M ASTER S OFTWARE
QUALITY SOFTWARE SINCE 1958
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M ASTER S OFTWARE QUALITY SOFTWARE SINCE 1958 |
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PROTECT YOUR PRIVACY -- PROTECT YOUR SECURITY
with the strongest line of data file and message encryption software available. |
MX-Crypt Message Encryption System Version 01
User Manual - January 30, 2009
1. MX-CRYPT MESSAGE ENCRYPTION SYSTEM
MX-Crypt is a completely new method for secure secret communication.
MX-Crypt requires no distribution of keys. The sender and the receiver of
a message have their own secret keys for each message, but neither party
knows the other party's key. The keys are never transmitted or
distributed.
This means that two parties who have never met, and have never had the
chance to exchange keys, can communicate securely using MX-Crypt. Even if
both parties were under constant surveillance, with every email, every
phone call and every letter obtained by their opponent, they can still
communicate secretly. Even if they say openly, "Let us send our messages
using MX-Crypt," and their opponent knows this, even if their opponent also
has MX-Crypt, and has every MX-Crypt message they have sent back and forth,
their messages still cannot be read.
TABLE OF CONTENTS
1. MX-CRYPT MESSAGE ENCRYPTION SYSTEM
1.1. Authentication and Validation
1.2. Private Key
1.3. Three-Pass Protocol
2. SENDING MESSAGES
2.1. Starting MX-Crypt
2.2. Creating the Attention message
2.3. Creating the Ready message
2.4. Encrypting and decrypting the message
3. AUTHENTICATION AND VALIDATION
3.1. Key administrator
3.2. Network users
3.3. Validation
4. ADDITIONAL FUNCTIONS
4.1. Checkpoints
4.2. Deleting cache entries
4.3. Deleting files
5. FILES
5.1. Identifying files
5.2. The MX directory
5.3. File identifiers.
6. KEYS
6.1. Key Do's and Don'ts
6.2. Letters, digits and punctuation
6.3. Key blocks
6.4. Pronounceable keys
6.5. Patterns
6.6. Secretaries and clerks
6.7. Key strength
6.8. Summary: Picking a key
Appendix A. DOS BASICS
A.1. Starting DOS
A.2. Sizing the DOS window
A.3. Directories
A.4. Current directory
A.5. Working with directories
A.6. Identifying files
A.7. Long names
A.8. File operations
A.9. Batch files
1.1. Authentication and Validation
MX-Crypt has two new features, authentication and validation. The
authentication feature prevents a third party from posing as a legitimate
user. Even if your opponent had the ability to forge all of the
identifying information and routing information in an email header, the
MX-Crypt authentication feature prevents your opponent from reading your
messages, and it detects and flags any bogus messages your opponent sends.
The validation feature prevents someone from altering a message
between two legitimate users. This means that someone cannot take a valid
message, change any part of it, and then send the altered message to any of
the legitimate receivers. It also detects any changes due to transmission
errors.
1.2. Private Key
Today there are two leading methods for secret communication, secret
key and public key. In secret key communication, each of the parties must
have a copy of the secret key or keys used for messages. The keys must be
distributed to all parties who need to communicate with one another, or who
might need to communicate in the future. When a new party joins the group,
it may be necessary for a courier to bring the secret keys in person, or
even to bring all parties new secret keys for communicating with the new
party.
In public key cryptography each party has both a private key, which is
never distributed, and a public key which must be distributed to everyone
who wishes to communicate with them. For widespread use, there must be one
or more key-issuing authorities which possess all of the keys, and which
distribute these keys to all participants.
With MX-Crypt's Private Key method of communication, you and you alone
have your secret keys. All of the key generation and handling is done
automatically by the MX-Crypt software. Your secret keys are never
distributed to anyone, so there is no chance that anyone will ever obtain
your keys. All of your keys are locked away on your computer under control
of a Master Key that you choose, and that you alone know. The Master Key
is never stored or recorded on the computer, so even if someone stole your
computer, or copied the entire contents of your hard drive, they still
would not be able to get your secret keys.
MX-Crypt is the latest in Master Software's extensive line of Private
Key encryption software. Our original NK-Crypt Message Encryption System
was introduced in 2005. It is based on time-tested large prime technology
for proven security.
MX-Crypt replaces all of our previous patent-pending matrix-based
encryption packages, namely N2-Crypt, N3-Crypt, NS-Crypt and MU-Crypt. All
of the users of these products have been upgraded to the new MX-Crypt.
MX-Crypt adds new feature called Randomized Matrix Selection which greatly
improves the security without sacrificing speed. MX-Crypt is more than
2000 times as fast as NK-Crypt.
1.3. Three-Pass Protocol
MX-Crypt uses the Three-Pass Protocol to transmit the message. It is
called the Three-Pass Protocol because three encrypted messages are sent.
The whole process takes 4 steps. The sender performs steps 1 and 3.
The receiver performs steps 2 and 4.
Step 1: The sender uses MX-Crypt to encrypt the message with the sender's
private key, and sends this Attention message to the receiver.
Step 2: The receiver uses MX-Crypt to encrypt the first message with the
receiver's private key, and sends this Ready message back to the sender.
This second message is encrypted by both the sender and the receiver.
Step 3: The sender uses MX-Crypt to remove the first encryption, and sends
this final message back to the receiver.
Step 4: The receiver uses MX-Crypt to decrypt the final message file.
None of the 3 transmitted files contain either the sender's private
key or the receiver's private key. None of the messages contain any data
that could be used to construct or determine these private keys. The
private keys are never transmitted, either openly or in any encrypted or
coded form, so there is no possibility of an eavesdropper obtaining a key
from an intercepted message.
