Consider the followingsubstitution-based cryptosystem. Input alphabet ={a,b,c,d,
ID: 3612303 • Letter: C
Question
Consider the followingsubstitution-based cryptosystem.
Input alphabet ={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z} (26symbols)
Output alphabet = Letters + Numbers;where:
Letters ={A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z};
Numbers = {0,1,2,3,4,5,6,7,8,9} (atotal of 26 + 10 = 36 symbols)
Substitutionmethod:
(1) The parties agree on a secretword, whose length is between 5 and 26 symbols taken from the"Letters" set. Let us call this word the KEY. The secret word mustbe such that no symbols of "Letters" are repeated in it. (Forinstance, KEY = ANALYSIS would notsatisfy this property.)
(2) The sender generates a randomnumber between 1 and 26. Let us call this number n.
(3) The sender aligns the secretword KEY under the n-th character of "Input alphabet" (depending onthe length of KEY, wrap-around might be needed)
(4) All remaining symbols in "Inputalphabet" are substituted by the symbols in "Letters" which are notin KEY, in the order they appear in "Letters".
(5) The sender uses thissubstitution to encrypt the message.
(6) The sender sends the randomnumber n and the encrypted message to the receiver.
For instance, consider KEY =SOCRATED. The following substitution and encryption result fromapplying steps (1)-(6) above, when the random number isn=10.
abcdefghijklmnopqrstuvwxyz
BFGHIJKLMSOCRATEDNPQUVWXYZ
Explanation / Answer
0m