Re: Pls help

manmohan2k@gmail.com wrote:
> Design and Implementation of LFSR based stream cipher
>
> Suppose in a system there are 4 shift register. Each shift register
> will generate a PN sequence. The length of sequence will depend on the
> size of shift register and the total period of the system will be the
> LCM of the periods of the 4 shift registers. The
> Keystream of the 4 shift registers are mixed with the input data
> bits(plain text) using the XOR operation. This will be the output
> sequence bit (ciphertext)
> Mathematically if X1,X2,X3,X4 are the output sequence bit of the 4
> shift registers R1, R2, R3 and R4 and P is the plain text(in ASCII)
> form . Then we can say
>
> PTK = C
> Where value of Keystream(K) is calculated by using the function
>
> (X1^X2)T X3TX4 = K

Your notation is not clear. What is T? Do you mean matrix transpose
in GF(2)?

Anyway LFSR is not secure. With one register, you trivially output the
state in you keystream, which is your secret. With multiple registers
where you have some combination (say, XOR), each keystream bit depends
on only a few bits of state. This is bad, because the attacker can
quickly narrow down the possibilities. If the attacker can guess a
small bit of plaintext and recover the corresponding keystream then he
can decrypt the rest.

-- Matt Mahoney
Received on Thu Sep 29 21:44:38 2005