Public Key System for Encryption and Digital Signatures
Copyright (c) Lloyd Miller, 1985
Calgary, Alberta
Version 0.0
and
DECRYPT. Worked fine
Distributed as three
programs, GENKEYS, ENCRYPT,
Version 0.01
Combined the three programs into one
executable,
PKSCRYPT, to save space on my disk. The same key files and encrypted
files are
used.
Version 0.01a
spelling
corrections.
Update to Version 2.0
of the compiler. Some
Version 0.01b
Eliminated last few remaining REALS. Added
a few
"Please Wait" messages. Documented the command line operation which has
always
been available.
Public Key Encryption is a system of encrypting messages
where the
encryption and decryption keys are different. The, so called,
public
encryption key, used to encrypt messages is related to the private
decrption
key but it is very difficult to figure out the decyrption key if you only
know
the encryption key. (If you think this is confusing, you're right.)
To send a secret message to someone you need to know only their
public
key. For anyone to send you a secret message, you need to let them know
your
public key. I have included my public key with these files so that anyone
who
gets this system can send me a secret message (I don't know why they
might
want to but they can). If someone wants me to reply (secretly) then they
would
have to send me their public key.
Messages can also be encrypted with the private key. This is
called a
signature. Anyone who knows the public key can decrypt the message but
only
the person who knows the corresponding private key could have
created the
message. I have included a message signed by me in this archive that can
only
be decoded with my private key and thus could only have been encrypted
by my
private key. Since I havn't given my private key to anyone and I don't
think
anyone could have figured it out, you should be able to safely assume
that I
wrote the orginal message. Signatures are not quite as useful as the
normal
encryption operation but they do have some very interesting capabilities.
Messages can be encrypted more than once. One use for this feature
is to
sign a secret message. To do this you encrypt the message with both the
your
private key and the recipient's public key. The combination is a message
which
only the recipient can read and only you could have created. In order to
read
the message it must be decrypted twice using the recipeint's private key
and
your public key in the reverse order that it was encrypted.
The difficulty of "breaking" the key is dependant on the length of
the
keys used. Keys of 8 digits (the smallest ones supported by this package)
are
actually quite easy to break. I havn't really tried it but a key of
even 50
digits is estimated that it could be decoded in about 4 hours using a
super
computer and the fastest factoring algoritm known. A key of 75 digits
would
take 104 days and a key of 200 digits would take 3,800,000,000 years. For
more
on this see the ACM article.
The disadvantage of longer keys is that it takes longer to find and
create
the key and the encryption/decryption processes are slower as well. A 8
digit
key will only take about 1/2 minute to generate with this software
on a
standard IBM-PC. A 20 digit key takes about 90 sec. A 50 digit key,
about 10
min. A 200 digit key will take about 6 to 12 hours. These times may be
longer
or shorter depending on your luck on the random number generator.
PKSCrypt is a program to perform the three basic functions reqired
for a
complete public key encryption system. The first question it asks is
what
function you want to perform, Genkeys, Encrypt, Decrypt, or Quit.
GENKEYS is the function to generate encryption and decryption keys
for the
RSA (Rivest, Shamir,
Adleman) system of
public key encryption.
See
Communications of the ACM, February, 1978.
GENKEYS takes, as the first input parameter, a single number from 8
to 200
designating the number of digits desired in the key. This number
determines
the level of security of the resulting cypher. It also determins the
amount of
time needed to generate a key.
Also input is a name for the
character
filename without extension.
key to be generated. This is an
8
Output of the function is two files (besides all the stuff
displayed on
the screen to keep you abrest of progress). The first is the public key
file
called keyname.KEY. It contains two numbers, each of which contains, at
most,
the number of digits requested. The first number is called "n" and
is the
product of two large prime numbers (p and q). The second number is called
"e"
and is a number which is relativly prime to (p-1)*(q-1). These numbers
are
the, so called, public key. This file is the one to be given out
and/or
published for people who want to send you secret messages.
The second file produced is called keyname.KDY. It also has two
numbers in
it. The first is "n" and is the same as the "n" in the public key. The
second
number in this file is called "d" and is the secret decryption key.
"d" is
constructed
your
private key
must be
kept secure
when
not in use,
such that (d * e) MOD ((p -
1) * (q - 1)) = 1. This file
is
for decrypting messages encrypted with your public key and
and private. Perhaps keep it
on a floppy disk and locked up
perhaps with extra copies in separate places.
