Lecture Notes for Computer Architecture II - St. Cloud State University

1. Following pseudo code represents BlowFish decryption algorithm used in
cryptographic applications.
For i =1 to 16 do
RDi = LDi-1 P19-i;
LDi = F[RDi] RDi-1;
LD17 = RD16 P1;
RD17 = LD16 P12;
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Where
REi and LEi refer to the 32-bit right half and 32-bit left half of the data after round i has
completed.
Pi represents the 32 bit sub key used at stage i.
The 32-bit input to function F is divided into 4 bytes a, b, c and d. Then the function F is
defined as F(a, b, c, d) = ((S1,a + S2,b) S3,c) + S4,d . Each S-box, S1,a , S2,b ,
S3,c,and S4,d generates 32-bit output from 8-bit input.
Draw the hardware organization for this special purpose algorithm.