Showing posts with label Computer Security. Show all posts
What are the requirements for the use of a public-key certificate scheme?
·
Any participant can read a certificate to
determine the name and public key of the certificate’s owner.
·
Any participant can verify that the certificate
originated from the certificate authority and is not counterfeit.
·
Only the certificate authority can create and
update certificates.
·
Any participant can verify the currency of the
certificate.
What is public-key certificate?
The public-key authority could be a bottleneck in the system, for a
user must appeal to
the authority for a public key for every other user that it wishes to contact. As before the
directory of names and public keys maintained by the authority is vulnerable to
tempering.
What are the essential ingredients of a public-key directory?
- The authority maintains a directory with a entry for each participant.
- Each participant registers a public key with the directory authority.
- A participant may replace the existing key with a new one at any time.
- Periodically, the authority publishes the entire directory or updates to the directory.
- Participants could also access the directory electronically.
List four general categories of schemes for the distribution of public keys.
Public
announcement
Publicly
available directory
Public-key
authority
Public-key
certificatesWhat are the two different uses of public-key cryptography related to key distribution?
There are two aspects to the use of public-key cryptography
In this regard:
- The distribution of public keys
- The use of public-key encryption to distribution secret keys
What is the difference between differential and linear cyptanalysis?
Explain the avalanche effect?
A desirable property
of any encryption algorithm is that a small change in either the plaintext or
the key should produce a significant change in the ciphertext.In particular, a
change I one of the plaintext or one bit of the key should produce a change in
many bits of the ciphertext.
What is the purpose of the S-boxes in DES?
The
role of the S-boxes in the function
F is that the substitution consists of a set of eight S-boxes ,each of
which accepts 6 bits as input and produces 4 bits as follows: The first and
last bits of the input to box Si form a 2-bit binary number to select one of
four substitutions defined by the four rows in the table for Si.The middle four
bits select one of the sixteen columns.The decimal value in the cell selected
by the row and column is then converted to its 4-bit representation to produce
the output. For example, in S1,for input 011001,the row is 01 and the column is
1100.The value in row 1,column 12 is 9,so the output is 1001.
Which parameters and design choices determine the actual algorithm of a Feistel cipher?
- Block size: Larger block sizes mean greater security but reduced encryption/decryption speed. A block size of 64 bits is a reasonable tradeoff and has been nearly universal in block cipher design.However, the new AES uses a 128-bit block size.
- Key size:Larger key size means greater security but may decrease encryption/decryption speed.Key sizes of 64 bits or less are now widely considered to be inadequate, and 128 bits has ecome a common size.
- Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate security but that multiple rounds offer increasing security.A typical size is 16 rounds.
- Subkey generation algorithm: Greater complexity in this algorithm should lead to greater difficulty of cryptanalysis.
- Round function: Again, greater complexity generally means greater resistance to cryptanalysis.
What is the difference between Diffusion and Confusion?
In Diffusion the statistical structure of the plaintext is
dissipated into long range statistics of the cipher text. This is achieved by
having each plaintext digit affect the value of many cipher text digits. Which
is equivalent to saying that each cipher text digit is affected by many
plaintext digits.
Confusion seeks to make a relationship between the statistics of
the cipher text and the value of the encryption key as complex as possible.
Thus even if the attacker can get some handle on the statistics of the cipher
text, the way in which the key was used to produce that cipher text is so
complex as to make it difficult to deduce the key.
What is the difference between a block cipher and a stream cipher?
A block cipher process the input one block of
elements at a time producing an output block for each input block.
A
stream cipher process the input elements continuously , producing output one
element at a time, as it goes along.
Why is it important to study the Feistel Cipher?
Feistel cipher using the concept of a product cipher, which is the
performing of two or more basic ciphers in
sequence in such a way that the final
result or product is cryptographically stronger then any of the
component ciphers.
Feistel
proposed the use of a cipher that alternates substitutions and permutations. So
Feistel cipher is considered to be an important one.
What is the difference between a monoalphabetic cipher and a polyalphabetic cipher?
In monoalphabetic
cipher single cipher alphabet is used per message. But in polyalphabetic cipher
there are multiple ciphertext letters for each plaintext letter, one for each
unique letter of keyword.
Briefly define the monoalphabetic cipher?
A dramatic increase in the key space is achieved by
allowing an arbitrary substitution. There are 26! Possible keys. It is referred to as monoalphabetic
substitution cipher, because a single cipher alphabet is used per message.
Briefly define the Caesar cipher?
The Caesar cipher involves replacing each letter of the alphabet
with the letter standing three places down the alphabet .The alphabet is
wrapped around, so that the letter following Z is A.
C
= E (p) = (p + 3) mod (26)
The general Caesar cipher algorithm is
C = E (p) = (p + k) mod (26)
where k takes the value in the range 1 to 25
The decryption algorithm is
p = D(C) = (C - k) mod (26)
What is the difference between an unconditionally secure cipher and a computationally secure cipher?
An encryption scheme is unconditionally secure if
the cipher text generated by the scheme does not contain enough information to
determine uniquely the corresponding plaintext, no matter how much cipher text
is available.
An
encryption scheme is said to be computationally secure if:
- The cost of breaking the cipher exceeds the value of the encrypted information.
- The time required to break the cipher exceeds the useful lifetime of the information.
What are the two general approaches to attacking a cipher?
The general two approaches for attacking a cipher
- Cryptanalysis: Cryptanalytic attacks rely on the nature of the algorithm plus perhaps some knowledge of the general characteristics of the plaintext or even some samples plaintext-cipher text pairs. This type of attack exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used. If the attack succeeds in deducing the key, the effect is catastrophic: All future and past messages encrypted with the key are compromised.
- Brute-force attack: The attacker tries every possible key on a piece of cipher text until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success.
What is the difference between a block cipher and a stream cipher?
A
block cipher processes the input one block at a time, producing an output block
for each input block.
A
stream cipher processes the input continuously, producing output one element at
a time, as it goes alone.
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 as symmetric,
single-key, secret-key or conventional encryption. If both sender and receiver
uses a different key, the system is referred as asymmetric, two-key or public
key encryption.
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