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                          Phil's Pretty Good Software
                                    Presents

                                      ===
                                      PGP
                                      ===

                              Pretty Good Privacy
                      Public Key Encryption for the Masses


                           -------------------------
                                PGP User's Guide
                           Volume II: Special Topics
                           -------------------------
                              by Philip Zimmermann
                                Revised 3 Dec 92


                           PGP Version 2.1 - 6 Dec 92
                              Software Written by
                               Philip Zimmermann
                                      with
                Hal Finney, Branko Lankester, and Peter Gutmann




     Synopsis:  PGP uses public-key encryption to protect E-mail and data
     files.  Communicate securely with people you've never met, with no
     secure channels needed for prior exchange of keys.  PGP is well
     featured and fast, with sophisticated key management, digital
     signatures, data compression, and good ergonomic design.


     Software and documentation (c) Copyright 1990-1992 Philip Zimmermann.
     For information on PGP licensing, distribution, copyrights, patents,
     trademarks, liability limitations, and export controls, see the
     "Legal Issues" section.



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     Contents
     ========

     Quick Overview
     Special Topics
       Selecting Keys via Key ID
       Separating Signatures from Messages
       Decrypting the Message and Leaving the Signature on it
       Sending ASCII Text Files Across Different Machine Environments
       Leaving No Traces of Plaintext on the Disk
       Displaying Decrypted Plaintext on Your Screen
       Making a Message For Her Eyes Only
       Preserving the Original Plaintext Filename
       Editing Your User ID or Pass Phrase
       Editing the Trust Parameters for a Public Key
       Checking If Everything is OK on Your Public Key Ring
       Verifying a Public Key Over the Phone
       Using PGP as a Unix-style Filter
       PGP Returns Exit Status to the Shell
       Environmental Variable for Pass Phrase
       Setting Configuration Parameters: CONFIG.TXT
         TMP - Directory Pathname for Temporary Files
         LANGUAGE - Foreign Language Selector
         MYNAME - Default User ID for Making Signatures
         TEXTMODE - Assuming Plaintext is a Text File
         CHARSET - Specifies Local Character Set for Text Files
         ARMOR - Enable ASCII Armor Output
         ARMORLINES - Size of ASCII Armor Multipart Files
         KEEPBINARY - Keep Binary Ciphertext Files After Decrypting
         VERBOSE - Enable Verbose Mode
         COMPRESS - Enable Compression
         BAKRING - Filename for Backup Secret Keyring
         COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed
         MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed
         CERT_DEPTH - How Deep May Introducers Be Nested
         PAGER - Selects Shell Command to Display Plaintext Output
         SHOWPASS - Echo Pass Phrase to User
         TZFIX - Timezone Adjustment
         CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text
       Protecting Against Bogus Timestamps
       A Peek Under the Hood
         Random Numbers
         PGP's Conventional Encryption Algorithm
         Data Compression
         Message Digests and Digital Signatures
       Compatibility with Previous Versions of PGP
     Vulnerabilities
       Compromised Pass Phrase and Secret Key
       Public Key Tampering
       "Not Quite Deleted" Files
       Viruses and Trojan Horses
       Physical Security Breach
       Tempest Attacks
       Exposure on Multi-user Systems
       Traffic Analysis

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       Cryptanalysis
     Legal Issues
       Trademarks, Copyrights, and Warranties
       Patent Rights on the Algorithms
       Licensing and Distribution
       Export Controls
     Recommended Readings
     To Contact the Author
     Where to Get PGP



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     Quick Overview
     =============

     Pretty Good(tm) Privacy (PGP), from Phil's Pretty Good Software, is a
     high security cryptographic software application for MSDOS, Unix,
     VAX/VMS, and other computers.  PGP combines the convenience of the
     Rivest-Shamir-Adleman (RSA) public key cryptosystem with the speed of
     conventional cryptography, message digests for digital signatures,
     data compression before encryption, good ergonomic design, and
     sophisticated key management.

     This volume II of the PGP User's Guide covers advanced topics about
     PGP that were not covered in the "PGP User's Guide, Volume I:
     Essential Topics".  You should first read the Essential Topics
     volume, or this manual won't make much sense to you.  Reading this
     Special Topics volume is optional.



     Special Topics
     ===============


     Selecting Keys via Key ID
     -------------------------

     In all commands that let the user type a user ID or fragment of a
     user ID to select a key, the hexadecimal key ID may be used instead.
     Just use the key ID, with a prefix of "0x", in place of the user ID.
     For example:

         pgp -kv 0x67F7

     This would display all keys that had 67F7 as part of their key IDs.

     This feature is particularly useful if you have two different keys
     from the same person, with the same user ID.  You can unambiguously
     pick which key you want by specifying the key ID.


     Separating Signatures from Messages
     -----------------------------------

     Normally, signature certificates are physically attached to the text
     they sign.  This makes it convenient in simple cases to check
     signatures.  It is desirable in some circumstances to have signature
     certificates stored separately from the messages they sign.  It is
     possible to generate signature certificates that are detached from
     the text they sign.  To do this, combine the 'b' (break) option with
     the 's' (sign) option.  For example:

         pgp -sb letter.txt

     This example produces an isolated signature certificate in a file
     called "letter.sig".  The contents of letter.txt are not appended to

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     the signature certificate.

     After creating the signature certificate file (letter.sig in the
     above example), send it along with the original text file to the
     recipient.  The recipient must have both files to check the signature
     integrity.  When the recipient attempts to process the signature
     file, PGP notices that there is no text in the same file with the
     signature and prompts the user for the filename of the text. Only
     then can PGP properly check the signature integrity.  If the
     recipient knows in advance that the signature is detached from the
     text file, she can specify both filenames on the command line:

         pgp letter.sig letter.txt
     or: pgp letter letter.txt

     PGP will not have to prompt for the text file name in this case.

     A detached signature certificate is useful if you want to keep the
     signature certificate in a separate certificate log.  A detached
     signature of an executable program is also useful for detecting a
     subsequent virus infection.  It is also useful if more than one party
     must sign a document such as a legal contract, without nesting
     signatures.  Each person's signature is independent.

     If you receive a ciphertext file that has the signature certificate
     glued to the message, you can still pry the signature certificate
     away from the message during the decryption.  You can do this with
     the -b option during decrypt, like so:

         pgp -b letter

     This decrypts the letter.pgp file and if there is a signature in it,
     PGP checks the signature and detaches it from the rest of the
     message, storing it in the file letter.sig.


     Decrypting the Message and Leaving the Signature on it
     ------------------------------------------------------

     Usually, you want PGP to completely unravel a ciphertext file,
     decrypting it and checking the nested signature if there is one,
     peeling away the layers until you are left with only the original
     plaintext file.

     But sometimes you want to decrypt an encrypted file, and leave the
     inner signature still attached, so that you are left with a decrypted
     signed message.  This may be useful if you want to send a copy of a
     signed document to a third party, perhaps re-enciphering it.  For
     example, suppose you get a message signed by Charlie, encrypted to
     you.  You want to decrypt it, and, leaving Charlie's signature on it,
     you want to send it to Alice, perhaps re-enciphering it with Alice's
     public key.  No problem.  PGP can handle that.

     To simply decrypt a message and leave the signature on it intact,
     type:


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         pgp -d letter

     This decrypts letter.pgp, and if there is an inner signature, it is
     left intact with the decrypted plaintext in the output file.

     Now you can archive it, or maybe re-encrypt it and send it to someone
     else.



     Sending ASCII Text Files Across Different Machine Environments
     --------------------------------------------------------------

     You may use PGP to encrypt any kind of plaintext file, binary 8-bit
     data or ASCII text.  Probably the most common usage of PGP will be for
     E-mail, when the plaintext is ASCII text.