A simple way to understand the three-pass protocol is to picture how
it was done 500 years ago. Imagine that each message is written on
parchment and placed in a strongbox which is carried by a courier on
horseback. The sender puts a secure lock on the strongbox and the courier
brings the locked box to the receiver. The sender keeps the key. It is
not sent with the courier, nor with a separate courier. The receiver puts
a second lock on the strongbox, and the courier takes it back to the
sender. The sender removes the first lock and sends the box back. When
the receiver gets the strongbox back, it has only the receiver's own lock,
so the receiver can unlock the box without ever opening the sender's lock.
2. SENDING MESSAGES
2.1. Starting MX-Crypt
To start MX-Crypt, you type the MX command at the DOS prompt. That
is, you type
mx
and then press the Enter key. The first time you run MX-Crypt it will
install itself. That will be explained in a later chapter. Let us
concentrate first on a how a message is conveyed from the sender to the
receiver.
The first thing you must do in an MX-Crypt session is to supply the
Master Key. This is the key you chose when you installed MX-Crypt. The
program prompts you for the key
Please enter the Master Key.
Key:
You type your Master Key exactly the way you entered it when you installed
MX-Crypt, for example,
Key: mozart pineapple wigwam
Remember that the Master Key is case-sensitive, so mozart and Mozart are
different keys. If you have entered the Master Key correctly, the session
can begin. If the wrong Master Key is given, MX-Crypt immediately shuts
down. This important security feature makes it difficult for an intruder
to try large numbers of keys.
MX-Crypt will then need to know what operation you want to perform.
The operation depends on whether you are sending or receiving a message.
MX-Crypt will display the session menu and prompt you for the next
operation.
Select an operation:
1 - Prepare a new file to transmit. Create the Attention message.
2 - Respond to a new received message. Create the Ready message.
3 - Create the encrypted message.
4 - Decrypt the message.
A - Change authentication key.
C - Checkpoint. Save the cache and Master File.
D - Delete entries from the cache.
M - Change Master Key.
Q - Quit.
Type your choice:
Steps 1 and 3 will be performed by the sender, while Steps 2 and 4 are
performed by the receiver. Let's go through these steps in sequence.
2.2. Creating the Attention message
Suppose you are ready to transmit a new message. You would choose
Step 1 by typing a 1. MX-Crypt now needs to know what file you want to
send. It will ask for the file name
Enter the name of the file to be encrypted, for example
\NEWPROD\DESIGN\ENGINE.ART or type Q to quit.
File name:
You would type in the name of the file that contains the message you want
to send, for example
File name: \recipes\rumcake
MX-Crypt will create a series of 3 messages that will be sent back and
forth. You need to choose an identifier that will distinguish the 3
message files for this message from all other message files that you and
the receiver are sending and receiving. Your message identifier must be 1
to 8 characters long and may contain letters, digits, or both. MX-Crypt
will prompt you for the message identifer like this
Choose an ID for this message. The ID must be
1 to 8 letters and/or digits.
Or, type Q to quit.
Message ID:
You would respond by typing the message ID that you wish, for example
Message ID: party
MX-Crypt will then create the Attention message and place it on your
computer under the name party.1. You then send the message file party.1 to
the receiver, for example by sending it as an attachment to an email, or by
uploading it to a website.
2.3. Creating the Ready message
When the receiver gets the Attention message, it should be placed in
the same directory as the MX-Crypt program. The receiver must then create
the Ready message to send back. The receiver goes through the same process
of typing MX to start MX-Crypt and then entering the Master Key. For
example,
mx
Please enter the Master Key.
Key: k407 d921 h368
This latter, of course, is the receiver's Master Key, and may be completely
different from the sender's Master Key. Neither party has any need to know
the other's Master Key.
Once the Master Key is entered, MX-Crypt will start the session by
displaying
Select an operation:
1 - Prepare a new file to transmit. Create the Attention message.
2 - Respond to a new received message. Create the Ready message.
3 - Create the encrypted message.
4 - Decrypt the message.
A - Change authentication key.
C - Checkpoint. Save the cache and Master File.
D - Delete entries from the cache.
M - Change Master Key.
Q - Quit.
Type your choice:
The receiver will type 2 to indicate that an Attention message has been
received, and MX-Crypt should prepare the Ready message. MX-Crypt will
prompt the receiver for the message ID by displaying
Enter the ID for the message. The ID is the same
as the name of the transmitted message.
Or, type Q to quit.
Message ID:
In this example the Attention message was called party.1 so the message ID
is party. The receiver types party after the prompt, like
Message ID: party
and presses Enter. MX-Crypt then creates the Ready message under the name
party.2. The receiver sends this message back to the sender.
2.4. Encrypting and decrypting the message
When the sender receives the Ready message, MX-Crypt is used to create
the final encrypted message file. The process is just as before. The
sender types MX to start MX-Crypt, types the Master Key, chooses Step 3,
and then types the message ID, which is still party. MX-Crypt creates the
encrypted message in the file party.3 which the sender must send to the
receiver.
Once the receiver has the encrypted message, MX-Crypt can be used to
decrypt it, so the message can be read. The process is the same as above.
The receiver types MX to start MX-Crypt, enters the Master Key, selects
Step 4, and types the message ID party. This time MX-Crypt will prompt the
receiver for the name of the output file where the decrypted message should
go.
Enter the name of the file where you want the decrypted
message to be placed, for example \PROJECT\SALES\2008.DOC
or type Q to quit.
File name:
The name that the receiver chooses for the file need not be the same as the
name the sender used. In this example, the sender called the file
\recipes\rumcake but the receiver might call it
\nuclear\treaty\revision.26. The receiver would therefore type the file
name as
File name: \nuclear\treaty\revision.26
and press Enter. MX-Crypt would then decrypt the message and place it in
that file on the receiver's computer.
3. AUTHENTICATION AND VALIDATION
Authentication and validation are two new features that protect the
users of MX-Crypt. The two terms are sometimes confused. You authenticate
people, you validate messages. These features make sure that you send and
receive only valid messages from authentic users.
