Cryptography Algorithm Interview Questions and Answers

1 :: How does one use DES securely?

If we use Triple DES it may be secure

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2 :: What are secret sharing schemes?

deffie hellman

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3 :: What are SHA and SHA-1?

SHA is a secure hash algorithm developed by national
institute of standard technology
SHA-1 is a type of SHA
message digest size of SHA 1 Is 160 bits and
message size is < 2^64
it uses block cipher
sha is a digital signature algorithm

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4 :: What is random number generation?

a random no. is a number used for checking freshness of
session. It is used to avoid replay attack

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5 :: What is the AES?

1)AES is ADVANCE ENCRYPTION STANDARD its is a symmetric key
block cipher .
2)AES has number of rounds each round makes a number of
transformations on a state and uses a round key derived
from encryption key.
3) number of block depends upon block size
4)an encryption of a block starts with a transformation
AddRoundkey this is followed by and odd number of regular
rounds and ends with a special final round.
5)the final round helps in decryption.
6) basic principle used is substitution permutation network.
7)aes has block size of 128 bits and key size of 128 192 or
256 bits

8)rounds are :
1)addroundkey-each byte of the state is combined with round
key using bitwise X-OR.
2)subbytes- each byte is replaced by another byte according
to lookup table
3)shiftRow-a transposition step where each row of the state
is shifted cyclically a certain number of steps
4)mix column-combination of 4 bytes takes palce in each column
5)final round

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