     ASCII text is sometimes represented differently on different
     machines.  For example, on an MSDOS system, all lines of ASCII text
     are terminated with a carriage return followed by a linefeed.  On a
     Unix system, all lines end with just a linefeed.  On a Macintosh, all
     lines end with just a carriage return.  This is a sad fact of life.

     Normal unencrypted ASCII text messages are often automatically
     translated to some common "canonical" form when they are transmitted
     from one machine to another.  Canonical text has a carriage return
     and a linefeed at the end of each line of text.  For example, the
     popular KERMIT communication protocol can convert text to canonical
     form when transmitting it to another system.  This gets converted
     back to local text line terminators by the receiving KERMIT.  This
     makes it easy to share text files across different systems.

     But encrypted text cannot be automatically converted by a
     communication protocol, because the plaintext is hidden by
     encipherment.  To remedy this inconvenience, PGP lets you specify
     that the plaintext should be treated as ASCII text (not binary data)
     and should be converted to canonical text form before it gets
     encrypted.  At the receiving end, the decrypted plaintext is
     automatically converted back to whatever text form is appropriate for
     the local environment.

     To make PGP assume the plaintext is text that should be converted to
     canonical text before encryption, just add the "t" option when
     encrypting or signing a message, like so:

        pgp -et message.txt her_userid

     This mode is automatically turned off if PGP detects that the
     plaintext file contains what it thinks is non-text binary data.

     For PGP users that use non-English 8-bit character sets, when PGP
     converts text to canonical form, it may convert data from the local
     character set into the LATIN1 (ISO 8859-1 Latin Alphabet 1) character
     set, depending on the setting of the CHARSET parameter in the PGP
     configuration file.  LATIN1 is a superset of ASCII, with extra
     characters added for many European languages.

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     Leaving No Traces of Plaintext on the Disk
     ------------------------------------------

     After PGP makes a ciphertext file for you, you can have PGP
     automatically overwrite the plaintext file and delete it, leaving no
     trace of plaintext on the disk so that no one can recover it later
     using a disk block scanning utility.  This is useful if the plaintext
     file contains sensitive information that you don't want to keep
     around.

     To wipe out the plaintext file after producing the ciphertext file,
     just add the "w" (wipe) option when encrypting or signing a message,
     like so:

         pgp -esw message.txt her_userid

     This example creates the ciphertext file "message.pgp", and the
     plaintext file "message.txt" is destroyed beyond recovery.

     Obviously, you should be careful with this option.  Also note that
     this will not wipe out any fragments of plaintext that your word
     processor might have created on the disk while you were editing the
     message before running PGP.  Most word processors create backup
     files, scratch files, or both.  Also, it overwrites the file only
     once, which is enough to thwart conventional disk recovery efforts,
     but not enough to withstand a determined and sophisticated effort to
     recover the faint magnetic traces of the data using special disk
     recovery hardware.



     Displaying Decrypted Plaintext on Your Screen
     ---------------------------------------------

     To view the decrypted plaintext output on your screen (like the
     Unix-style "more" command), without writing it to a file, use the -m
     (more) option while decrypting:

          pgp -m ciphertextfile

     This displays the decrypted plaintext display on your screen one
     screenful at a time.



     Making a Message For Her Eyes Only
     ----------------------------------

     To specify that the recipient's decrypted plaintext will be shown
     ONLY on her screen and cannot be saved to disk, add the -m option:

          pgp -sem message.txt her_userid


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     Later, when the recipient decrypts the ciphertext with her secret key
     and pass phrase, the plaintext will be displayed on her screen but
     will not be saved to disk.  The text will be displayed as it would if
     she used the Unix "more" command, one screenful at a time.  If she
     wants to read the message again, she will have to decrypt the
     ciphertext again.

     This feature is the safest way for you to prevent your sensitive
     message from being inadvertently left on the recipient's disk.  This
     feature was added at the request of a user who wanted to send
     intimate messages to his lover, but was afraid she might accidentally
     leave the decrypted messages on her husband's computer.



     Preserving the Original Plaintext Filename
     ------------------------------------------

     Normally, PGP names the decrypted plaintext output file with a name
     similar to the input ciphertext filename, but dropping the
     extension.  Or, you can override that convention by specifying an
     output plaintext filename on the command line with the -o option.
     For most E-mail, this is a reasonable way to name the plaintext file,
     because you get to decide its name when you decipher it, and your
     typical E-mail messages often come from useless original plaintext
     filenames like "to_phil.txt".

     But when PGP encrypts a plaintext file, it always saves the original
     filename and attaches it to the plaintext before it compresses and
     encrypts the plaintext.  Normally, this hidden original filename is
     discarded by PGP when it decrypts, but you can tell PGP you want to
     preserve the original plaintext filename and use it as the name of
     the decrypted plaintext output file.  This is useful if PGP is used
     to on files whose names are important to preserve.

     To recover the original plaintext filename while decrypting, add
     the -p option, like so:

          pgp -p ciphertextfile

     I usually don't use this option, because if I did, about half of my
     incoming E-mail would decrypt to the same plaintext filenames of
     "to_phil.txt" or "prz.txt".



     Editing Your User ID or Pass Phrase
     -----------------------------------

     Sometimes you may need to change your pass phrase, perhaps because
     someone looked over your shoulder while you typed it in.

     Or you may need to change your user ID, because you got married and
     changed your name, or maybe you changed your E-mail address.  Or
     maybe you want to add a second or third user ID to your key, because
     you may be known by more than one name or E-mail address or job

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     title.  PGP lets you attach more than one user ID to your key, any
     one of which may be used to look up your key on the key ring.

     To edit your userid or pass phrase for your secret key:

          pgp -ke userid [keyring]

     PGP prompts you for a new user ID or a new pass phrase.



     Editing the Trust Parameters for a Public Key
     ---------------------------------------------

     Sometimes you need to alter the trust parameters for a public key on
     your public key ring.  For a discussion on what these trust
     parameters mean, see the section "How Does PGP Keep Track of Which
     Keys are Valid?" in the Essential Topics volume of the PGP User's
     Guide.

     To edit the trust parameters for a public key:

          pgp -ke userid [keyring]



     Checking If Everything is OK on Your Public Key Ring
     ----------------------------------------------------

     Normally, PGP automatically checks any new keys or signatures on your
     public key ring and updates all the trust parameters and validity
     scores.  In theory, it keeps all the key validity status information
     up to date as material is added to or deleted from your public key
     ring.  But perhaps you may want to explicitly force PGP to perform a
     comprehensive analysis of your public key ring, checking all the
     certifying signatures, checking the trust parameters, updating all
     the validity scores, and checking your own ultimately-trusted key
     against a backup copy on a write-protected floppy disk.  It may be a
     good idea to do this hygienic maintenance periodically to make sure
     nothing is wrong with your public key ring.  To force PGP to perform
     a full analysis of your public key ring, use the -kc (key ring check)
     command:

          pgp -kc

     You can also make PGP check all the signatures for just a single
     selected public key by:

          pgp -kc userid [keyring]

     For further information on how the backup copy of your own key is
     checked, see the description of the BAKRING parameter in the
     configuration file section of this manual.




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     Verifying a Public Key Over the Phone
     -------------------------------------

     If you get a public key from someone that is not certified by anyone
     you trust, how can you tell if it's really their key?  The best way
     to verify an uncertified key is to verify it over some independent
     channel other than the one you received the key through.  One
     convenient way to tell, if you know this person and would recognize
     them on the phone, is to call them and verify their key over the
     telephone.  Rather than reading their whole tiresome (ASCII-armored)
     key to them over the phone, you can just read their key's
     "fingerprint" to them.  To see this fingerprint, use the -kvc
     command:

          pgp -kvc userid [keyring]

     This will display the key with the 16-byte digest of the public key
     components.  Read this 16-byte fingerprint to the key's owner on the
     phone, while she checks it against her own, using the same -kvc
     command at her end.

