Question

(a) What is the difference between synchronous and asynchronous stream ciphers.

(b) Who was Horst Feistel?

(c) What is the main weakness of affine ciphers?

(d) What is the Kerckhoffs’ principle?

(e) Suppose a block cipher uses m-to-m bits S-boxes. How many bits are required to store the look-up table of k different such m-to-m bits S-boxes? Give a formula in terms of k and m. Hint: A look-up table for one DES S-box requires 256 bits of storage.

Answer 1

Ans(a)- synchronous stream ciphers-

• synchronous stream cipher a stream of pseudo- random digit generated independemtly of the plaintext and cipher text messages.
• the plaintext using the exclusive or operation(xor)
• they are combined plaintext and ciphertext.
• plaintext basedon to encrypt and ciphertext based on decrypt.
• synchronous stream using self-synchronising steam cipher update their state based on previous ciphertext digits.

Asynchronous stream cipher

asynchronous stream cipher depends on ciphertext.

asynchronous called self synchronizing steam cipher.

there are using limit error transmission.

asynchronous depands on security of stream cipher.

Ans(b)-Horst Feistel

Horst Feistel was a german-american cryptographer it worked on design of cipher at ibm. the development of data encryption standard in the 1970. the des another called feistel network is commonly used many block cipher.the use of the des algorithm was made mandatory for all financial transaction of us. government involving electronic fund transfer,including those conducted bu member banks of federal reserve system. des used standard for business and commerical data security as well.

des is product block cipher in which 16 iterations or rounds of substitution and transposition process are cascaded the block size 64bits.and the remaining 8 are parity check bits.

Ans(c)- weakness of affine ciphers-   

decryption is possible if the affine function is injective. the affine cipher involes case of monoalphaabetic sustitution cipher.

weakness of affine ciphers is less than secure than a subsitution cipher .it is vulnerable of all of the attacks that work against substitution ciphers,

the cipher's is mainly weakness facts the cryptanalyst can mean of frequency analysi bute force ,guessing the plaintext of two ciphertext character then the key can be obtaing solving as equestion.

Ans(d) Kerchoff's Principle

the security of crptosystem should not rely on a secret that 's hard to change.

• don't have secret encryption methods.
• have a large class of encryption methods instead .
• make the class public information.
• use a secret key to specify which one you are using .
• it is easy to change the key .
• the only secrecy involved with cryptography system should be the key.
• the algorithrm should pubicly known.
• if security were based on many secrets.

Ans(e)-DES - is a symmetric key block cipher published by (nist) National institute of standards and technology.

it is based on feistel cipher. it uses 16 round in feistel structure and the block size is 64 bit key length is 64 bit des has effective key length of 56 nit .64 bits of the key are not used by the encryption algorithm .

• it is required to des-
• round function
• key schedule
• any additional processing

Round function- it is based on des function f .the des function applied a 48 bit key to rightmost 32 bit and output 32 bit .

Key Schedule- key schedule based on round key generrator creates 16 ,48 bit keys out of 56 bit cipher key. this process of key generation.

Any additional Processing-(initial and final permutation)-

the initial and final permutations are straight permutation boxes (p) . it is not use of crptography significance in des.

S-box- is a based on symmetric key algorithm . they are involve between key and the ciphertext - shannon's property of cunfusion .s-box gives some number of bit ,m taranforms them some number of output n .an m*n s- box implemented as lookup table with 2^m words of n bit each .