MacFormat 1994 November/macformat-018.iso/Utility Spectacular/Security/MacEncrypt ƒ/MacEncrypt Doc (Text)

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MacEncrypt
File Encryption for the Macintosh



Version 2.0






by
J. Clarke Stevens

MacEncrypt


What Is It?

MacEncrypt is an application that uses the Data Encryption Standard (DES) to scramble
the bits of your files until they are totally unrecognizable.
Why Would I Want It?

Scrambling your bits (or encrypting) is useful if you don’t want other people knowing
what is in your files.  The bits are scrambled with a secret key that you type in.  Unless
someone discovers your key it is practically impossible for them to unscramble (decrypt)
your files.

MacEncrypt is intuitive and convenient.  You can encrypt files from within the
application using standard file dialog boxes or use the “drag and drop” technique to
encrypt one or many files automatically.  You can decrypt a file from within the
application or by simply double-clicking on an encrypted file.

How Do I Use It?

You can probably run MacEncrypt without reading this document, but just for the record,
here is what you need to do.

Encryption

Encryption is the process of reading an unsuspecting file, scrambling its bits with an
encryption password, and writing the scramble bits out to the file again.  MacEncrypt
uses the Data Encryption Standard (DES) for file encryption.  If you want to know more
about DES, a brief overview is included in this document.
Selecting the File

The first step in the encryption process is selecting a file or group of files to be encrypted.
This can be done in two ways with MacEncrypt.  The first method is to open the file from
within MacEncrypt.  The second method is to use the “drag and drop” technique from the
finder.

From the Application

To select a file for encryption from the application, open MacEncrypt by double-clicking
on its icon (the TOP SECRET manila folder with an “application” hand).  You should see
a menu bar appear with “File” and “Options” menus.  There is also an “About
MacEncrypt” item under the apple menu.
Choose the “Encrypt…” item from the “File” menu and you should get a standard file
dialog box.  Use the regular techniques for selecting a file to be encrypted.
From the Finder

You can also select a file or group of files to be encrypted using the “drag and drop”
technique.  To do this, use the shift-click combination to select any files you want to
encrypt.  Release the shift key and click on one of the hilighted files.  While holding
down the mouse button, drag the files and drop them (release the mouse button) on the
MacEncrypt application icon.  MacEncrypt will open automatically and ask for an
encryption key.

Entering the Key

The Data Encryption Standard requires a user-supplied encryption key.  After you have
selected a file to encrypt, a dialog box will appear requesting that you type an encryption
key.  The encryption key is a string of typable (and possibly some untypable) characters.
All the standard letters, numbers, and symbols on the keyboard can be used.  In addition,
some of the keys that have no “character” associated with them (such as the “home” key)
can be used to make your key particularly hard to guess.

It is important to note that the Data Encryption Standard only uses 64 bit (i.e. eight
character) keys.  Anything you type for the key that is beyond eight characters is ignored,
but longer strings are supported so that any length of password (up to 255 characters) can
be used for convenience.  A password of zero length is also allowed.

Visibility

You can have the passwords you type be visible or invisible.  In the visible password
mode, characters will appear on the screen as you type them.  In the invisible mode, a
bullet character (•) will appear instead of the character you type.  MacEncrypt remembers
the characters you type, but someone looking over your shoulder will not be able to see
the password.  You can turn password visibility on and off by selecting the “Visible
Passwords” item under the “Options” menu.

Verification

There is a danger in using invisible passwords that you might accidentally type a
character in your password and not realize it.  If a file were encrypted with such a
password, you would probably not be able to type the right password during decryption
and your file would be lost forever.  In order to avoid this situation, you are required to
type the password twice before encrypting a file.  If the passwords match, the file will be
encrypted.  If they don’t match, you will be asked to type them again.  This is true for
visible as well as invisible passwords because there are some characters which can be
typed but not drawn on the screen.  These can be useful in passwords, but you need to
type them intentionally.  Password verification can help ensure that you know your
password.

Reusing Passwords

Multiple files can be selected for encryption from the finder.  If the “Reuse Passwords”
option is selected (from the options menu in MacEncrypt), a single password will be used
on all the selected files.  If “ReusePasswords” is not selected, you will be prompted to
type a password for each file.  It is important to note that whenever you change selections
in the option menu within MacEncrypt those changes are remembered when MacEncrypt
is closed.  The same options will be used when files are “dropped” on MacEncrypt for
encryption or decryption.

Encrypting

When your password is verified, MacEncrypt immediately begins to encrypt the selected
file.  During encryption, an animated cursor (a Morse code key set) will indicate that the
file is being encrypted.  MacEncrypt encrypts about three kilobytes per second on my
Mac IIsi.  Your mileage may vary.  When the encryption is complete, the cursor will
return to normal.  If you started the encryption from within MacEncrypt, you can select
another file to encrypt or decrypt or quit the application.  If you started from the finder,
MacEncrypt will automatically quit and return you to the finder.

