decrypt.m

elgamal elgrithm by matlab

function message = decrypt(y1, y2)

% Uses y1 and y2 to decrypt message

% Get y1 and y2 value
maple('y1:=',y1);
maple('y2:=',y2);

% Calculate the numberic plain text using y1 and y2 from the sender
maple('num_plaintext :=', '(((y1&^a)&^(-1))*y2) mod p');
num_plaintext = maple('num_plaintext');

% Convert numberic plain text to characters
num_plaintext_length = numel(num_plaintext);
odd = rem(num_plaintext_length,2);

% If the length of numberic plain text is even number
% This means the numberic value of the first character is equal or greater than 10
% We can convert every 2 consecutive numbers to one character
if odd==0,;
z=1;
plaintext = '';
for n=1:num_plaintext_length/2,;
    q = ((num_plaintext(z)-48)*10)+(num_plaintext(z+1)-48);
    plaintext = strcat(plaintext,char(q+'a'-1));
    z=z+2;
end;
  message = plaintext
end;

% If the length of numberic plain text is odd number
% This means the numberic value of the first character is less the than 10
% We convert first single number to the first character
% For the rest of them, we convert every 2 consecutive numbers to one character
if odd==1,
m = floor(num_plaintext_length/2) + 1;
q = num_plaintext(1)-48;
plaintext = char(q+'a'-1);
z=2;
for n=2:m,
    q = ((num_plaintext(z)-48)*10)+(num_plaintext(z+1)-48);
    plaintext = strcat(plaintext,char(q+'a'-1));
    z=z+2;
end;
  message = plaintext
end;