     You can both verify each other's keys this way, and then you can sign
     each other's keys with confidence.  This is a safe and convenient way
     to get the key trust network started for your circle of friends.



     Using PGP as a Unix-style Filter
     --------------------------------

     Unix fans are accustomed to using Unix "pipes" to make two
     applications work together.  The output of one application can be
     directly fed through a pipe to be read as input to another
     application.  For this to work, the applications must be capable of
     reading the raw material from "standard input" and writing the
     finished output to "standard output".  PGP can operate in this mode.
     If you don't understand what this means, then you probably don't need
     this feature.

     To use a Unix-style filter mode, reading from standard input and
     writing to standard output, add the -f option, like so:

          pgp -feast her_userid <inputfile >outputfile

     This feature makes it easier to make PGP work with electronic mail
     applications.

     When using PGP in filter mode to decrypt a ciphertext file, you may
     find it useful to use the PGPPASS environmental variable to hold the
     pass phrase, so that you won't be prompted for it.  The PGPPASS
     feature is explained below.



     PGP Returns Exit Status to the Shell
     ------------------------------------

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     To facilitate running PGP in "batch" mode, such as from an MSDOS
     ".bat" file or from a Unix shell script, PGP returns an error exit
     status to the shell.  An exit status code of zero means normal exit,
     while a nonzero exit status indicates some kind of error occurred.
     Different error exit conditions return different exit status codes to
     the shell.



     Environmental Variable for Pass Phrase
     --------------------------------------

     Normally, PGP prompts the user to type a pass phrase whenever PGP
     needs a pass phrase to unlock a secret key.  But it is possible to
     store the pass phrase in an environmental variable from your
     operating system's command shell.  The environmental variable PGPPASS
     can be used to hold the pass phrase that PGP will attempt to use
     first.  If the pass phrase stored in PGPPASS is incorrect, PGP
     recovers by prompting the user for the correct pass phrase.

     For example, on MSDOS, the shell command:

         SET PGPPASS=zaphod beeblebrox for president

     would eliminate the prompt for the pass phrase if the pass phrase
     were indeed "zaphod beeblebrox for president".

     This dangerous feature makes your life more convenient if you have to
     regularly deal with a large number of incoming messages addressed to
     your secret key, by eliminating the need for you to repeatedly type
     in your pass phrase every time you run PGP.

     I added this feature because of popular demand.  However, this is a
     somewhat dangerous feature, because it keeps your precious pass
     phrase stored somewhere other than just in your brain.  Even worse,
     if you are particularly reckless, it may even be stored on a disk on
     the same computer as your secret key.  It would be particularly
     dangerous and stupid if you were to install this command in a batch
     or script file, such as the MSDOS AUTOEXEC.BAT file.  Someone could
     come along on your lunch hour and steal both your secret key ring and
     the file containing your pass phrase.

     I can't emphasize the importance of this risk enough.  If you are
     contemplating using this feature, be sure to read the sections
     "Exposure on Multi-user Systems" and "How to Protect Secret Keys from
     Disclosure" in this volume and in the Essential Topics volume of the
     PGP User's Guide.

     If you must use this feature, the safest way to do it would be to
     just manually type in the shell command to set PGPPASS every time you
     boot your machine to start using PGP, and then erase it or turn off
     your machine when you are done.  And you should definitely never do
     it in an environment where someone else may have access to your
     machine.  Someone could come along and simply ask your computer to
     display the contents of PGPPASS.

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     Setting Configuration Parameters: CONFIG.TXT
     ============================================

     PGP has a number of user-settable parameters that can be defined in a
     special configuration text file called "config.txt", in the directory
     pointed to by the shell environmental variable PGPPATH.  Having a
     configuration file enables the user to define various flags and
     parameters for PGP without the burden of having to always define
     these parameters in the PGP command line.

     Configuration parameters may be assigned integer values, character
     string values, or on/off values, depending on what kind of
     configuration parameter it is.  A sample configuration file is
     provided with PGP, so you can see some examples.

     In the configuration file, blank lines are ignored, as is anything
     following the '#' comment character.  Keywords are not
     case-sensitive.

     Here is a short sample fragment of a typical configuration file:

        # TMP is the directory for PGP scratch files, such as a RAM disk.
        TMP = "e:\"    # Can be overridden by environment variable TMP.
        Armor = on     # Use -a flag for ASCII armor whenever applicable.
        # CERT_DEPTH is how deeply introducers may introduce introducers.
        cert_depth = 3

     If some configuration parameters are not defined in the configuration
     file, or if there is no configuration file, or if PGP can't find the
     configuration file, the values for the configuration parameters
     default to some reasonable value.

     Note that it is also possible to set these same configuration
     parameters directly from the PGP command line, by preceding the
     parameter setting with a "+" character.  For example, the following
     two PGP commands produce the same effect:

          pgp -e +armor=on message.txt smith
     or:  pgp -ea message.txt smith


     The following is a summary of the various parameters than may be
     defined in the configuration file.


     TMP - Directory Pathname for Temporary Files
     --------------------------------------------

     Default setting:  TMP = ""

     The configuration parameter TMP specifies what directory to use for
     PGP's temporary scratch files.  The best place to put them is on a
     RAM disk, if you have one.  That speeds things up quite a bit, and

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     increases security somewhat.  If TMP is undefined, the temporary
     files go in the current directory.  If the shell environmental
     variable TMP is defined, PGP instead uses that to specify where the
     temporary files should go.


     LANGUAGE - Foreign Language Selector
     ------------------------------------

     Default setting:  LANGUAGE = "en"

     PGP displays various prompts, warning messages, and advisories to the
     user on the screen.  For example, messages such as "File not found.",
     or "Please enter your pass phrase:".  These messages are normally in
     English.  But it is possible to get PGP to display its messages to
     the user in other languages, without having to modify the PGP
     executable program.

     A number of people in various countries have translated all of PGP's
     display messages, warnings, and prompts into their native languages.
     These hundreds of translated message strings have been placed in a
     special text file called "language.txt", distributed with the PGP
     release.  The messages are stored in this file in English, Spanish,
     Dutch, German, French, Italian, Russian, Latvian, and Lithuanian.
     Other languages may be added later.

     The configuration parameter LANGUAGE specifies what language to
     display these messages in.  LANGUAGE may be set to "en" for English,
     "es" for Spanish, "de" for German, "nl" for Dutch, "fr" for French,
     "it" for Italian, "ru" for Russian, "lt3" for Lithuanian, "lv" for
     Latvian, "esp" for Esperanto.  For example, if this line appeared in
     the configuration file:

        LANGUAGE = "fr"

     PGP would select French as the language for its display messages.
     The default setting is English.

     When PGP needs to display a message to the user, it looks in the
     "language.txt" file for the equivalent message string in the selected
     foreign language and displays that translated message to the user.
     If PGP can't find the language string file, or if the selected
     language is not in the file, or if that one phrase is not translated
     into the selected language in the file, or if that phrase is missing
     entirely from the file, PGP displays the message in English.

     To conserve disk space, most foreign translations are not included
     in the standard PGP release package, but are available separately.


     MYNAME - Default User ID for Making Signatures
     ----------------------------------------------

     Default setting:  MYNAME = ""

     The configuration parameter MYNAME specifies the default user ID to

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 14


     use to select the secret key for making signatures.  If MYNAME is not
     defined, the most recent secret key you installed on your secret key
     ring will be used.  The user may also override this setting by
     specifying a user ID on the PGP command line with the -u option.


     TEXTMODE - Assuming Plaintext is a Text File
     --------------------------------------------

     Default setting:  TEXTMODE = off

     The configuration parameter TEXTMODE is equivalent to the -t command
     line option.  If enabled, it causes PGP to assume the plaintext is a
     text file, not a binary file, and converts it to "canonical text"
     before encrypting it.  Canonical text has a carriage return and a
     linefeed at the end of each line of text.

