This document discusses block ciphers, including their definition, structure, design principles, and avalanche effect. A block cipher operates on fixed-length blocks of bits and uses a symmetric key. It encrypts bits in blocks rather than one by one. Block ciphers have advantages like high diffusion but are slower than stream ciphers. They are built using the Feistel cipher structure with a number of rounds and keys. Important design principles for block ciphers include the number of rounds, design of the round function, and key schedule algorithm. The avalanche effect causes a small input change to result in a significant output change.

1. Block Cipher and its Design
Principles
Prof. Neeraj Bhargava
Mrs. Shubha Chaturvedi
Department of Computer Science, School of Engineering &
System Sciences
MDS University Ajmer, Rajasthan

2. Block Cipher
• In cryptography a block cipher is a deterministic
algorithm operating on fixed-length groups
of bits, called blocks. It uses an unvarying
transformation, that is, it uses a symmetric key.
• Block ciphers take a number of bits and encrypt
them as a single unit, padding the plaintext so
that it is a multiple of the block size.

3. • As in the case of stream ciphers, most encryption methods encrypt
bits one by one (stream ciphers). Block ciphers, on the other hand,
encrypt 128 bit blocks with a key of predetermined length: 128,
192, or 256 bits.
• A 128-bit block cipher brings 128 bits of plaintext and encrypts it
into 128 bits of ciphertext. Where the amount of plaintext is less
than 128 bits, in this example, the cipher will employ methods to
reconcile the difference (padding schemes).
• Block ciphers have the advantage of high diffusion and strong
tamper resistance without detection. They have the disadvantage
of slower encryption speed since the entire block must be captured
for encryption/decryption. Block ciphers also breed errors since a
mistake in just one symbol could alter the whole block.

4. Diagram for Block Cipher

5. Block Cipher Design Principles
• Block ciphers are built in the Feistel Cipher
structure. Block cipher has a specific number
of rounds and keys for generating ciphertext.
For defining the complexity level of an
algorithm few design principles are to be
considered.

6. These are explained as following below :
• Number of Rounds –
The number of Rounds is regularly considered in design criteria, it
just reflects the number of rounds to be suitable for an algorithm to
make it more complex, in DES we have 16 rounds ensuring it to be
more secure while in AES we have 10 rounds which makes it more
secure.
• Design of function F –
The core part of the Feistel Block cipher structure is the Round
Function. The complexity of cryptanalysis can be derived from the
Round function i.e. the increasing level of complexity for the round
function would be greatly contributing to an increase in
complexity.To increase the complexity of the round function, the
avalanche effect is also included in the round function, as the
change of a single bit in plain text would produce a mischievous
output due to the presence of avalanche effect.
• Key schedule algorithm –
In Feistel Block cipher structure, each round would generate a sub-
key for increasing the complexity of cryptanalysis. The Avalanche
effect makes it more complex in deriving sub-key. Decryption must
be done very carefully to get the actual output as the avalanche
effect is present in it.

7. Avalanche Effect
• In cryptography, the avalanche effect is the desirable property
of cryptographic algorithms, typically block
ciphers and cryptographic hash functions, wherein if 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.

8. ASSIGNMENT
• Q. List the types of Block Cipher methods ?
• Q. Explain the Feistel Cipher Structure?