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KeyTerm
Explain
1.
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. In cryptography,
the avalanche effect is the desirable property of cryptographic algorithms,
typically block ciphers and cryptographic hash functions wherein if
when an input is changed slightly (for example, flipping a single bit) the
output changes significantly (e.g., half the output bits flip). In the case of
high-quality block ciphers, such a small change in either the key or the plaintext
should cause a drastic change in the ciphertext.
The actual term was first used by Horst Feistel,[1]
although the concept dates back to at least Shannon's
diffusion.
The SHA-1 hash function exhibits good
avalanche effect. When a single bit is changed the hash sum becomes completely
different.
If a block cipher or cryptographic
hash function does not exhibit the avalanche effect to a significant degree,
then it has poor randomization, and thus a cryptanalyst can make predictions about the input, being given only the
output. This may be sufficient to partially or completely break the algorithm.
Thus, the avalanche effect is a desirable condition from the point of view of the
designer of the cryptographic algorithm or device.
Constructing a cipher or hash to
exhibit a substantial avalanche effect is one of the primary design objectives,
and mathematically the construction takes advantage of butterfly effect.[2][3] This is why most block ciphers are product ciphers. It is also why hash functions have large data blocks. Both
of these features allow small changes to propagate rapidly through iterations
of the algorithm, such that every bit of the output should depend on
every bit of the input before the algorithm terminates
2. Explain
About Block Cipher
In cryptography,
a block cipher is a deterministic algorithm operating on
fixed-length groups of bits,
called blocks, with an unvarying transformation that is specified by a symmetric key.
Block ciphers operate as important elementary components in the design of
many cryptographic protocols, and are widely
used to implement encryption of bulk data.
The modern design
of block ciphers is based on the concept of an iterated product
cipher. In his seminal 1949 publication, Communication Theory of Secrecy
Systems, Claude Shannon analyzed product ciphers and
suggested them as a means of effectively improving security by combining simple
operations such as substitutions and permutations.[1]
Iterated product ciphers carry out encryption in multiple rounds, each of which
uses a different subkey derived from the original key. One widespread
implementation of such ciphers, named a Feistel
network after Horst Feistel, is notably implemented in the DES cipher.[2]
Many other realizations of block ciphers, such as the AES, are classified as substitution-permutation networks.[3]
The publication of
the DES cipher by the United States National Bureau of Standards (subsequently
the U.S. National Institute of Standards and
Technology, NIST) in 1977 was fundamental in the public
understanding of modern block cipher design. It also influenced the academic
development of cryptanalytic attacks. Both differential and linear cryptanalysis arose out of studies
on the DES design. As of 2016 there is a palette of attack techniques against
which a block cipher must be secure, in addition to being robust against brute force attacks.
Even a secure
block cipher is suitable only for the encryption of a single block under a
fixed key. A multitude of modes of operation have
been designed to allow their repeated use in a secure way, commonly to achieve
the security goals of confidentiality and authenticity.
3.
Explain About Product Cipher
In cryptography,
a product cipher combines two or more transformations in a manner
intending that the resulting cipher is more secure than the individual
components to make it resistant to cryptanalysis.[1]
The product cipher combines a sequence of simple transformations such as substitution (S-box), permutation (P-box), and modular arithmetic. The concept of product
ciphers is due to Claude Shannon, who presented the idea in his
foundational paper, Communication Theory of Secrecy
Systems.
For transformation
involving reasonable number of n message symbols, both of the foregoing cipher
systems (the S-box and P-box)
are by themselves wanting. Shannon suggested using a combination of S-box and
P-box transformation—a product cipher. The combination could yield a cipher
system more powerful than either one alone. This approach of alternatively
applying substitution and permutation transformation has been used by IBM in
the Lucifer cipher system, and has become the
standard for national data encryption standards such as the Data Encryption Standard and the Advanced Encryption Standard. A product
cipher that uses only substitutions and permutations is called a SP-network.
Feistel
ciphers are an important class of product ciphers.
Review Question
1. 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.
2.
What
is the difference between a block cipher and a stream cipher?
3. 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.
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Posted by Samini on Monday 17 July 2017 - Rating: 4.5
Title : Answer Question cryptographic
Description : KeyTerm Explain 1. Explain the avalanche effect? A desirable property of any encryption algorithm is that a small change in ei...
Description : KeyTerm Explain 1. Explain the avalanche effect? A desirable property of any encryption algorithm is that a small change in ei...
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