There is a possibility with Private Key encryption that an opponent
could pose as a legitimate user of the system. For example, your opponent
might have the capability of forging emails so that they appear to be from
a legitimate correspondent. MX-Crypt uses a new authentication feature
which prevents third parties from posing as legitimate correspondents.
The authentication feature requires an authentication key which must
be distributed to all legitimate users of your network, and kept strictly
secret from all other people. With MX-Crypt, all message keys are
generated internally by the software, and are never seen by the users. The
authentication key, however, will be chosen by a person and distributed to
all of the users, who must type it into their systems.
The authentication feature is optional. If you choose not to use the
authentication feature, it will still protect you against accidental
garbling of the message during transmission, but it will not protect you
against a deliberate bogus message.
3.1. Key administrator
In larger networks, where many users need to communicate with each
other, the easiest way to manage authentication keys is for one person to
be designated as the key administrator. The key administrator will be
responsible for creating and distributing the initial authentication key,
and periodic replacement keys.
The administrator should choose the key according to the principles in
the Keys chapter of this manual. The key administrator enters the
authentication key by selecting option A on the main MX-Crypt menu.
Select an operation:
1 - Prepare a new file to transmit. Create the Attention message.
2 - Respond to a new received message. Create the Ready message.
3 - Create the encrypted message.
4 - Decrypt the message.
A - Change authentication key.
C - Checkpoint. Save the cache and Master File.
D - Delete entries from the cache.
M - Change Master Key.
Q - Quit.
Type your choice:
MX-Crypt will respond by displaying
The Authentication Key is case-sensitive. For example, the 6 words
sample Sample SAMple
SAMPLE SamPle SamplE
are all considered different keys. All characters are taken as
part of the key, including leading and trailing blanks.
Please enter the Authentication Key.
Key:
The administrator should carefully type in the chosen key. MX-Crypt
will respond with
The verification code is xxxxx
Please check the verification code carefully.
Is the verification code correct? (Y or N):
The verification code will be a 5-digit number. The administrator
should carefully check that the authentication key was typed correctly. If
the key is correct, the administrator should record the 5-digit
verification code. When the authentication key is distributed to the
authorized users, the verification code should be included.
3.2. Network users
Each network user should follow the same steps to enter the
authentication key. The user should check the verification code. If the
verification code does not match the distributed code, that means the
authentication key has not been entered correctly. The user should check
the code carefully, paying attention to capitalization, missing characters,
extra characters, or transposed characters. The user should then retype
the authentication key correctly. If the verification code does not match
after several tries, it is likely that the user has received an incorrect
key.
The authentication key needs to typed only once. As soon as the
authentication key has been verified using the verification code, the paper
containing the key can be destroyed. There is no need to remember the
authentication key or the verification code, so there is no need to use an
easy-to-remember key format.
All network users must use the same authentication key when they
communicate with each other. In particular, it is not possible for one
user who is using the network authentication key to communicate with
another user who is not using the authentication key. If this were
possible, that would allow an imposter to pose as a legitimate network
user.
This means that whenever the authentication key is changed, all users
must change keys at the same time. MX-Crypt will not allow a user to
change keys while there are any messages in the cache. Changing keys would
make these messages unreadable. Therefore changing authentication keys
should be done only when the network is quiescent.
3.3. Validation
MX-Crypt contains a validation feature which protects you against any
accidental or intentional changes to the messages you send and receive. If
a message is accidentally changed during transmission, or if some third
party attempts to alter your message, the validation feature detects the
change.
The validation feature is completely automatic. You do not have to do
anything, enter any keys or check any verification codes. If anything goes
wrong during the transmission of a message, you will get the following
message
WARNING!! The validation code does not match. The message
may have been accidentally damaged or deliberately altered
during transmission.
4. ADDITIONAL FUNCTIONS
4.1. Checkpoints
MX-Crypt automatically saves the cache and the Master File at the end
of each session. If the session is long, either because you are processing
many messages, or because some of the message files are large, you may want
to save the cache and Master File more frequently.
If your computer stops for any reason, such as a hardware error, or a
momentary power interruption, you would need to repeat all of the
operations in the last session. However, if you take checkpoints, then you
would need to repeat only the operations that were done since the last
checkpoint. To perform a checkpoint, on the session menu
Select an operation:
1 - Prepare a new file to transmit. Create the Attention message.
2 - Respond to a new received message. Create the Ready message.
3 - Create the encrypted message.
4 - Decrypt the message.
A - Change authentication key.
C - Checkpoint. Save the cache and Master File.
D - Delete entries from the cache.
M - Change Master Key.
Q - Quit.
Type your choice:
you would type C.
4.2. Deleting cache entries
If encryption and decryption proceed smoothly, with Steps 1 through 4
always performed in order, then MX-Crypt will automatically remove
unneeeded entries from the cache. However, sometimes operations may not
proceed in order. For example, a sender might notify the receiver, "Ignore
the last message, here is a new one." In such cases, the receiver might
not decrypt the old message, and the corresponding entry would remain in
the cache. The cache could fill up over time, and have no room for new
entries.
To prevent this problem, you may want to check the cache for obsolete
or outdated entries. To do this, you would type D at the session menu
Select an operation:
1 - Prepare a new file to transmit. Create the Attention message.
2 - Respond to a new received message. Create the Ready message.
3 - Create the encrypted message.
4 - Decrypt the message.
A - Change authentication key.
C - Checkpoint. Save the cache and Master File.
D - Delete entries from the cache.
M - Change Master Key.
Q - Quit.
Type your choice:
This will start the cache clean-up operation. MX-Crypt will display
the message IDs in the cache, and allow you to delete any message IDs that
are no longer needed. The display might look like this:
Cache entries 1 to 8
ROME LONDON NEWYORK PARIS CAIRO MOSCOW
MUNICH TOKYO
Enter a message ID to be deleted, or type Q to quit.
Message ID:
You would then type the ID that you wanted to delete and press Enter, for
example
Message ID: cairo
You may delete as many message IDs as you wish.
