PUBLIC KEY ENCRYPTION demonstration software by D. Canright



Use the scroll bar or PgUp, PgDn, and arrow keys to scroll through 

this information.  To resize the window, drag lower right corner.



OVERVIEW:

This demo illustrates the basic mathematics behind the RSA public-

key encryption method.  The essence of public-key schemes is that 

two keys (numbers) are chosen such that information encoded using 

one key can only be decoded using the other key.  (Actually, a third 

number is also used, called the modulus, the same in both encoding 

and decoding.)  So a person, say Alice, can make public one key (and 

the modulus), so anybody else can encode a message to Alice that 

only she can decode with her secret key.  Another application is 

that Alice can write a message and encode it using her secret key, 

then anybody else can decode it using Alice's public key, and so be 

sure the message really came from Alice (this is called signature 

verification).



Basic Steps for Alice to send a secret message to Bob:

(1)  Bob 

     (a)  generates a keyset, 

     (b)  saves it to a file, and

     (c)  gives the key file to Alice.

[Note: Bob only needs to do this ONCE; the same keyset can be used 

for all the messages addressed to Bob, from anyone.]

(2)  Then Alice

     (a)  opens Bob's key file,

     (b)  writes a new message,

     (c)  encodes it with BOB's PUBLIC key,

     (d)  saves it to a file, and

     (e)  gives the message file to Bob.

(3)  So then Bob

     (a)  opens the message file from Alice, and

     (b)  decodes it with his PRIVATE key.

[Note: nobody but Bob needs to know his private key.]



COMMAND SUMMARY: [Most commands have Alt- or F- key shortcuts]

File menu [Alt-F]:

   New Key [Alt-K] create a new key set

   Save Key [Alt-S] save the current key set to a .KEY file

      (NOTE: this does NOT save the private key, so remember it!)

   Open Key [Alt-O] open a .KEY file

   New Msg [Alt-M] create a new message

   Save Msg [Alt-V] save the current message to a .MES file

   Open Msg [Alt-P] open a .MES file

   Exit [Alt-X] terminate this program

Code menu [Alt-C]:

   Clip Key [Alt-L] put the current keyset on the key clipboard

   Encode Msg [Alt-E] encode the text of the current message

   Decode Msg [Alt-D] decode the secret of the current message

Advanced menu [Alt-A]: (see below for more details)

   Double Encode [Alt-B] encode secret again with outer key

      (NOTE: do this AFTER encoding text with inner key, see below)

   Double Decode [Alt-U] decode secret once with outer key

      (NOTE: do this BEFORE decoding with inner key, see below)

   Encode File [F7] encode ASCII text file (.TXT), save to .PKE file

   Decode File [F8] decode secret file (.PKE), save to .TXT file

   Double Encode File [F9] double encode .TXT file to .PKE file

   Double Decode File [F10] double decode .PKE file to .TXT file

Windows menu [Alt-W]:

   Move [Ctrl-F5] move current window by arrow keys

   Next [F6] switch to next window in sequence

   Previous [Shift-F6] switch to previous window in sequence

   Close [Alt-F3] close the current window

Help menu [Alt-H]:

   Help Window [F1] show the help window (this window)

   How PKE Works [F2] show the window about how the program works

   About PKE [F3] show the window about this program (in general)



HOW TO USE THIS PROGRAM:

(NOTE: all the files created in this program are plain ASCII text 

files, and can be sent through e-mail either as attachments or cut 

and pasted.)



First:

Choose a secret key (some odd number from 3 to 30001) using the New 

Key command. This generates a new keyset (private key, public key, 

and modulus) in a Key Window. REMEMBER the number you chose; this is 

your private key, so you should not tell it to anyone. Then Save 

this keyset to a .KEY file (your private key does NOT get saved, so 

if you forget it, you can't use this keyset any more). Then give 

copies of your .KEY file to your friends so they can use PKE to 

encode messages to you (using your public key), and the messages can 

only be decoded with your private key.



To pass a secret message to a friend:

Open the .KEY file that your friend gave you; this will show your 

friend's public key (and modulus) in a Key Window. Use the Clip 

command to put the key on the Key Clipboard. Then use the New 

Message command to get a blank Message Window. Type your message in 

the Message Text line (one line only; you can edit the message if 

you change your mind). Then Encode the message using your friend's 

public key from the clipboard, and Save the secret to a .MES message 

file. Give your friend the .MES file.



To read a secret message from a friend:

Open the .MES file that your friend gave you (this should have been 

encoded using your public key, as above). Decode the secret using 

your private key. (If you still have the new Key Window open from 

when you created your keyset, then you can Clip it to the clipboard 

before you decode, and paste your private key. Otherwise, you have 

to type in your private key. You do remember it, don't you?)



ADVANCED APPLICATIONS:

Besides sending short secret messages, this program can be used for 

some other typical applications of public key encryption, such as 

signature verification and encryption of files.



Signature Verification:

This is where you encode a message using YOUR PRIVATE key. Then 

anyone else can decode it using your PUBLIC key. So if your friend 

decodes it with your public key, he/she knows that the message must 

have come from you, because nobody else knows your private key. That 

way, you have used your private key as a digital signature. (This 

idea is important when doing business over the internet, to make 

sure people are who they say they are!) But since your public key is 

public (known to many people), your message is not secret; to keep 

it secret and digitally signed, use double encoding (see below).



Double Encoding:

To send a secret message to a friend and digitally sign it (see 

above), you must encode it twice, once with your private key, once 

with your friend's public key: First encode the message with your 

own private key (using the Encode command as usual). Then use the 

Double Encode command to encode the secret again, and use your 

friend's public key. Save the message to a file, and give your 

friend a copy. Then your friend uses the Double Decode command with 

his/her private key, and then the Decode command with your public 

key, to read the message. That way, nobody else can read the 

message, and your friend knows it could only have come from you.



File Encoding:

PKE can also encode files in ASCII text (.TXT) format (not other 

word-processor formats). So you can use the NotePad program, for 

example, to write a long letter, and save it to a .TXT file in the 

PKE directory. Then in the PKE program, use the Encode File command 

(with your friend's public key, for example) to encode it into a 

.PKE file. Give the .PKE file to your friend, who would use the 

Decode File command (with his/her private key) to get back the 

original .TXT file to read. (Note: you could also use file encoding 

to write secret diary files; encode it with your own public key and 

then only you can decode it with your private key, which you never 

tell anyone.)

You can also double-encode files (to digitally sign secret letters, 

for example) using the Double Encode File command, and your friend 

would use the Double Decode File command.



DISCLAIMER: This demonstration software was created for educational 

purposes only, to show how public-key encryption (in particular the 

RSA algorithm) works, and to be fun. This program is NOT intended 

for serious encryption of sensitive information. See the About PKE 

window for information on serious encryption products.