     This mode will be automatically turned off if PGP detects that the
     plaintext file contains what it thinks is non-text binary data.

     For VAX/VMS systems, the current version of PGP defaults TEXTMODE=ON.

     For further details, see the section "Sending ASCII Text Files Across
     Different Machine Environments".


     CHARSET - Specifies Local Character Set for Text Files
     ------------------------------------------------------

     Default setting:  CHARSET = NOCONV

     Because PGP must process messages in many non-English languages with
     non-ASCII character sets, you may have a need to tell PGP what local
     character set your machine uses.  This determines what character
     conversions are performed when converting plaintext files to and from
     canonical text format.  This is only a concern if you are in a
     non-English non-ASCII environment.

     The configuration parameter CHARSET selects the local character set.
     The choices are NOCONV (no conversion), LATIN1 (ISO 8859-1 Latin
     Alphabet 1), KOI8 (used by most Russian Unix systems), ALT_CODES
     (used by Russian MSDOS systems), ASCII, and CP850 (used by most
     western European languages on standard MSDOS PCs).

     LATIN1 is the internal representation used by PGP for canonical text,
     so if you select LATIN1, no conversion is done.  Note also that PGP
     treats KOI8 as LATIN1, even though it is a completely different
     character set (Russian), because trying to convert KOI8 to either
     LATIN1 or CP850 would be futile anyway.  This means that setting
     CHARSET to NOCONV, LATIN1, or KOI8 are all equivalent to PGP.

     If you use MSDOS and expect to send or receive traffic in western
     European languages, set CHARSET = "CP850".  This will make PGP
     convert incoming canonical text messages from LATIN1 to CP850 after
     decryption.  If you use the -t (textmode) option to convert to
     canonical text, PGP will convert your CP850 text to LATIN1 before

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 15


     encrypting it.

     For further details, see the section "Sending ASCII Text Files Across
     Different Machine Environments".


     ARMOR - Enable ASCII Armor Output
     ---------------------------------

     Default setting:  ARMOR = off

     The configuration parameter ARMOR is equivalent to the -a command
     line option.  If enabled, it causes PGP to emit ciphertext or keys in
     ASCII Radix-64 format suitable for transporting through E-mail
     channels.  Output files are named with the ".asc" extension.

     If you tend to use PGP mostly for E-mail, it may be a good idea to
     enable this parameter.

     For further details, see the section "Sending Ciphertext Through
     E-mail Channels: Radix-64 Format" in the Essential Topics volume.


     ARMORLINES - Size of ASCII Armor Multipart Files
     ------------------------------------------------

     Default setting:  ARMORLINES = 720

     When PGP creates a very large ".asc" radix-64 file for sending
     ciphertext or keys through the E-mail, it breaks the file up into
     separate chunks small enough to send through Internet mail
     utilities.  Normally, Internet mailers prohibit files larger than
     about 50000 bytes, which means that if we restrict the number of
     lines to about 720, we'll be well within the limit.  The file chunks
     are named with suffixes ".as1", ".as2", ".as3", ...

     The configuration parameter ARMORLINES specifies the maximum number
     of lines to make each chunk in a multipart ".asc" file sequence.  If
     you set it to zero, PGP will not break up the file into chunks.

     Fidonet email files usually have an upper limit of about 32K bytes,
     so 450 lines would be appropriate for Fidonet environments.

     For further details, see the section "Sending Ciphertext Through
     E-mail Channels: Radix-64 Format" in the Essential Topics volume.


     KEEPBINARY - Keep Binary Ciphertext Files After Decrypting
     ----------------------------------------------------------

     Default setting:  KEEPBINARY = off

     When PGP reads a ".asc" file, it recognizes that the file is in
     radix-64 format and will convert it back to binary before processing
     as it normally does, producing as a by-product a ".pgp" ciphertext
     file in binary form.  After further processing to decrypt the ".pgp"

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 16


     file, the final output file will be in normal plaintext form.

     You may want to delete the binary ".pgp" intermediate file, or you
     may want PGP to delete it for you automatically.  You can still rerun
     PGP on the original ".asc" file.

     The configuration parameter KEEPBINARY enables or disables keeping
     the intermediate ".pgp" file during decryption.

     For further details, see the section "Sending Ciphertext Through
     E-mail Channels: Radix-64 Format" in the Essential Topics volume.


     VERBOSE - Enable Verbose Mode
     -----------------------------

     Default setting:  VERBOSE = off

     The configuration parameter VERBOSE enables "verbose" diagnostic
     messages during PGP's operation, which is mainly useful for debugging
     PGP.  Otherwise, there is not much use for it.


     COMPRESS - Enable Compression
     -----------------------------

     Default setting:  COMPRESS = on

     The configuration parameter COMPRESS enables or disables data
     compression before encryption.  It is used mainly for debugging PGP.
     Normally, PGP attempts to compress the plaintext before it encrypts
     it.  Generally, you should leave this alone and let PGP attempt to
     compress the plaintext.


     COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed
     ------------------------------------------------------------------

     Default setting:  COMPLETES_NEEDED = 1

     The configuration parameter COMPLETES_NEEDED specifies the minimum
     number of completely trusted introducers required to fully certify a
     public key on your public key ring.  This gives you a way of tuning
     PGP's skepticism.

     For further details, see the section "How Does PGP Keep Track of
     Which Keys are Valid?" in the Essential Topics volume.


     MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed
     ------------------------------------------------------------------

     Default setting:  MARGINALS_NEEDED = 2

     The configuration parameter MARGINALS_NEEDED specifies the minimum
     number of marginally trusted introducers required to fully certify a

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 17


     public key on your public key ring.  This gives you a way of tuning
     PGP's skepticism.

     For further details, see the section "How Does PGP Keep Track of
     Which Keys are Valid?" in the Essential Topics volume.


     CERT_DEPTH - How Deep May Introducers Be Nested
     -----------------------------------------------

     Default setting:  CERT_DEPTH = 4

     The configuration parameter CERT_DEPTH specifies how many levels deep
     you may nest introducers to certify other introducers to certify
     public keys on your public key ring.  For example, If CERT_DEPTH is
     set to 1, there may only be one layer of introducers below your own
     ultimately-trusted key.  If that were the case, you would be required
     to directly certify the public keys of all trusted introducers on
     your key ring.  If you set CERT_DEPTH to 0, you could have no
     introducers at all, and you would have to directly certify each and
     every key on your public key ring in order to use it.  The minimum
     CERT_DEPTH is 0, the maximum is 8.

     For further details, see the section "How Does PGP Keep Track of
     Which Keys are Valid?" in the Essential Topics volume.


     BAKRING - Filename for Backup Secret Keyring
     --------------------------------------------

     Default setting:  BAKRING = ""

     All of the key certification that PGP does on your public key ring
     ultimately depends on your own ultimately-trusted public key (or
     keys).  To detect any tampering of your public key ring, PGP must
     check that your own key has not been tampered with.  To do this, PGP
     must compare your public key against a backup copy of your secret key
     on some tamper-resistant media, such as a write-protected floppy
     disk.  A secret key contains all the information that your public key
     has, plus some secret components.  This means PGP can check your
     public key against a backup copy of your secret key.

     The configuration parameter BAKRING specifies what pathname to use
     for PGP's trusted backup copy of your secret key ring.  On MSDOS, you
     could set it to "a:\secring.pgp" to point it at a write-protected
     backup copy of your secret key ring on your floppy drive.  This check
     is performed only when you execute the PGP -kc option to check your
     whole public key ring.

     If BAKRING is not defined, PGP will not check your own key against
     any backup copy.

     For further details, see the sections "How to Protect Public Keys
     from Tampering" and "How Does PGP Keep Track of Which Keys are
     Valid?" in the Essential Topics volume.


