elgencrypt.m

elgamal elgrithm by matlab

function ciphertext = ELGencrypt(p,Alpha,Betaa,m)

global y1 y2 

% This function encrypts the message typed by the sender using the 
% public key Betaa the primitive root Alpha and the large prime p.

% Get the message length
message_length = numel(m);

% Convert all characters in the message to number and store in mnum_array
n = 1;
j = 2;
mnum = '';

% For the first character, we don't need any zero infront of the number (if the number is less than 10)
h =text2int1(m(1));
mnum_array(1) = h;

% For the rest of the message, if the number is less than 10 we need to put 0 in front of that number
n= 2;
while (j <= message_length)
  h = text2int1(m(j));
  if ( h <= 9 )
    mnum_array(n) = 0;
    mnum_array(n+1) = h;
    n = n + 2;
  else
    mnum_array(n) = h;
    n = n + 1;
  end;
  j = j + 1;
end;

% Put every values in the array together and assign to variable mnum (numberic message)
mnum_array_length = numel(mnum_array);
for n=1:mnum_array_length,
mnum = strcat(mnum,num2str(mnum_array(n)));
end;

maple('mnum:=',mnum);
mnum_length = numel(mnum);

% Compare mnum with p. If mnum is greater than p then set flag to 1
p_length = numel(p);
flag = 0;
if (mnum_length == p_length)
   for index=1: p_length,
     if(mnum(index) > p(index))
        flag = 1;
        break;
     end;
   end;
   
end;           
if(mnum_length > p_length)
      flag =1;
end;
  
% If mnum is greater than p, display the error message
if(flag ==1)
   disp('The message is too long, please make it shorter.')

% If mnum is less than p, do the encryption process
else
   % Generate random integer k
   maple('k:=','rand()');
   k = maple('k');
   
   % Calculate y1
   maple('y1:=','Alpha&^k mod p');
   y1 = maple('y1')
   % Calculate y2
   maple('y2:=','((Betaa&^k)*mnum) mod p');
   y2 = maple('y2')
end;