When you are finished with this group of message IDs, type Q to quit.
If there are more message IDs in the cache, the next group will be
displayed, otherwise you will be taken back to the session menu.
4.3. Deleting files
MX-Crypt does not automatically delete the intermediate message files
xx.1, xx.2 and xx.3. These files are kept in case of problems such as
power interruptions or transmission errors.
The user may wish to delete old files to prevent the hard disk from
getting cluttered with outdated files. Alternatively, the user could keep
reusing the same file names, so that old files get continually replaced by
current files. For example, the files SMITH.1, SMITH.2 and SMITH.3 could
always contain the latest message from Smith.
5. FILES
In order to use MX-Crypt effectively you need to know two basic
concepts, files and keys. This chapter and the next one will deal with
these important topics.
The files on your computer are organized into directories or folders.
Directories and folders are two names for the same thing. When you are in
Windows, the computer will show you lists of files organized as folders.
When you are in DOS, the computer will show you lists of those same files
in directories. Directories and folders are equivalent.
You need to identify which files contain the messages that you want to
send. Often these files will be in directories that are named for the
program that created them. For example, if you create drawings using a
program called EZ-Draw, then the drawings are likely to be in a directory
with a name such as
\EZ-DRAW\
or
\PROGRAMS\EZ-DRAW\
or in a subdirectory of these directories, say
\EZ-DRAW\MYFILES\
You should always give your data files and folders names that clearly
identify what they contain. That way, you can easily find the files that
you want to send, or that you have received. You can find all of the files
on your computer by clicking the My Computer icon on the Windows desktop.
Equivalently, you can find all of your files by using the DIR command in
DOS.
You can encrypt any file you wish, even system files. It is safe to
do this because the file itself is not changed. MX-Crypt creates new files
each time you encrypt a file for transmission. These files are named
XXX.1, XXX.2 and XXX.3, where XXX is the message ID that you have chosen.
Even if you encrypted one of these XXX files, the encrypted version would
have a different message ID from the original, so the file names would be
different and there would be no danger.
5.1. Identifying files
Each time you send or receive a message you must identify the file to
MX-Crypt. MX-Crypt will prompt you for the file name or message identifier
at the appropriate time. You identify files to MX-Crypt the same way that
you identify files to DOS, namely by specifying the drive, path, filename
and extension. (If you already know DOS, you can skip or just skim this
section.)
drive is the device where your file is stored, usually
C for your hard drive, A or B for a floppy drive,
D or E for a CDROM drive.
path is the directory on your drive where the file is
located.
filename is the name that you gave your file. The name
usually indicates the contents or purpose of the
file.
extension is a suffix that indicates the kind of file, such
as TXT for a text file, JPEG for a picture file,
EXE for an executable file, etc.
A full file identifier might look like this,
c:\mycompany\mydepartment\2005\sales.wp
In this example, c: identifies that your file is on the C drive, which
is your hard drive. \mycompany\mydepartment\2005\ is the path to your
data. It shows that the data file is located in the 2005 folder, which is
inside the mydepartment folder, in the mycompany folder. So the path
consists of nested folders, or a list of directories. sales.wp is the file
with the data. The filename is sales, and the extension is wp, which
indicates that it is a WordPerfect document.
In a file identifier all of the fields except the filename are
optional.
drive can be omitted if the file is on the current drive,
that is, the drive where you are now working.
path can be omitted if the file is on the current
directory of the drive.
extension can be omitted if the file does not have an
extension on its name. For example, if the file
is just named oldstuff then no extension is
needed.
Here are some examples of valid file identifiers:
a:budget
identifies the file budget in the current directory
of the A drive.
\jones\commissions
identifies the file commissions in the \jones directory
on the current drive.
late\requests.txt
identifies the file requests.txt in the late
subdirectory of the current directory.
5.2. The MX directory
Messages created by MX-Crypt for transmission are named by a simple
identifier without a drive, path, or file extension. MX-Crypt reads the
message files from the current directory, and writes them to the current
directory. It is recommended that MX-Crypt be installed in its own
directory and that these files reside in this same directory along with the
MX-Crypt program files.
The MX directory might be \mx or \windows\mx or whatever directory you
prefer. If you have installed MX-Crypt in its own directory, let's say \mx
then you would make \mx the current directory each time you use MX-Crypt.
You do this by typing
cd \mx
at the DOS prompt, and then pressing Enter. To start MX-Crypt you would
then issue the MX command. The sequence of DOS commands would be
cd \mx
mx
and then MX-Crypt would start.
When you receive MX-Crypt message files from the Internet, or from a
website, they should be downloaded into the \mx directory, or else copied
into the \mx directory after downloading.
Having all of your MX-Crypt message files in a single directory makes
it easy to keep track of them, so you can erase old MX-Crypt files once the
message has been sent and received.
5.3. File identifiers.
If you are sending only a few MX-Crypt messages, then you can use
pretty much any message IDs you like, and keep them all straight in your
head. However, if you have a large message traffic, with many senders and
receivers, you may wish to adopt a systematic approach. One possible
convention would be to break the message ID into 3 separate fields, sender,
receiver, and message number. For example, a message ID PTRS0117 could
designate message number 117 from sender PT to receiver RS.
6. KEYS
Choosing the Master Key for encrypting your files is one of the most
critical steps in using the MX-Crypt package. If you choose a short or
weak key, it may be easy to remember and easy to type each time you need
it, but your data will not be secure. It is a serious mistake to think
that you can use a weak key simply because you are using such a strong
encryption package. A strong safe with a weak lock is not secure.
If you choose a long strong key your data will be more secure, but it
will be harder for you to remember it and to type it accurately each time
it is needed. This chapter will describe techniques for choosing keys that
are both secure and easy to remember and to type accurately.
6.1. Key Do's and Don'ts
Many people try to take shortcuts in order to have keys that are easy
for them to remember. You need to assume that any opponent will also be
aware of the same shortcuts. Here are some simple rules that can help
prevent a costly error.