     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 18



     PAGER - Selects Shell Command to Display Plaintext Output
     ---------------------------------------------------------

     Default setting:  PAGER = ""

     PGP lets you view the decrypted plaintext output on your screen (like
     the Unix-style "more" command), without writing it to a file, if you
     use the -m (more) option while decrypting.  This displays the
     decrypted plaintext display on your screen one screenful at a time.

     If you prefer to use a fancier page display utility, rather than
     PGP's built-in one, you can specify the name of a shell command that
     PGP will invoke to display your plaintext output file.  The
     configuration parameter PAGER specifies the shell command to invoke
     to display the file.  For example:

        PAGER = "more"

     However, if the sender specified that this file is for your eyes
     only, and may not be written to disk, PGP always uses its own
     built-in display function.

     For further details, see the section "Displaying Decrypted Plaintext
     on Your Screen".


     SHOWPASS - Echo Pass Phrase to User
     -----------------------------------

     Default setting:  SHOWPASS = off

     Normally, PGP does not let you see your pass phrase as you type it
     in.  This makes it harder for someone to look over your shoulder
     while you type and learn your pass phrase.  But some typing-impaired
     people have problems typing their pass phrase without seeing what
     they are typing, and they may be typing in the privacy of their own
     homes.  So they asked if PGP can be configured to let them see what
     they type when they type in their pass phrase.

     The configuration parameter SHOWPASS enables PGP to echo your typing
     during pass phrase entry.


     TZFIX - Timezone Adjustment
     ---------------------------

     Default setting:  TZFIX = 0

     PGP provides timestamps for keys and signature certificates in
     Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC), which
     means the same thing for our purposes.  When PGP asks the system for
     the time of day, the system is supposed to provide it in GMT.

     But sometimes, because of improperly configured MSDOS systems, the
     system time is returned in US Pacific Standard Time time plus 8

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 19


     hours.  Sounds weird, doesn't it?  Perhaps because of some sort of US
     west-coast jingoism, MSDOS presumes local time is US Pacific time,
     and pre-corrects Pacific time to GMT.  This adversely affects the
     behavior of the internal MSDOS GMT time function that PGP calls.
     However, if your MSDOS environmental variable TZ is already properly
     defined for your timezone, this corrects the misconception MSDOS has
     that the whole world lives on the US west coast.

     The configuration parameter TZFIX specifies the number of hours to
     add to the system time function to get GMT, for GMT timestamps on
     keys and signatures.  If the MSDOS environmental variable TZ is
     defined properly, you can leave TZFIX=0.  Unix systems usually
     shouldn't need to worry about setting TZFIX at all.  But if you are
     using some other obscure operating system that doesn't know about
     GMT, you may have to use TZFIX to adjust the system time to GMT.

     On MSDOS systems that do not have TZ defined in the environment, you
     should make TZFIX=0 for California, -1 for Colorado, -2 for Chicago,
     -3 for New York, -8 for London, -9 for Amsterdam.  In the summer,
     TZFIX should be manually decremented from these values.  What a mess.

     It would be much cleaner to set your MSDOS environmental variable TZ
     in your AUTOEXEC.BAT file, and not use the TZFIX correction.  Then
     MSDOS gives you good GMT timestamps, and will handle daylight savings
     time adjustments for you.  Here are some sample lines to insert into
     AUTOEXEC.BAT, depending on your time zone:

     For Los Angeles:  SET TZ=PST8PDT
     For Denver:       SET TZ=MST7MDT
     For Arizona:      SET TZ=MST7
        (Arizona never uses daylight savings time)
     For Chicago:      SET TZ=CST6CDT
     For New York:     SET TZ=EST5EDT
     For London:       SET TZ=GMT0BST
     For Amsterdam:    SET TZ=MET-1DST
     For Moscow:       SET TZ=MSK-3MSD
     For Aukland:      SET TZ=NZT-13


     CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text
     ------------------------------------------------------------------

     Default setting:  CLEARSIG = off

     Normally, unencrypted PGP signed messages have a signature
     certificate prepended in binary form.  To send this through a 7-bit
     E-mail channel, radix-64 ASCII armor is applied (see the ARMOR
     parameter), rendering the message unreadable to casual human eyes,
     even though the message is not actually encrypted.  The recipient
     must use PGP to strip the armor off before reading the message.

     If the original plaintext message is in text (not binary) form, there
     is a way to send it through an E-mail channel in such a way that the
     ASCII armor is applied only to the binary signature certificate, but
     not to the plaintext message.  This makes it possible to read the
     signed message with human eyes, without the aid of PGP.  Of course,

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 20


     you still need PGP to actually check the signature.

     To enable this feature, set CLEARSIG=ON, and set ARMOR=ON (or use
     the -a option), and set TEXTMODE=ON (or use the -t option).  For
     example, you can set CLEARSIG directly from the command line:

          pgp -sta +clearsig=on message.txt

     This message representation is analogous to the MIC-CLEAR message type
     used in Internet Privacy Enhanced Mail (PEM).  It is important to
     note that since this method only applies ASCII armor to the binary
     signature certificate, and not to the message text itself, there is
     some risk that the unarmored message may suffer some accidental
     molestation while en route.  This can happen if it passes through
     some E-mail gateway that performs character set conversions, or in
     some cases extra spaces may be added to or stripped from the ends of
     lines.  If this occurs, the signature will fail to verify, which may
     give a false indication of intentional tampering.  But since PEM
     lives under a similar vulnerability, it seems worth having this
     feature despite the risks.



     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 21



     Protecting Against Bogus Timestamps
     ===================================

     A somewhat obscure vulnerability of PGP involves dishonest users
     creating bogus timestamps on their own public key certificates and
     signatures.  You can skip over this section if you are a casual user
     and aren't deeply into obscure public key protocols.

     There's nothing to stop a dishonest user from altering the date and
     time setting of his own system's clock, and generating his own public
     key certificates and signatures that appear to have been created at a
     different time.  He can make it appear that he signed something
     earlier or later than he actually did, or that his public/secret key
     pair was created earlier or later.  This may have some legal or
     financial benefit to him, for example by creating some kind of
     loophole that might allow him to repudiate a signature.

     A remedy for this could involve some trustworthy Certifying Authority
     or notary that would create notarized signatures with a trustworthy
     timestamp.  This might not necessarily require a centralized
     authority.  Perhaps any trusted introducer or disinterested party
     could serve this function, the same way real notary publics do now.
     A public key certificate could be signed by the notary, and the
     trusted timestamp in the notary's signature would have some legal
     significance.  The notary could enter the signed certificate into a
     special certificate log controlled by the notary.  Anyone can read
     this log.

     The notary could also sign other people's signatures, creating a
     signature certificate of a signature certificate.  This would serve
     as a witness to the signature the same way real notaries do now with
     paper.  Again, the notary could enter the detached signature
     certificate (without the actual whole document that was signed) into
     a log controlled by the notary.  The notary's signature would have a
     trusted timestamp, which might have greater credibility than the
     timestamp in the original signature.  A signature becomes "legal" if
     it is signed and logged by the notary.

     This problem of certifying signatures with notaries and trusted
     timestamps warrants further discussion.  This can of worms will not
     be fully covered here now.  There is a good treatment of this topic
     in Denning's 1983 article in IEEE Computer (see references).  There
     is much more detail to be worked out in these various certifying
     schemes.  This will develop further as PGP usage increases and other
     public key products develop their own certifying schemes.



     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 22



     A Peek Under the Hood
     =====================

     Let's take a look at a few internal features of PGP.


     Random Numbers
     --------------

     PGP uses a cryptographically strong pseudorandom number generator for
     creating temporary conventional session keys.  The seed file for this
     is called  "randseed.bin".  It too can be kept in whatever directory
     is indicated by the PGPPATH environmental variable.  If this random
     seed file does not exist, it is automatically created and seeded with
     truly random numbers derived from timing your keystroke latencies.