It is handy to put MacEncrypt on the desktop near the garbage can.  When you want to
encrypt a file, simply drag it to the MacEncrypt Icon and type the encryption password.
The encryption will take place and you will be returned to the finder ready to continue
your tasks.

Decryption

Decryption is the process of reading an encrypted file and unscrambling its bits using a
decryption password.  If the password is correct, the file will be returned to its original
unencrypted state.

Selecting the File

The process for selecting a file to encrypt is similar to the process used to select a file to
encrypt.  You can select the file from within MacEncrypt using standard file dialog boxes
or start MacEncrypt from the finder by using the “drag and drop” technique or by double-
clicking an encrypted file.

From the Application

To select a file for decryption from within MacEncrypt, first open MacEncrypt by
double-clicking on its icon.  The MacEncrypt menu bar should appear.  Choose the
“Decrypt…” item from the “File” menu.  A standard file dialog box should pop up.  You
can then select the file to be Decrypted using the standard file dialog navigation
techniques.

From the Finder

There are two ways to start the decryption process from the finder.  The first process is
using the “drag and drop” technique.  Simply use the shift-click method to select the files
to decrypt, then drag and drop the files on the MacEncrypt application icon.  Each of the
files will ask for a password and then be decrypted.  The encryption process can also be
started this way.

The second way to initiate decryption from the finder is to double-click on an encrypted
file or to select a file and choose “Open” from the finder “File” menu.  Since encrypted
files belong to the MacEncrypt application, double-clicking on an encrypted file will start
the process to open the file using its creating application (MacEncrypt).
Entering the Key

After the file is selected, you will be requested to supply a decryption key.  The MUST be
the exact same key used to originally encrypt the file.  If you type the right key, the file
will be decrypted, if you type the wrong key, nothing will happen.

Visibility

The visibility option in decryption works just as it does in encryption.  If the visibility
option is on, the characters will appear as you type them.  If it is switched off, only bullet
characters (•) will appear.  You can toggle the visibility option by selecting the “Visible
Passwords” item under the “Options” menu.

Verification

It is only necessary to type the decryption password once since typing the wrong
password will only mean that you have to try again.  If you type the wrong password, a
dialog box will appear indicating that the password entered was incorrect.  If you type the
correct password, decryption will begin immediately.

Decrypting

As in the encryption process, during decryption an animated cursor will indicate that the
file is being decrypted.  Decryption occurs at the same rate as encryption (about 3
kBytes/sec on my Mac IIsi).  The cursor will return to normal when encryption is
complete.  If the decryption process was started from within MacEncrypt, you will be
returned to MacEncrypt and can encrypt or decrypt other files.  If you started from the
finder, either the next file in the group will be processed (encrypted or decrypted) or
control will be returned to the finder.

Options

There is five items in the “Options” menu.  The status of these options is saved when you
quit MacEncrypt, so you don’t need to worry about adjusting the options each time the
application is started.  The options selected when you quit MacEncrypt are the options
that will be used if you “drag and drop” items on MacEncrypt from the finder.
Visible Passwords

This is the “Visible Passwords” item.  If the item is marked with a check mark,
passwords will be visible and characters will appear as you type them from the keyboard.
If the item is not check-marked, password will show up as a string of bullet-characters (•).
Reusable Passwords

If the “Reusable Passwords” option is checked, the same password will be used to
encrypt all selected files.  This is handy if you want to encrypt several files at once and
you don’t want to retype the same password for each file.  If the item is not checked, you
will be prompted to enter a password for each file.

Extra Security

The “Extra Security” option is handy if you need protection against particularly snoopy
people who will use disk level editors to try to read bits from the disk where your file was
before encryption.  If this option is checked, MacEncrypt will “shred” the old file to
ensure that it can’t be read by a disk editing program.  Selecting this option will make the
encryption process take longer.  (Nothing is free.)

Multi-Item Archives

Multi-item archives are new to version 2.0.  If this option is checked, multiple files can be
encrypted to a single archive file (the icon looks like a safe).  This is handy if you want to
encrypt all the files in a directory at once and you don’t need to access them individually.
It is also nice if you don’t want people to know the names of your files.  When an archive
is decrypted, all the files in the archive are decrypted.

Delete Archived Items

When an archive is created the encrypted files can be deleted or left alone.  If “Delete
Archived Items” is checked, items will be deleted (and can be shredded) as they are put in
the archive.  If the item is not checked, the files will remain in place.

How Do I Pay For It?

MacEncrypt is NOT free.  It is shareware.  If you keep it for more than a month (30 days)
you are obligated to pay for it.  Please send the shareware fee of $10.00 (U.S.) to:

J. Clarke Stevens
MacEncrypt
1118 Forrest Blvd.
Decatur, GA  30030
U.S.A.

As a benefit of registration, I will provide limited support (limited to e-mail or U.S. Mail)
and will be more likely to implement suggestions you have for future versions.  I will also
notify you of all future versions of the program as well as other programs I produce.
Support shareware to encourage the development of quality, try and buy, affordable
software.  Site licenses are available and terms are negotiable.  If you would like to see a
particular new feature in MacEncrypt, let me know.  If it’s a good idea, I will probably
put it in eventually.  If it is a good idea and you are willing to bribe me, I will put it in
sooner.