When you choose a key, do not base the key on your personal
information. Assume that your opponent knows all of your personal data.
DO NOT base your key on
Your birthday
Your telephone number
Your Social Security number
Your license plate number
Your spouse's, child's, parent's, sibling's or even
your pet's name, birthday, phone number, etc.
DO NOT base your key on commonplace phrases
Nursery rhymes
Song titles or lyrics
Folk sayings
Names of famous people, groups, places or events
Names of books, plays or TV shows
Punchlines from jokes
Well-known dates
Tongue twisters
Words or phrases in other languages
DO NOT use data widely known within your specialized field
Digits of pi or e
Names of bones, nerves, or organs
Names of stars, minerals, geological features, bacteria,
ancient cultures, alloys, proteins, theorems, etc.
Mnemonics
Names of people, schools, companies, places, etc.
The speed of light, Avogadro's number, the Golden Ratio, etc.
DO NOT choose sequences of consecutive letters from the alphabet or from
the keyboard, whether forwards, backwards or diagonally.
DO NOT use the keys that appear in this manual. Always assume that your
opponent has read it, too.
DO use a long key.
DO try to make your key as random as possible.
DO read this entire chapter on picking keys.
DO evaluate the strength of your key according to the principles in the
following sections.
DO make your Master Key extra long and strong.
6.2. Letters, digits and punctuation
If there are several people who need access to the data, and who are
trusted with the Master Key, then the problem of recording or memorizing
the Master Key becomes multiplied. Some people have the capacity to
memorize long strings of random-looking letters and/or digits, but most
people cannot do this. The safest course is to write down your key, and
keep it in a secure place, such as a locked safe. Other techniques will be
discussed in a later section. It is advisable to have several copies, in
case one copy gets lost, stolen or destroyed.
The strength of an encryption key is measured in bits, the binary
digits that are used by your computer's hardware. Here is a rough guide to
how many bits you get from each character in an encryption key when the
characters are chosen randomly.
Table 1. Strength of each character in a key.
Decimal digits = 3.3 bits
Single case letters = 4.7 bits
Mixed case letters = 5.7 bits
Mixed letters and digits = 5.9 bits
Mixed letters, digits and punctuation = 6.3 bits
Based on this chart, here is the strength of some sample 10-character keys
Table 2. Strength of 10-character blocks.
5835701483 = 33 bits Decimal digits
CIWMRPTNZX = 47 bits Upper case letters
tyuhbivxks = 47 bits Lower case letters
DmbHaqREkV = 57 bits Mixed case letters
ku8Je94Lg7 = 59 bits Mixed letters and digits
g"p5WZc4%F = 63 bits Mixed letters, digits, punctuation
As you can see, the strength of the key increases when you choose
randomly from a larger set of characters. However, the difficulty of
memorizing the keys and typing them accurately becomes much greater as the
keys get more random.
Note that all of the keys illustrated above are too short to be
considered secure.
6.3. Key blocks
There are several methods for producing keys that are secure, yet
easier for people to manage. The first technique is to break your keys
into blocks. It has been a common practice for many years to break coded
messages into blocks of 5 characters each so that they can be transcribed
more accurately. The same idea works for keys, too. Notice how the key
CNWIALVMXBTEPOSBXRNH
becomes much easier to read when it is broken into groups of 5 letters
CNWIA LVMXB TEPOS BXRNH
For longer keys it may be advisable to use additional punctuation to
organize the blocks into groups of blocks. For example,
48591-04528-16392, 35207-31654-74925, 09482-71653-42570
GBXTL=PRBUI=LVZEW..BXGMN=LUIQT=SPFAE..VZJOQ=HUKBW=OZCND
The second technique is to use groups that have the same structure.
Here are some examples, and the strength of each key block
91486 61872 94373 16 bits per block 5 digits
T3708 D6204 F5193 18 bits per block 1 letter, 4 digits
GS437 BR092 LX528 19 bits per block 2 letters, 3 digits
UHM15 XTN63 MYA74 21 bits per block 3 letters, 2 digits
QRILC PJRMS OVDZK 23 bits per block 5 letters
The strength remains the same when the letters are placed in different
positions. For example, all of the following keys have the same strength,
namely 2 letters and 3 digits
GS437 BR092 LX528 Letters at the start of each block
943KP 471GQ 205YL Letters at the end of each block
V107J X219C F738L Letters at both ends of each block
6WF52 9TU48 7JN13 Letters in the middle of each block
One advantage of using key blocks that always have the same structure
is that there is no confusion between letters and digits. Some letters and
digits that may get confused are
Letters B G I l O S T Z
Digits 8 6 1 1 0 5 7 2
Its position in the block tells you whether the character is a letter or a
digit, so there is no need to avoid these characters when you use blocks
with a fixed structure.
Another variation on this idea is to make each key block uniform, but
to vary the types of blocks randomly. Here are two 30-character keys with
uniform blocks. Each block consists of all digits, or all uppercase
letters, or all lowercase letters.
KNUHW 50258 fewrz 39274 gyakf obqnk
doztc 81463 69917 AGNDL rdefo PUIZH
6.4. Pronounceable keys
Another technique that can be used to produce keys which are secure,
yet easy to remember, is to make the keys pronounceable. That is, you
would use pronounceable combinations of vowels and consonants to form
syllables, and combine these syllables to form artificial words. This
method may be valuable in situations where it is unsafe to write down the
keys, and they must be memorized. Here are some examples.
shambu dilp prelec oltu domex sarbuti shum obior
Yotz doruc flean jadmek pra kerazi, Lagatu limbrazon.
You can burn the key into your memory by starting with just a few
artificial words, say DOZEK ULM HAPLICO, and repeat these to yourself for a
day or two. Then add another few words, say DOZEK ULM HAPLICO GRUX ANTIAM,
and repeat those in your head for a few more days. You can add some more
words the following day.
dozek ulm haplico grux antiam ludovesk gur amesqi
You can complete the process by adding capitalization and punctuation,
like
Dozek ulm Haplico "Grux Antiam" ludo-vesk gur a'mesqi.
