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UNIT-2
Conventional encryption principles, conventional encryption algorithms, cipher block
modes of operation, location of encryption devices, key distribution approaches of message
authentication, secure hash functions and hmac
Conventional Encryption principles
A Symmetric encryption scheme has five ingredients
1. Plain Text: This is the original message or data which is fed into the algorithm as input.
2. Encryption Algorithm: This encryption algorithm performs various substitutions and transformations on
the plain text.
3. Secret Key: The key is another input to the algorithm. The substitutions and transformations performed
by algorithm depend on the key.
4. Cipher Text: This is the scrambled (unreadable) message which is output of the encryption algorithm.
This cipher text is dependent on plaintext and secret key. For a given plaintext, two different keys produce
two different cipher texts.
5. Decryption Algorithm: This is the reverse of encryption algorithm. It takes the cipher text and secret key
as inputs and outputs the plain text.
Two main requirements are needed for secure use of conventional encryption:
(i). A strong encryption algorithm is needed. It is desirable that the algorithm should be in such a way that,
even the attacker who knows the algorithm and has access to one or more cipher texts would be unable to
decipher the cipher text or figure out the key.
(ii).The secret key must be distributed among the sender and receiver in a very secured way. If in any way
the key is discovered and with the knowledge of algorithm, all communication using this key is readable.
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Cryptography
A cipher is a secret method of writing, as by code. Cryptography, in a very broad sense, is the
study of techniques related to aspects of information security. Hence cryptography is concerned with the
writing (ciphering or encoding) and deciphering (decoding) of messages in secret code. Cryptographic
systems are classified along three independent dimensions:
The type of operations used for performing plaintext to ciphertext
All the encryption algorithms make use of two general principles; substitution and transposition through
which plaintext elements are rearranged. Important thing is that no information should be lost.
The number of keys used
If single key is used by both sender and receiver, it is called symmetric, single-key, secret-key or
conventional encryption. If sender and receiver each use a different key, then it is called asymmetric, twokey or public-key encryption.
The way in which plaintext is processed
A block cipher process the input as blocks of elements and generated an output block for each input block.
Stream cipher processes the input elements continuously, producing output one element at a time as it goes
along.
Cryptanalysis
The process of attempting to discover the plaintext or key is known as cryptanalysis. It is very difficult
when only the cipher text is available to the attacker as in some cases even the encryption algorithm is not
known. The most common attack under these circumstances is brute-force approach of trying all the possible
keys. This attack is made impractical when the key size is considerably large. The table below gives an idea on
types of attacks on encrypted messages.
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Cryptology covers both cryptography and cryptanalysis. Cryptology is a constantly evolving science; ciphers are
invented and, given time, are almost certainly breakable. Cryptanalysis is the best way to understand the subject
of cryptology. Cryptographers are constantly searching for the perfect security system, a system that is both fast
and hard and a system that encrypts quickly but is hard or impossible to break. Cryptanalysts are always
looking for ways to break the security provided by a cryptographic system, mostly though
mathematical understanding of the cipher structure.
Cryptography can be defined as the conversion of data into a scrambled code that can be deciphered and sent
across a public or a private network.
A Ciphertext-only attack is an attack with an attempt to decrypt ciphertext when only the ciphertext itself
is available.
A Known-plaintext attack is an attack in which an individual has the plaintext samples and its encrypted
version(ciphertext) thereby allowing him to use both to reveal further secret information like the key
A Chosen- plaintext attack involves the cryptanalyst be able to define his own plaintext, feed it into the
cipher and analyze the resulting ciphertext.
A Chosen-ciphertext attack is one, where attacker has several pairs of plaintext-ciphertext and ciphertext
chosen by the attacker.
An encryption scheme is unconditionally secure if the ciphertext generated by the scheme does not contain
enough information to determine uniquely the corresponding plaintext, no matter how much ciphertext and time
is available to the opponent. Example for this type is One-time Pad.
An encryption scheme is computationally secure if the ciphertext generated by the scheme meets the following
criteria:
Cost of breaking cipher exceeds the value of the encrypted information.
Time required to break the cipher exceeds the useful lifetime of the information. The average
time required for exhaustive key search is given below:
Key Size
Number of
Time required at 1
Time required at
(bits)
Alternative Keys
decryption/µs
106 decryptions/µs
32
232
56
256
128
2128
= 4.3
= 7.2
= 3.4
109
1016
1038
231 µs
= 35.8 minutes
255 µs = 1142 years
2127 µs
= 5.4
1024 years
2.15 milliseconds
10.01 hours
5.4
1018 years
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168
2168
= 3.7
1050
2167 µs
= 5.9
1036 years
5.9
1030 years
Feistel Cipher Structure
Most symmetric block ciphers are based on a Feistel Cipher Structure. It was first described by Horst Feistel of
IBM in 1973 and is still forms the basis for almost all conventional encryption schemes. It makes use of two
properties namely diffusion and confusion; identified by Claude Shannon for frustrating statistical cryptanalysis.
Confusion is basically defined as the concealment of the relation between the secret key and the cipher text. On
the other hand, diffusion is regarded as the complexity of the relationship between the plain text and the cipher
text.
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