     This generator reseeds the disk file each time it is used by mixing
     in new key material partially derived with the time of day and other
     truly random sources.  It uses the conventional encryption algorithm
     as an engine for the random number generator.  The seed file contains
     both random seed material and random key material to key the
     conventional encryption engine for the random generator.

     If you feel uneasy about trusting any algorithmically derived random
     number source however strong, keep in mind that you already trust the
     strength of the same conventional cipher to protect your messages.
     If it's strong enough for that, then it should be strong enough to
     use as a source of random numbers for temporary session keys.  Note
     that PGP still uses truly random numbers from physical sources
     (mainly keyboard timings) to generate long-term public/secret key
     pairs.



     PGP's Conventional Encryption Algorithm
     ---------------------------------------

     As described earlier, PGP "bootstraps" into a conventional single-key
     encryption algorithm by using a public key algorithm to encipher the
     conventional session key and then switching to fast conventional
     cryptography.  So let's talk about this conventional encryption
     algorithm.  It isn't the DES.

     The Federal Data Encryption Standard (DES) is a good algorithm for
     most commercial applications.  However, the Government does not trust
     the DES to protect its own classified data.  Perhaps this is because
     the DES key length is 56 bits, short enough for a brute force attack
     with a special purpose machine built from massive numbers of DES
     chips.  Also, Biham and Shamir have had some success recently on
     attacking the full 16-round DES.

     PGP does not use the DES as its conventional single-key algorithm to
     encrypt messages.  Instead, PGP uses a different conventional
     single-key block encryption algorithm, called IDEA(tm).  A future
     version of PGP may support the DES as an option, if enough users

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 23


     ask for it.  But I suspect IDEA is better than DES.

     For the cryptographically curious, the IDEA cipher has a 64-bit block
     size for the plaintext and the ciphertext.  It uses a key size of 128
     bits.  It is based on the design concept of "mixing operations from
     different algebraic groups".  It runs much faster in software than
     the DES.  Like the DES, it can be used in cipher feedback (CFB) and
     cipher block chaining (CBC) modes.  PGP uses it in 64-bit CFB mode.

     The IPES/IDEA block cipher was developed at ETH in Zurich by James L.
     Massey and Xuejia Lai, and published in 1990.  This is not a
     "home-grown" algorithm.  Its designers have a distinguished
     reputation in the cryptologic community.  Early published papers on
     the algorithm called it IPES (Improved Proposed Encryption Standard),
     but they later changed the name to IDEA (International Data
     Encryption Algorithm).  So far, IDEA has resisted attack much better
     than other ciphers such as FEAL, REDOC-II, LOKI, Snefru and Khafre.
     And preliminary evidence suggests that IDEA may be more resistant
     than the DES to Biham & Shamir's highly successful differential
     cryptanalysis attack.  Biham and Shamir have been examining the IDEA
     cipher for weaknesses, without success.  Academic cryptanalyst groups
     in Belgium, England, and Germany are also attempting to attack it, as
     well as the military services from several European countries.  As
     this new cipher continues to attract attack efforts from the most
     formidable quarters of the cryptanalytic world, confidence in IDEA is
     growing with the passage of time.

     A famous hockey player once said, "I try to skate to where I think
     the puck will be."  A lot of people are starting to feel that the
     days are numbered for the DES.  I'm skating toward IDEA.

     It is not ergonomically practical to use pure RSA with large keys to
     encrypt and decrypt long messages.  Absolutely no one does it that way
     in the real world.  But perhaps you are concerned that the whole
     package is weakened if we use a hybrid public-key and conventional
     scheme just to speed things up.  After all, a chain is only as strong
     as its weakest link.  Many people less experienced in cryptography
     mistakenly believe that RSA is intrinsically stronger than any
     conventional cipher.  It's not.  RSA can be made weak by using weak
     keys, and conventional ciphers can be made strong by choosing good
     algorithms.  It's usually difficult to tell exactly how strong a good
     conventional cipher is, without actually cracking it.  A really good
     conventional cipher might possibly be harder to crack than even a
     "military grade" RSA key.  The attraction of public key cryptography
     is not because it is intrinsically stronger than a conventional
     cipher-- its appeal is because it helps you manage keys more
     conveniently.



     Data Compression
     ----------------

     PGP normally compresses the plaintext before encrypting it.  It's too
     late to compress it after it has been encrypted; encrypted data is
     incompressible.  Data compression saves modem transmission time and

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 24


     disk space and more importantly strengthens cryptographic security.
     Most cryptanalysis techniques exploit redundancies found in the
     plaintext to crack the cipher.  Data compression reduces this
     redundancy in the plaintext, thereby greatly enhancing resistance to
     cryptanalysis.  It takes extra time to compress the plaintext, but
     from a security point of view it seems worth it, at least in my
     cautious opinion.

     Files that are too short to compress or just don't compress well are
     not compressed by PGP.

     If you prefer, you can use PKZIP to compress the plaintext before
     encrypting it.  PKZIP is a widely-available and effective MSDOS
     shareware compression utility from PKWare, Inc.  Or you can use ZIP,
     a PKZIP-compatible freeware compression utility on Unix and other
     systems, available from Jean-Loup Gailly.  There is some advantage in
     using PKZIP or ZIP in certain cases, because unlike PGP's built-in
     compression algorithm, PKZIP and ZIP have the nice feature of
     compressing multiple files into a single compressed file, which is
     reconstituted again into separate files when decompressed.  PGP will
     not try to compress a plaintext file that has already been
     compressed.  After decrypting, the recipient can decompress the
     plaintext with PKUNZIP.  If the decrypted plaintext is a PKZIP
     compressed file, PGP automatically recognizes this and advises the
     recipient that the decrypted plaintext appears to be a PKZIP file.

     For the technically curious readers, the current version of PGP uses
     the freeware ZIP compression routines written by Jean-loup Gailly,
     Mark Adler, and Richard B. Wales.  This ZIP software uses
     functionally-equivalent compression algorithms as those used by
     PKWare's new PKZIP 2.0.  This ZIP compression software was selected
     for PGP mainly because of its free portable C source code
     availability, and because it has a really good compression ratio, and
     because it's fast.



     Message Digests and Digital Signatures
     --------------------------------------

     To create a digital signature, PGP encrypts with your secret key.
     But PGP doesn't actually encrypt your entire message with your secret
     key-- that would take too long.  Instead, PGP encrypts a "message
     digest".

     The message digest is a compact (128 bit) "distillate" of your
     message, similar in concept to a checksum.  You can also think of it
     as a "fingerprint" of the message.  The message digest "represents"
     your message, such that if the message were altered in any way, a
     different message digest would be computed from it.  This makes it
     possible to detect any changes made to the message by a forger.  A
     message digest is computed using a cryptographically strong one-way
     hash function of the message.  It would be computationally infeasible
     for an attacker to devise a substitute message that would produce an
     identical message digest.  In that respect, a message digest is much
     better than a checksum, because it is easy to devise a different

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 25


     message that would produce the same checksum.  But like a checksum,
     you can't derive the original message from its message digest.

     A message digest alone is not enough to authenticate a message.  The
     message digest algorithm is publicly known, and does not require
     knowledge of any secret keys to calculate.  If all we did was attach
     a message digest to a message, then a forger could alter a message
     and simply attach a new message digest calculated from the new
     altered message.  To provide real authentication, the sender has to
     encrypt (sign) the message digest with his secret key.

     A message digest is calculated from the message by the sender.  The
     sender's secret key is used to encrypt the message digest and an
     electronic timestamp, forming a digital signature, or signature
     certificate.  The sender sends the digital signature along with the
     message.  The receiver receives the message and the digital
     signature, and recovers the original message digest from the digital
     signature by decrypting it with the sender's public key.  The
     receiver computes a new message digest from the message, and checks
     to see if it matches the one recovered from the digital signature.  If
     it matches, then that proves the message was not altered, and it came
     from the sender who owns the public key used to check the signature.