Using mixed-case letters and punctuation increases the strength of your
key.
You can imagine the key to be a saying in some private language, and
make up a translation, in order to fix it more firmly in your mind. For
example,
Wise king Haplico "Lion of Antioch" out-witted a sorcerer.
In a pronounceable key each letter has a strength of about 3.3 bits if
the words are fairly uniform in length, and about 3.5 bits if the words are
more variable in length. For example, the first key below is fairly
uniform in length, while the second is more variable.
panek dilbap greho drung fasdop ulben bukty crivan
lobykar elb dixiat glem urbiqeo dhorsh uz vilagump
6.5. Patterns
When choosing a key, avoid creating any patterns, such as repeated
letters or syllables. Patterns weaken the keys by making them easier to
guess. Here are some examples of keys with patterns.
BBXXTT KKUUVV WWYYCC The letters are all in pairs.
aaa3gg5yyyy9ccc7uu2 There are runs of equal letters.
10704 20906 50803 The second and fourth digit in
each group is zero.
51615 38183 29092 Each group has an ABCBA pattern.
zampana reveske flogoto The vowels in each group are all
the same.
tuntam memescu saksoli The first and second syllable
start with the same letter.
debendik devogi delakt Every group starts with de.
ABC ghi LMN def XYZ Each group has 3 consecutive
letters of the alphabet.
500XD 711TJ 822GN The second and third digits in
each group are the same.
31734 23839 30376 Every group has two 3's.
dobaku levoti wafigo Consonants and vowels alternate.
vgy7 2wdc zse4 7ujm Has diagonal runs on the keyboard.
KAZ VEK CIF ZOP HUQ The vowels run in order AEIOU.
Once you have chosen a key, inspect it for patterns, and change it to
remove them. If your key is a long string of letters or digits, look to
see if there are any letters or digits that are used too often, or that are
missing. You may want to make some changes. However, don't overdo it. If
you use every letter or every digit exactly the same number of times, or if
all the letters and digits in each block of your Master Key are always
different, those are also patterns which weaken the key.
6.6. Secretaries and clerks
Sometimes lower-echelon employees will not safeguard file keys as
zealously as other workers. It is common for these employees to write down
keys in places that are easily accessible, such as on the computer itself,
on their desk pads or wall calendars, or on slips of paper on a bulletin
board. Anybody could see the keys and write them down. It is absurd for
the company president to keep the Master Key in a locked box inside a
walk-in vault, and for the secretary's assistant to write the Master Key on
a gummed label on the wall next to the computer.
The employee might assume that nobody will ever guess that those
cryptic letters and digits are actually the Master Key that unlocks all of
the company's secret files. The employee might assume incorrectly. If
these employees must be trusted with the Master Key then it is essential
that they be educated to avoid such security breaches.
Keys should never be written or pasted on the computer itself, the
computer desk, a desk pad or calendar, the cover of a notebook or steno
pad, the bottom of a stapler, telephone or flowerpot, the back of a
clipboard, letter tray or desk organizer, or any similar place. Intruders
know to look in such places. Don't make their job easy.
6.7. Key strength
The following table is a guide to how long a key must be in order to
achieve various levels of security. For example, if you want a key
strength of 200 bits, and you use a decimal key, then you need 60 digits.
With the speed of current computers 100 bits is the lowest level of
security that can be considered safe.
The table assumes that the letters or digits of the key are chosen
completely randomly. If the letters or digits follow some pattern then
your key needs to be longer. For example, a key such as
TC174 JF296 BH583 KD629
would be measured as 8 single-case letters and 12 digits, for a total
strength of 77 bits. Because of the LLDDD pattern it would not be
considered to be 20 mixed letters and digits, which would have a strength
of 118 bits.
Table 3. For each type of key, this table shows how long to make
the key in order to achieve the desired strength.
Desired key strength measured in bits
Type of key 100 125 150 200 250 300 400
---------------------------------------------------------------
Decimal digits 30 38 45 60 75 90 120
Single-case letters 21 27 32 43 53 64 85
Mixed-case letters 18 22 26 35 44 53 70
S-C letters + digits 19 24 29 39 48 58 77
M-C letters + digits 17 21 25 34 42 50 67
Letters, digits, punc 16 20 24 32 40 47 63
Uniform blocks 22 27 33 44 55 66 88
Pronounceable, uniform 30 38 45 60 75 90 120
Pronounceable, variable 29 36 43 57 71 86 114
For example, if you wanted a decimal key you would read across the top row
of this table. If you wanted the decimal key to have a strength of 125
bits, you would look at the second column in the top row to find that you
would need 38 decimal digits. If you wanted a key of mixed-case letters
and digits with a strength of 250 bits, you would need 42 letters and
digits.
Note that the longest input line you can enter is 126 characters.
(This is a limitation of DOS, not a limit set by MX-Crypt.) So if you
wanted 400 bits of strength, and you chose to have a decimal key which
requires 120 digits, then you would have only 6 characters left to separate
the blocks. Your blocks would need to average over 17 characters each. (A
pattern of 17, 17, 17, 17, 17, 17, 18 would fit.)
6.8. Summary: Picking a key
The best way to pick a key is to follow these steps.
(1) Decide how strong you want your key to be, say 200 bits.
(2) Choose the type of key, say blocks of letters and digits.
(3) Use the tables above to determine the key length.
(4) Randomly choose a key of the required length.
(5) Inspect the key for patterns.
(6) Adjust the key to remove or reduce the patterns.
(7) If you will need the key again, write down the key and keep
a copy in a secure place.
(8) Type the key when MX-Crypt asks for it.