How Does It Work?

Data Encryption Standard (DES)

For those of you who are curious, I have included the following information on the
history of the Data Encryption Standard and the encryption methods used in DES.  Much
of this information is a summary of the presentation by Dominic Welsh in his book Codes
and Cryptography (Oxford Scientific Publications, 1989).

DES History

Motivated by the increase in digital data communications in the early seventies, the U.S.
government decided to adopt a standard for data security.  The National Bureau of
Standards sponsored a search for a scheme that could be implemented on a single chip
and mass-produced.  The winner of the search was IBM.

The scheme proposed by IBM was an adaptation of a scheme they had previously
developed called Lucifer.  The main difference between Lucifer and the Data Encryption
Standard is the size of the encryption key.  Lucifer uses a 128 bit key while DES uses a
64 bit key (eight of which are discarded by the algorithm).

The size of the key has generated quite a controversy.  A paper by M.E. Hellman and W.
Diffie of Stanford University pointed out that a key length of 56 bits is uncomfortably
short.  A subsequent New York Times article by D. Kahn fueled the fire.  As a result, the
National Bureau of Standards sponsored workshops to discuss the issue.  The problem
was further complicated by the refusal of IBM do disclose some of the design principles
used because they were classified.

The conclusion of most attending the workshops was that DES would be acceptably
secure for about 10 years (DES was adopted in 1977).  However, the only known method
for breaking the code is an exhaustive search.  That means 255 attempts.  Since with
current computing power, this is not practical, your data is reasonably safe.  You don’t
need to worry about someone breaking you code with a personal computer.

Encryption Methods in DES

The following methods are used in DES:

1.    A 64-bit block is broken into left (L) and right (R) halves.
2.    The right half of the block (R) becomes the input to the left half (L’) of the next stage.
3.    The input to the right half of the next stage (R’) is calculated by
a.    scrambling and diffusing R to make it 48 bits,
b.    forming the modulo-2 sum with one bit of the key,
c.    passing it through the infamous S-boxes (a non-linear transform) to make it 32
bits,
d.    permuting it by the operation P,
e.    summing it modulo-2 with L to get R’.
4.    Finally, the left and right halves are combined again using the inverse of the initial
permutation matrix.

Each of the S-boxes transforms six input bits to four output bits.  There is an S-box for
each bit of the key.  For a more detailed explanation, read the section on DES in the book
by Dominic Welsh.

How Hard Is It To Break?

Breaking the DES algorithm is equivalent to solving an NP-hard (nondeterministic
polynomial time) problem.  Solving algebraic equations modulo 2 is an NP-hard problem.
There is no fast algorithm known for solving any NP-hard problem.  In other words,
breaking a cryptosystem based on an NP-hard problem is equivalent to finding a fast
algorithm to solve the problem.  DES is based on an NP-hard problem.  The only way
known to break it is to use an exhaustive search of all the 255 possible passwords.  This
would take the fastest known computer years to complete.  Your data is pretty safe, but
theoretically not totally safe, if you encrypt it using DES.  The weak link is the password,
if someone discovers your password, your data is at risk!

Who Is Responsible?

MacEncrypt was written by J. Clarke Stevens using knowledge of the Data Encryption
Standard gleaned from a graduate class on codes and cryptography in the department of
Electrical Engineering at Georgia Tech.  I was disappointed when I could find no simple
shareware program for implementing the algorithm on the Macintosh, so I decided to fill
the void myself.  The DES engine is based on public domain DES code written by Phil
Karn and Jim Gillogly.  MacEncrypt was written using Symantec’s THINK C.

Once you register, I will be happy to respond to bug reports and suggestions you may
have for future versions of MacEncrypt.

Bug Reports and Suggested Improvements

You can send bug reports and suggestions for enhancements to MacEncrypt to me using
the following methods.

Internet

This is my preferred means of communication.  I can usually respond very quickly.  My
address is:

[e-mail]

U.S. Mail (SnailMail)

If you can’t send e-mail, you can always reach me eventually with conventional mail at:

J. Clarke Stevens
MacEncrypt
1118 Forrest Blvd.
Decatur, GA  30030
U.S.A.

Liability

I have tested this program quite extensively and have used it on my own files.  I believe it
to be quite safe and have found no evidence to the contrary.  That notwithstanding, the
following statement applies.

Legal Junk follows:

MACENCRYPT IS A COPYRIGHTED COMPUTER PROGRAM OF J. CLARKE
STEVENS AND SUPPLIED AS-IS.  J. CLARKE STEVENS MAKES NO
WARRANTIES, EITHER EXPRESS OR IMPLIED REGARDING THE ENCLOSED
SOFTWARE PROGRAM OR ITS SUITABILITY FOR ANY PARTICULAR
PURPOSE.  J. CLARKE STEVENS WILL NOT BE LIABLE FOR ANY DAMAGES
THAT MAY OCCUR AS A RESULT OF USING THIS PROGRAM.