The key files are only significant up to the end of the second
number in
them so comments may be added at the end of the file. This is
particularily
usefull for the public key files where the name and/or address can be
added to
the key to indicate the owner of the key. For an example of this, see the
file
LLOYD.KEY included in this archive. That file is my public key and can be
used
to send me secret messages. When you edit these files make sure your
editor
does not break any long lines into shorter lines. For instance a 200
digit key
would have over 200 characters in each of the first two lines. These
lines
must not be broken up into shorter lines.
The other two functions, ENCRYPT and DECRYPT, use the key files
created by
GENKEYS to encrypt and decrypt other files. The time required for
these
functions to do their job depends on the length of the key used and the
size
of the file. Any file can be encrypted with any key. They can
only be
decrypted with the opposite key of the pair.
ENCRYPT, when selected, will ask for the name of the file to be
encrypted
and the full name of the key file to be used for encryption (ie
"LLOYD.KEY").
Third and last it will ask for a file to store the encrypted file in.
ENCRYPT
saves the name of the key file used to encrypt in the output file in
plain
text so that the decrypt system can easily figure out which key is
needed. It
does not store any keys in the encrypted file, just the key file name.
Encrypt
also saves the input file name and size in the encrypted file. This is so
the
decryption can reproduce the origninal file. The input file name and size
are
encrypted so no extra information is obtainable from the encrypted
file
without the decryption key.
DECRYPT first asks for name of the encrypted file. It reads the
header and
offers you the default name for the decryption key. (I.E. if the
file was
encrypted with a key called "test.key" it will ask if you want to decrypt
with
"test.kdy".) Normally you would anser "yes" to this question. If you
have
renamed the key file you want to use then answer no and type in the name
you
want to use. If you tell the program the wrong key to use then nothing
will
work right after that. DECRYPT then decodes the first part of the
file to
obtain the original file name and size. Once it has decoded this
information
it will ask you if you want to use the original name for the decoded
file or
you can enter a new filename. DECRYPT will then procede to decrypt the
rest of
the file.
PKSCrypt can be operated from the command line if you know in advance
the
answers to ALL the questions which will be asked just type them following
the
command to start up the program. ie:
pkscrypt genkeys 10 temp quit
pkscrypt e temp.key temp.dt1 temp.dt2 q
pkscrypt decrypt temp.dt2 yes yes quit
Anyone can make as many copies of this software as they want and give
them
away so long as the programs and this documentation are included in any
copies
and all files are in their original unmodified form. In fact
you are
encouraged to distribute this software as much as possible by whatever
means
you can find, electronic, magnetic, optical or via any future
technologies
which may be developed. If you find this system usefull then you can
encourage
me to write and distribute more of my software by sending $30 to:
Lloyd Miller
2420 Capitol Hill Crescent, N.W.
Calgary, Alberta
T2M-4C2
If you are a corporate or governmental user then the $30 is not
optional.
If you need an invoice in order to pay for this please feel free to make
one
out. If you send me a SSAE I will send you a real invoice.
This software and documentaion is protected by copyright and if you
don't
think that means much then ask a lawyer.
If you have comments, complaints (heaven forbid) or questions then
you can
either send a SASE to the above address or send a message to me via
Fidonet(TM
of Tom Jennings) node 134/1, "The Calgary Fido". If you want to send
encrypted
messages via Fidonet then you will need your sysop's co-operation to
send
"attached files". If you want to send encrypted messages via traditional
mail
then you would have to send floppy disks.
This software is designed to run on an IBM-PC using MS-DOS version
2.00 or
later. I have tried to avoid using any features of the IBM-PC
which are
machine dependant so I believe it should work on any system which uses
MS-DOS
or PC-DOS but it has not been tested on many other types of systems. I
would
like to hear from you if you have tried it on any other systems and if
you
have had any probelms or not. I have tested the system on a Sanyo MBC 555
and
it works fine.
These programs are written in Modula-2 using the Logitech
Modula-2/86
compiler system. I would be willing to consider converting these programs
for
use with other Modula-2 systems if anyone wants to pay me to
do the
convertion. I would not consider converting the programs to another
language.
The nature of these programs would probably eliminate from consideration
any
M-CODE or P-CODE interpreter style compiler systems since they would
become
prohibitivly slow.
Lloyd
Miller
1986
February 25