Appendix A. DOS BASICS
MX-Crypt runs under DOS, not under Windows. DOS was the primary
operating system for personal computers from about 1975 to 1995. Older
versions of Windows, prior to the introduction of Windows 95, ran as tasks
under DOS. Since 1995 the situation has reversed, and DOS now runs as a
task under Windows. Every computer user before 1995 knew DOS well.
However, newer computer users may not be familiar with DOS, so that a
little basic orientation may be helpful.
A.1. Starting DOS
On newer computers it may be difficult even to find DOS in order to
use it. There are two methods for running DOS. The first method is to
click on a DOS icon from your desktop, or from a taskbar at the top or
bottom edge of the desktop. The icon may say DOS, or MSDOS, or possibly
CMD or COMMAND. Clicking any one of these icons will start DOS. If there
is a DOS icon on your desktop or in a taskbar, you can skip the rest of
this section.
If there is no DOS icon on your desktop or taskbar you may find one
elsewhere. Start by clicking on "Start" in the corner of the screen. This
will bring up a menu listing various programs and options. If there is a
DOS icon there, you can use it directly, or you could drag it onto the
desktop for future use. If it is not there, click on "Programs" or "All
Programs." This will bring up a long list of various programs that are on
your computer. If one of these is DOS, you can click it, or you can drag
it to the desktop.
If you still don't see a DOS or CMD icon, put your mouse on each of
the icons that you see. Don't click, just let the mouse cursor rest on the
icon. This will often bring up another list of programs, and DOS may be
among them.
If DOS still is not there, don't give up. You just need to search
deeper. In the list of All Programs there will be some folders with names
such as "Applications" or "System Utilities." Click to open each of these
folders. In those folders you may find DOS or CMD. Or, you may find more
folders. Again, rest the mouse on the names of programs, and click on
folders to find even more well-hidden programs and folders.
Once you find the DOS icon, drag it to the desktop. Put the mouse
cursor on the DOS icon and hold down the left button. Move the mouse to
drag the cursor onto the desktop, and then release it to drop the icon on
the desktop. Click the desktop to close all of the other windows. Then
drag the DOS icon to wherever you want it on the desktop.
If all of this fails, it is time to try the second method. Go back to
the desktop, and click on "Start" again. In the list of options click on
"Run" or "Run Program." This will open a small window with a box where you
can type the name of a program that you wish to run. Type CMD in this box,
and then press Enter. This will open a DOS window.
A.2. Sizing the DOS window
The DOS window will often be a small window in the middle of the
screen, probably off-center. It is easier to work with DOS in full-screen
mode, with no distracting windows or borders. To do this, right click on
the top border of the DOS window, and select "Properties" from the pop-up
window that appears. Use the various options to select full-screen mode.
This may take several tries before it works, so don't get frustrated if the
next time you use DOS you get the same small window, and need to resize it
again.
When you do get the full screen mode, the screen is likely to be set
to 50-line mode. This makes the characters small and crudely formed. You
may be more comfortable using 25-line mode. To switch, you can type the
command
mode con lines=25
This will double the size of the characters and make them easier to read.
A.3. Directories
In DOS your computer's hard disk is organized into directories. All
of the files on your computer are in directories. These correspond to the
folders in Windows. Directories and folders are the same thing. A
directory or a folder can contain files and more directories or folders, so
that the folders or directories are nested one inside the other in a
hierarchy.
The top of the hierarchy is called the "root directory." Typically
the root directory does not contain any files. Rather, it contains all of
the principal directories on the computer, such as
\Windows
\Program Files
\Documents and Settings
and so forth. The backslash \ in front of these directory names shows that
they are directories within the root directory.
A directory within another directory is sometimes called a
subdirectory. In the example above the directory Windows would be a
subdirectory of the root directory.
A.4. Current directory
Files are identified in DOS by using a path, a filename and a
filetype. For example,
direc1\direc2\file1.doc
Here the path is direc1\direc2, the filename is file1 and the filetype is
doc. The path consists of the sequence of nested directories which contain
the desired file.
If the path starts with a \ backslash, then the sequence of
directories start from the root directory. If the backslash is omitted,
then the path starts from the current directory. For example, if the
current directory is Windows, then the file identifier
direc1\direc2\file1.doc would refer to the file
\Windows\direc1\direc2\file1.doc
By setting the current directory you can shorten the names of programs
and files that you must type. For example, if you want to use the program
\direc1\direc2\prog1.exe
to process the data files
\direc1\direc2\file1.dat
and
\direc1\direc2\file2.dat
you could type
\direc1\direc2\prog1 \direc1\direc2\file1.dat \direc1\direc2\file2.dat
If you changed the current directory to \direc1\direc2 then this could be
shortened to
prog1 file1.dat file2.dat
The command to change the current directory is cd. To change the
current directory to \direc1\direc2 you would type
cd \direc1\direc2\
If you later wanted to change the current directory to
\direc1\direc2\direc3 it is sufficient to type
cd direc3
since you were already in the directory \direc1\direc2.
A.5. Working with directories
You can make your own directories by using the Make Directory command.
For example, if the current directory is \direc1\direc2 and you wanted to
make a subdirectory called direc3, then you could type
md direc3
Starting from the root directory, the new directory would be
\direc1\direc2\direc3.
To remove a directory, you can use the Remove Directory command. For
example, to remove the directory \direc1\direc2\direc3 you would type
rd \direc1\direc2\direc3
As a safety precaution, you cannot remove a directory until you have
deleted all of the files in the directory, and removed all of its
subdirectories. This prevents you from accidentally deleting files that
you meant to keep.
To list the contents of a directory, you can use the Directory
command. The basic format is
dir mydirec /options
Here mydirec is the directory you want to list. There are many possible
options. Here are a few of the most useful:
/s List the contents of all subdirectories
/on Sort the files by name
/os Sort the files, smallest to largest
/o-s Sort the files, largest to smallest
/od Sort the files, oldest to newest
/o-d Sort the files, newest to oldest
/p Pause after every 20 lines
You can use several options in the same command. For example,
dir \direc1 /s /od /p
would list the files in \direc1 and all of its subdirectories sorted from
oldest to newest, and pausing after every 20 lines.
