الفهرس 
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Abstract
This thesis proposes a new secure Blowfish algorithm for generating sub
keys (S-boxes and P-arrays) which are basic components of symmetric
key algorithm and are used to make the relationship between the key and
the cipher text non linear based on Linear Feedback Shift Register
(LFSR) which generates the S-boxes and P-arrays.
Blowfish algorithm has 64 bits block size and a variable key length from
32 up to 448 bits. One of the main disadvantages of Blow fish algorithm is
the time required to initialize the algorithm with the key (P-arrays and
S-boxes) with the hexadecimal fraction of pi. The P-array consists of 18
32-bit values while the 4 S-boxes consist of 256 32-bit values. So, the
Blowfish algorithm requires 521 encryptions of itself to generate the sub
keys namely 18 entries in P-arrays and 1024 entries in S-boxes.
The proposed design replaces the 521 encryptions by adapting linear
feedback shift register (LFSR) in order to generate key dependant
P-arrays and S-boxes and speed up the Blowfish algorithm, which
requires over 4Kbytes of memory. So, Blowfish algorithm is not suitable
for applications in embedded systems, in which the secret key changes
frequently. The proposed speech encryption system has been
implemented using Matlab and the output is analyzed using Avalanche
test.
The results show that the modified design of the Blowfish algorithm
continues to offer the same level of security as the original Blowfish
cipher with a less computational overhead in key generation. The
modified Blowfish algorithm is used to transfer voice, text, and image
,
over the intemet because it does not require memory.