1.
CSS is introduced in 1996. It is a 40 bit stream cipher that uses 2 linear feedback shift registers
(LFSRs) of 17-bit and 25-bit and denoted by R 1 and R 2. The feedback polynomials for R 1 and R
2 are respectively:
C 1 (x) = x 17
+ x 2
+1 and C 2 (x) =x 25
+ x 21
+ x 20
+ x 10
+ 1
The LFSRs are initialized with the 40-bit key. One bit of each LFSR is set to 1 to prevent the 0
state which generate only 0 keystream. A byte of the keystream is generated by 8 shifts of both
registers and the use of an 8-bit adder with carry.
Encryption and decryption are done by bit-xor input bytes with keystream bytes. Write an
application implement CSS with given specifications to do both encrypt and decrypt of any
binary file.
Write a report to document your CSS application. Also, discuss the possible attacks on this
cryptosystem
Solution
Ans;
FOLLOW THIS ou have two vectors r(t) .- Each one defines a space curve , r1 = (t , (1-t)
,(3+t^2) ) r2 = ( (3-s), (s-2), s^2 ) they intercept when r1=r2 , ie , t= 3-s (1-t) = s-2 3+t^2 = s^2
s+t= 3 s+t= 3 ( coupled) t= 3-s 3+(3-s)^2 =s^2 3+9-6s+s^2 =s^2 6s= 12 s=2 t=1 plug in r1 Point
is P(1,0,4) The angle , dr1/dt = (1,-1, 2t) at t=1 dr1/dt = (1,-1,2) = U dr2/ds = (-1,1,2s ) at s= 2
dr2/ds = (-1,1,4) = V U dot V = IUI IVI cos T (-1 -1+8) = (2)(sqrt18) cos T = 6 sqrt 2 cos T cos
T = 1/sqrt2 = (1/2) sqrt2 T= pi/4