elgamal.m

elgamal elgrithm by matlab

%
%                               ElGamal PKC
%
% Functions:
%
% pub_key_gen
% encrypt(p, Alpha, Betaa, m)
% decrypt(y1, y2)
%
%
%
% Use:
% 
% pub_key_gen - Generates the public key for the receiver.  This function
%		requires no arguments and simply chooses a large prime p
%		of a maximum size of 128 bits.  The receiver is to type 
%               this at the beginning to compute his or her publick key.
%
% ELGencrypt()- Encrypts the sender's message.  This function takes 4 inputs.
%		These inputs are the published values of p, Alpha, and Betaa.
%		The 4th input is the message that the sender plans to send.
%
%		This function is called from within the encrypt.m script as
%		the sender does not need to call it explicitly but only needs
%		to type encrypt followed by the message after being prompted.
%
%		The message cannot contain any spaces and if it is too long
%		will generate an error.  If a long message needs to be sent
%		the sender will need to break it up into manageable portions
%		and send each portion individually by calling the encrypt 
%		function for each piece of the message.
%		
%		The sender must allow the receiver to decrypt each part of 
%		the message before sending the next part.
%
%		The size of the message depends on the chosen p.
%
% decrypt()   - Decrypts the message encrypted by the sender.  The receiver
%		supplies this function with the values produced by encrypt().
%		These values are y1 and y2 and are the only arguments needed
% 		for the function to decrypt the ciphertext.	
%
% Note:  	The user simply calls these functions and uses the variables 
%	 	within the calls (i.e p, Alpha, Betaa, y1, y1) instead of 
%	 	copying the values of those into the arguments.  
% 
% Secret key  - It is important to note that the secret key for this PKC is 
%		already set to a predetermined value.  For this to change you
%		must manually edit the code of pub_key_gen.  The private key
%		is stored in variable 'a'. 
%		

greeting

disp(' ')
disp(' ')
answer = input('Would you like to continue and call the public key generator \nfunction? y/n\n\n','s')

if (answer == 'y')
   pub_key_gen
else
   disp('If you want to run this program again simply type "elgamal" at the')
   disp('MATLAB prompt.  To avoid the start screen you may directly type') 
   disp('"pub_key_gen" to generate your public key.')
end