     A potential forger would have to either produce an altered message
     that produces an identical message digest (which is infeasible), or
     he would have to create a new digital signature from a different
     message digest (also infeasible, without knowing the true sender's
     secret key).

     Digital signatures prove who sent the message, and that the message
     was not altered either by error or design.  It also provides
     non-repudiation, which means the sender cannot easily disavow his
     signature on the message.

     Using message digests to form digital signatures has other advantages
     besides being faster than directly signing the entire actual message
     with the secret key.  Using message digests allows signatures to be
     of a standard small fixed size, regardless of the size of the actual
     message.  It also allows the software to check the message integrity
     automatically, in a manner similar to using checksums.  And it allows
     signatures to be stored separately from messages, perhaps even in a
     public archive, without revealing sensitive information about the
     actual messages, because no one can derive any message content from a
     message digest.

     The message digest algorithm used here is the MD5 Message Digest
     Algorithm, placed in the public domain by RSA Data Security, Inc.
     MD5's designer, Ronald Rivest, writes this about MD5:

     "It is conjectured that the difficulty of coming up with two messages
     having the same message digest is on the order of 2^64 operations,
     and that the difficulty of coming up with any message having a given
     message digest is on the order of 2^128 operations.  The MD5
     algorithm has been carefully scrutinized for weaknesses.  It is,
     however, a relatively new algorithm and further security analysis is
     of course justified, as is the case with any new proposal of this

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 26


     sort.  The level of security provided by MD5 should be sufficient for
     implementing very high security hybrid digital signature schemes
     based on MD5 and the RSA public-key cryptosystem."



     Compatibility with Previous Versions of PGP
     ===========================================

     I'm sorry, PGP version 2.0 is not compatible with PGP version 1.0.
     If you have keys generated with version 1.0, you will have to
     generate new keys to use with this version.  This version of PGP uses
     all new algorithms for conventional cryptography, compression, and
     message digests, as well as using a much better approach to key
     management.  There were just too many changes to make it compatible
     with the old format messages, signatures, and keys.  Perhaps we could
     have provided a special conversion utility to convert old keys into
     new keys, but we were all tired and wanted to get the new release out
     the door.  Besides, converting the old keys into new keys would
     probably create more problems than it would solve, because we have
     changed to a new recommended uniform style for the user ID that
     includes the full name and E-mail address in a particular syntax.

     We made some effort to design the internal data structures of this
     version of PGP to be adaptable to future changes, so that hopefully
     you will not be required to discard and regenerate your keys in future
     versions.



     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 27



     Vulnerabilities
     ===============

     No data security system is impenetrable.  PGP can be circumvented in
     a variety of ways.  In any data security system, you have to ask
     yourself if the information you are trying to protect is more
     valuable to your attacker than the cost of the attack.  This should
     lead you to protecting yourself from the cheapest attacks, while not
     worrying about the more expensive attacks.

     Some of the discussion that follows may seem unduly paranoid, but
     such an attitude is appropriate for a reasonable discussion of
     vulnerability issues.


     Compromised Pass Phrase and Secret Key
     --------------------------------------

     Probably the simplest attack is if you leave your pass phrase for
     your secret key written down somewhere.  If someone gets it and also
     gets your secret key file, they can read your messages and make
     signatures in your name.

     Don't use obvious passwords that can be easily guessed, such as the
     names of your kids or spouse.  If you make your pass phrase a single
     word, it can be easily guessed by having a computer try all the words
     in the dictionary until it finds your password.  That's why a pass
     phrase is so much better than a password.  A more sophisticated
     attacker may have his computer scan a book of famous quotations to
     find your pass phrase.  An easy to remember but hard to guess pass
     phrase can be easily constructed by some creatively nonsensical
     sayings or very obscure literary quotes.

     For further details, see the section "How to Protect Secret Keys from
     Disclosure" in the Essential Topics volume of the PGP User's Guide.


     Public Key Tampering
     --------------------

     A major vulnerability exists if public keys are tampered with.  This
     may be the most crucially important vulnerability of a public key
     cryptosystem, in part because most novices don't immediately
     recognize it.  The importance of this vulnerability, and appropriate
     hygienic countermeasures, are detailed in the section "How to Protect
     Public Keys from Tampering" in the Essential Topics volume.

     To summarize:  When you use someone's public key, make certain it has
     not been tampered with.  A new public key from someone else should be
     trusted only if you got it directly from its owner, or if it has been
     signed by someone you trust.  Make sure no one else can tamper with
     your own public key ring.  Maintain physical control of both your
     public key ring and your secret key ring, preferably on your own
     personal computer rather than on a remote timesharing system.  Keep a
     backup copy of both key rings.

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 28




     "Not Quite Deleted" Files
     -------------------------

     Another potential security problem is caused by how most operating
     systems delete files.  When you encrypt a file and then delete the
     original plaintext file, the operating system doesn't actually
     physically erase the data.  It merely marks those disk blocks as
     deleted, allowing the space to be reused later.  It's sort of like
     discarding sensitive paper documents in the paper recycling bin
     instead of the paper shredder.  The disk blocks still contain the
     original sensitive data you wanted to erase, and will probably
     eventually be overwritten by new data at some point in the future.
     If an attacker reads these deleted disk blocks soon after they have
     been deallocated, he could recover your plaintext.

     In fact this could even happen accidentally, if for some reason
     something went wrong with the disk and some files were accidentally
     deleted or corrupted.  A disk recovery program may be run to recover
     the damaged files, but this often means some previously deleted files
     are resurrected along with everything else.  Your confidential files
     that you thought were gone forever could then reappear and be
     inspected by whomever is attempting to recover your damaged disk.
     Even while you are creating the original message with a word
     processor or text editor, the editor may be creating multiple
     temporary copies of your text on the disk, just because of its
     internal workings.  These temporary copies of your text are deleted
     by the word processor when it's done, but these sensitive fragments
     are still on your disk somewhere.

     Let me tell you a true horror story.  I had a friend, married with
     young children, who once had a brief and not very serious affair.
     She wrote a letter to her lover on her word processor, and deleted
     the letter after she sent it.  Later, after the affair was over, the
     floppy disk got damaged somehow and she had to recover it because it
     contained other important documents.  She asked her husband to
     salvage the disk, which seemed perfectly safe because she knew she
     had deleted the incriminating letter.  Her husband ran a commercial
     disk recovery software package to salvage the files.  It recovered
     the files alright, including the deleted letter.  He read it, which
     set off a tragic chain of events.

     The only way to prevent the plaintext from reappearing is to somehow
     cause the deleted plaintext files to be overwritten.  Unless you know
     for sure that all the deleted disk blocks will soon be reused, you
     must take positive steps to overwrite the plaintext file, and also
     any fragments of it on the disk left by your word processor.  You can
     overwrite the original plaintext file after encryption by using the
     PGP -w (wipe) option.  You can take care of any fragments of the
     plaintext left on the disk by using any of the disk utilities
     available that can overwrite all of the unused blocks on a disk.  For
     example, the Norton Utilities for MSDOS can do this.

     Even if you overwrite the plaintext data on the disk, it may still be
     possible for a resourceful and determined attacker to recover the

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 29


     data.  Faint magnetic traces of the original data remain on the disk
     after it has been overwritten.  Special sophisticated disk recovery
     hardware can sometimes be used to recover the data.


     Viruses and Trojan Horses
     -------------------------

     Another attack could involve a specially-tailored hostile computer
     virus or worm that might infect PGP or your operating system.  This
     hypothetical virus could be designed to capture your pass phrase or
     secret key or deciphered messages, and covertly write the captured
     information to a file or send it through a network to the virus's
     owner.  Or it might alter PGP's behavior so that signatures are not
     properly checked.  This attack is cheaper than cryptanalytic attacks.