You can also list specific files, files that have a given filename or
filetype, or files whose filenames and filetypes begin with specific
letters. Here are some examples
dir tax.ref Lists the file tax.ref.
dir tax.* Lists all files with the name tax.
dir *.doc Lists all files of type doc.
dir st*.c* Lists all files whose filename starts with st
and whose filetype begins with c, such as
startup.cfg, study.com or state.core.
The * asterisks in these commands are called wildcards because they can be
replaced by any set of letters. These commands can tell you whether these
files exist, their sizes, and the date they were last updated.
A.6. Identifying files
All of the data in your computer resides in files. Files contain the
operating system, all of the application programs, and all of the data that
they use and create. Files are identified to DOS by four fields, namely
the drive, path, filename and extension.
drive is the device where your file is stored, usually
C for your hard drive, A or B for a floppy drive,
D or E for a CDROM drive.
path is the directory on your drive where the file is
located.
filename is the name that you gave your file. The name
usually indicates the contents or purpose of the
file.
extension is a suffix that indicates the kind of file, such
as TXT for a text file, JPEG for a picture file,
EXE for an executable file, etc.
A full file identifier might look like this,
c:\mycompany\mydepartment\2005\sales.wp
In this example, c: identifies that your file is on the C drive, which
is your hard drive. \mycompany\mydepartment\2005\ is the path to your
data. It shows that the data file is located in the 2005 folder, which is
inside the mydepartment folder, in the mycompany folder. So the path
consists of nested folders, or a list of directories. sales.wp is the file
with the data. The filename is sales, and the extension is wp, which
indicates that it is a WordPerfect document.
In a file identifier all of the fields except the filename are
optional.
drive can be omitted if the file is on the current
drive, that is, the drive where you are now
working.
path can be omitted if the file is on the current
directory of the drive.
extension can be omitted if the file does not have an
extension on its name. For example, if the file
is just named oldstuff then no extension is
needed.
Here are some examples of valid file identifiers:
a:budget
identifies the file budget in the current directory
of the A drive.
\jones\commissions
identifies the file commissions in the jones directory
on the current drive.
late\requests.txt
identifies the file requests.txt in the late
subdirectory of the current directory.
A.7. Long names
Some Windows files and directories have long names, or names
containing blanks or dots, such as
Documents and Settings
My Music
Microsoft.Net
SharedReg12.dll
Microsoft has made the naming of files and directories incompatible between
Windows and DOS. DOS limits directory names to 8 characters, and does not
allow blanks in names.
To refer to these directories, you need to shorten the names down to 8
characters. The short name is formed by taking the first 6 non-blank
characters of the name plus the combination ~1. When the name of a
directory contains a . dot character, each of the parts of the name is
treated separately. For example, for the directories above,
Documents and Settings would be called Docume~1
My Music would be called MyMusi~1
Microsoft.Net would be called Micros~1.Net
SharedReg12.dll would be called Shared~1.dll
Thus a full path and file name such as
\Windows\Microsoft.Net\Framework\SharedReg12.dll
in DOS would be called
\Windows\Micros~1.Net\Framew~1\Shared~1.dll
It is a good idea to give all of your own files and directories names
that are compatible with DOS. The names should be no more than 8
characters long and should not contain blanks.
A.8. File operations
Besides the encryption and decryption operations that you perform
using MX-Crypt, it can be useful to know several other common file
operations.
There is no DOS operation to create a file. Files are created by
application programs such as word processors, picture editors,
spreadsheets, etc. Once created, files can be copied, renamed and deleted.
It is important to remember that encrypted files should not be
renamed, and files should not be copied into or out of a group of encrypted
files. It is safest to decrypt files before renaming or copying.
To copy a file to a new location, the command is
copy oldfile newfile
The old file and new file identifiers can be fully qualified, that is, they
may have drive, path, filename and filetype. So the copy command can be
used to copy files to other directories or to other drives.
Wildcards can be used in the copy command to copy groups of files.
For example, the command
copy \oldpath\*.doc \newpath\*.*
would copy all files of type doc from the \oldpath directory to the
\newpath directory.
The rename command works similarly to the copy command. The form is
ren oldfile newname
Here oldfile can be fully qualified, with drive, path, filename and
filetype. However, newname can have only a new filename and filetype.
There cannot be a new drive or new path because the file does not change
its location, only its name and/or type. For example,
ren target\x3*.jpg x4*.*
would rename all of the jpg files in the target directory that start with
x3 to start with x4.
The command to delete files takes the form
del file
Here, file can be a fully-qualified file identifier, with drive, path,
filename and filetype. It can also have wildcards so that you can delete
several files with a single command. For example,
del a:old*.*
would delete all files in the current directory of the a drive whose
filenames start with old.
Note that deleting a file does not erase it. The file still exists on
the disk, where it can be read by various utility programs that are
available for that purpose. The file will remain there until some other
file eventually gets written on top of it.
A.9. Batch files
Batch files are a useful way to reduce the number and complexity of
the DOS commands that you must type. Each batch file can contain any
number of DOS commands. You execute the entire sequence of DOS commands
just by typing the name of the batch file.
Here is a simple example. Suppose that you frequently use the program
MX-Crypt. If the current directory is \plans\tower but MX-Crypt is in the
directory \programs\download then to use MX-Crypt you would type
\programs\download\MX
To make this easier, you could create a batch file named MX.bat on the
current directory. This file would contain the single line
\programs\download\MX
Now when you wanted to execute MX-Crypt all you would need to type is
MX
You could place a copy of the batch file MX.bat in every directory
where you usually work. Then you could run MX-Crypt from anywhere just by
typing MX. You would not need to have multiple copies of MX-Crypt.
There are many other DOS commands and options. This is just a small
sample of useful DOS commands.