM
Computer Science and Engineering, 7th Semester 2 Marks Question and Answer
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UNIT-I
1. What is cryptology?
Cryptology is the study of cryptography and cryptanalysis.
2. What is the difference between an unconditionally secure cipher and a
computationally secure cipher?
_ An unconditionally secure cipher is a scheme such that if the cipher text generated by the
scheme does not contain enough information to determine uniquely the corresponding plain
text, no matter how much cipher text is available. A computationally secure scheme is such
that the cost of breaking the cipher exceeds the value of the encrypted information and the
time required to break the cipher exceeds the useful lifetime of the information.
3. Briefly define the Caesar cipher.
The Caesar cipher involves replacing each letter of the alphabet with the letter standing
three places further down the alphabet. For example:
Plain: meet me after the toga party
Cipher: PHHW PH DIWHU WKH WRJD SDUWB
4. Briefly define the monoalphabetic cipher?
A monoalphabetic cipher maps from a plain alphabet to cipher alphabet. Here a single cipher
alphabet is used per message.
5. Briefly define the playfair cipher.
The best-known multiple-letter encryption cipher is the playfair, which treats diagrams in the
plain text as single units and translates these units into cipher text diagrams.
6. What are the two problems with one-time pad?
1.It makes the problem of making large quantities of random keys.
2.It also makes the problem of key distribution and protection.
7. What is a transposition cipher?
Transposition cipher is a cipher, which is achieved by performing some sort of
permutation on the plaintext letters.
8. What are the two basic functions used in encryption algorithms?
The two basic functions used in encryption algorithms are
Substitution
Transposition
9. How many keys are required for two people to communicate via a cipher?
If both sender and receiver use the same key, the system is referred to as symmetric, single
key, secret key, or conventional encryption. If the sender and receiver each use a different
key, the system is referred to as asymmetric, two-key, or public-key encryption.
10. What is the difference between a block cipher and a stream cipher?
A block cipher processes the input one block of elements at a time, producing an output
block for each input block. A stream cipher processes the input elements continuously,
producing output one element at a time, as it goes along.