2. STREAM CIPHER
A stream cipher is a symmetric key cipher where
plaintext digits are combined with
a pseudorandom cipher digit stream (keystream)
In a stream cipher each plain text is encrypted one at a time with the
corresponding digit of the key stream, to give a digit of the cipher text
stream.
An alternative name is a state cipher, as the encryption of each digit is
dependent on the current state.
In practice, a digit is typically a bit and the combining operation
an exclusive -or (XOR).
4. BLOCK CIPHER
A block of plaintext is treated as a wholeand used to
producea ciphertext block of equal length
Typicallya block size of 64 or 128 bits is used
As with astream cipher, the two usersshare a
symmetric encryption key
7. A block cipheroperateson a plaintext block of n bits toproduce
a ciphertext blockof n bits. Thereare 2n possible different
plaintext blocks and, for the encryption to be reversible (i.e., for
decryption to be possible), each must produce a unique
ciphertext block. Such a transformation is called reversible, or
nonsingular.
9. THE FEISTAL CIPHER
Feistel cipher is theexecution of twoor moresimple
ciphers in sequence in such a way that the final
result or product is cryptographically stronger than
any of the componentciphers.
Feistel ciphers are a special class of iterated block
cipherswhere theciphertext is calculated from the
plaintext by repeated application of the same
transformation or roundfunction.
10. In a Feistel cipher, the text being encrypted is split into
two halves. The round function f is applied to one half
using a subkey and theoutputof f is exclusive ORed with
other half.
The two halves are then swapped. Eachround follows the
same pattern except forthe last round there is no swap
A nice feature of a feistel cipher is that encryption and
decryption are structurally identical, though thesubkeys
used during encryption at each round are taken in reverse
order duringdecryption
11. DIFFUSION AND CONFUSION
Diffusion:
Diffusion means that if we change a character of the
plaintext, then several characters of the ciphertext
should change, and similarly, if we change a
characterof theciphertext, then several characters of
the plaintext shouldchange
diffusion refers to dissipating(unwanted) thestatistical
structureof plaintextover bulk of ciphertext.
12. Confusion:
Confusion means that each binary digit (bit) of the
ciphertextshould depend on several partsof the key,
obscuring the connections between thetwo.
Confusion means that each character of the
ciphertextshould depend on several partsof the
key.
13. DESIGN FEATURES IN FISTEL
CIPHER
eBlock size
Larger block sizes mean greater security but reduced encryption/decryption
speed for a givenalgorithm
Keysize
Larger keysize meansgreatersecurity but maydecreaseencryption/decryption
speeds
Number of rounds
Theessenceof the Feistel cipher is thata singleround offers inadequatesecurity
but that multipleroundsoffer increasing security. A typical size is 16 rounds.
Subkey generationalgorithm
Greatercomplexity in thisalgorithmshould lead togreaterdifficultyof
cryptanalysis
eatures
14. DESIGN FEATURES IN FISTEL
CIPHER
Round function F
Greater complexity generally means greater resistance to
cryptanalysis
Fast softwareencryption/decryption
In many cases, encrypting is embedded in applications or utility
functions in such a way as to make impossible a hardware
implementation; accordingly, the speed of execution of the
algorithm becomes a concernsoftware
Ease of Analysis
If thealgorithmcan be conciselyand clearly explained, it is easierto
analyze that algorithm for cryptanalytic vulnerabilities and
thereforedevelopa higher level of assuranceas to its strength