     Defending against this falls under the category of defending against
     viral infection generally.  There are some moderately capable
     anti-viral products commercially available, and there are hygienic
     procedures to follow that can greatly reduce the chances of viral
     infection.  A complete treatment of anti-viral and anti-worm
     countermeasures is beyond the scope of this document.  PGP has no
     defenses against viruses, and assumes your own personal computer is a
     trustworthy execution environment.  If such a virus or worm actually
     appeared, hopefully word would soon get around warning everyone.

     Another similar attack involves someone creating a clever imitation
     of PGP that behaves like PGP in most respects, but doesn't work the
     way it's supposed to.  For example, it might be deliberately crippled
     to not check signatures properly, allowing bogus key certificates to
     be accepted.  This "Trojan horse" version of PGP is not hard for an
     attacker to create, because PGP source code is widely available, so
     anyone could modify the source code and produce a lobotomized zombie
     imitation PGP that looks real but does the bidding of its diabolical
     master.  This Trojan horse version of PGP could then be widely
     circulated, claiming to be from me.  How insidious.

     You should make an effort to get your copy of PGP from a reliable
     source, whatever that means.  Or perhaps from more than one
     independent source, and compare them with a file comparison utility.

     There are other ways to check PGP for tampering, using digital
     signatures.  If someone you trust signs the executable version of
     PGP, vouching for the fact that it has not been infected or tampered
     with, you can be reasonably sure that you have a good copy.  You
     could use an earlier trusted version of PGP to check the signature on
     a later suspect version of PGP.  But this will not help at all if
     your operating system is infected, nor will it detect if your
     original copy of PGP.EXE has been maliciously altered in such a way
     as to compromise its own ability to check signatures.  This test also
     assumes that you have a good trusted copy of the public key that you
     use to check the signature on the PGP executable.


     Physical Security Breach
     ------------------------

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 30



     A physical security breach may allow someone to physically acquire
     your plaintext files or printed messages.  A determined opponent
     might accomplish this through burglary, trash-picking, unreasonable
     search and seizure, or bribery, blackmail or infiltration of your
     staff.  Some of these attacks may be especially feasible against
     grassroots political organizations that depend on a largely volunteer
     staff.  It has been widely reported in the press that the FBI's
     COINTELPRO program used burglary, infiltration, and illegal bugging
     against antiwar and civil rights groups.  And look what happened at
     the Watergate Hotel.

     Don't be lulled into a false sense of security just because you have
     a cryptographic tool.  Cryptographic techniques protect data only
     while it's encrypted-- direct physical security violations can still
     compromise plaintext data or written or spoken information.

     This kind of attack is cheaper than cryptanalytic attacks on PGP.


     Tempest Attacks
     ---------------

     Another kind of attack that has been used by well-equipped opponents
     involves the remote detection of the electromagnetic signals from
     your computer.  This expensive and somewhat labor-intensive attack is
     probably still cheaper than direct cryptanalytic attacks.  An
     appropriately instrumented van can park near your office and remotely
     pick up all of your keystrokes and messages displayed on your
     computer video screen.  This would compromise all of your passwords,
     messages, etc.  This attack can be thwarted by properly shielding all
     of your computer equipment and network cabling so that it does not
     emit these signals.  This shielding technology is known as "Tempest",
     and is used by some Government agencies and defense contractors.
     There are hardware vendors who supply Tempest shielding commercially,
     although it may be subject to some kind of Government licensing.  Now
     why do you suppose the Government would restrict access to Tempest
     shielding?


     Exposure on Multi-user Systems
     ------------------------------

     PGP was originally designed for a single-user MSDOS machine under
     your direct physical control.  I run PGP at home on my own PC, and
     unless someone breaks into my house or monitors my electromagnetic
     emissions, they probably can't see my plaintext files or secret keys.

     But now PGP also runs on multi-user systems such as Unix and VAX/VMS.
     On multi-user systems, there are much greater risks of your plaintext
     or keys or passwords being exposed.  The Unix system administrator or
     a clever intruder can read your plaintext files, or perhaps even use
     special software to covertly monitor your keystrokes or read what's
     on your screen.  On a Unix system, any other user can read your
     environment information remotely by simply using the Unix "ps"
     command.  Similar problems exist for MSDOS machines connected on a

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 31


     local area network.  The actual security risk is dependent on your
     particular situation.  Some multi-user systems may be safe because
     all the users are trusted, or because they have system security
     measures that are safe enough to withstand the attacks available to
     the intruders, or because there just aren't any sufficiently
     interested intruders.  Some Unix systems are safe because they are
     only used by one user-- there are even some notebook computers
     running Unix.  It would be unreasonable to simply exclude PGP from
     running on all Unix systems.

     PGP is not designed to protect your data while it is in plaintext
     form on a compromised system.  Nor can it prevent an intruder from
     using sophisticated measures to read your secret key while it is
     being used.  You will just have to recognize these risks on
     multi-user systems, and adjust your expectations and behavior
     accordingly.  Perhaps your situation is such that you should consider
     only running PGP on an isolated single-user system under your direct
     physical control.  That's what I do, and that's what I recommend.


     Traffic Analysis
     ----------------

     Even if the attacker cannot read the contents of your encrypted
     messages, he may be able to infer at least some useful information by
     observing where the messages come from and where they are going, the
     size of the messages, and the time of day the messages are sent.
     This is analogous to the attacker looking at your long distance phone
     bill to see who you called and when and for how long, even though the
     actual content of your calls is unknown to the attacker.  This is
     called traffic analysis.  PGP alone does not protect against traffic
     analysis.  Solving this problem would require specialized
     communication protocols designed to reduce exposure to traffic
     analysis in your communication environment, possibly with some
     cryptographic assistance.


     Cryptanalysis
     -------------

     An expensive and formidable cryptanalytic attack could possibly be
     mounted by someone with vast supercomputer resources, such as a
     Government intelligence agency.  They might crack your RSA key by
     using some new secret factoring breakthrough.  Perhaps so, but it is
     noteworthy that the US Government trusts the RSA algorithm enough in
     some cases to use it to protect its own nuclear weapons, according to
     Ron Rivest.  And civilian academia has been intensively attacking it
     without success since 1978.

     Perhaps the Government has some classified methods of cracking the
     IDEA(tm) conventional encryption algorithm used in PGP.  This is
     every cryptographer's worst nightmare.  There can be no absolute
     security guarantees in practical cryptographic implementations.

     Still, some optimism seems justified.  The IDEA algorithm's designers
     are among the best cryptographers in Europe.  It has had extensive

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 32


     security analysis and peer review from some of the best cryptanalysts
     in the unclassified world.  It appears to have some design advantages
     over the DES in withstanding differential cryptanalysis, which has
     been used to crack the DES.

     Besides, even if this algorithm has some subtle unknown weaknesses,
     PGP compresses the plaintext before encryption, which should greatly
     reduce those weaknesses.  The computational workload to crack it is
     likely to be much more expensive than the value of the message.

     If your situation justifies worrying about very formidable attacks of
     this caliber, then perhaps you should contact a data security
     consultant for some customized data security approaches tailored to
     your special needs.  Boulder Software Engineering, whose address and
     phone are given at the end of this document, can provide such
     services.


     In summary, without good cryptographic protection of your data
     communications, it may have been practically effortless and perhaps
     even routine for an opponent to intercept your messages, especially
     those sent through a modem or E-mail system.  If you use PGP and
     follow reasonable precautions, the attacker will have to expend far
     more effort and expense to violate your privacy.

     If you protect yourself against the simplest attacks, and you feel
     confident that your privacy is not going to be violated by a
     determined and highly resourceful attacker, then you'll probably be
     safe using PGP.  PGP gives you Pretty Good Privacy.



     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 33



     Legal Issues
     ============


     Trademarks, Copyrights, and Warranties
     --------------------------------------

     "Pretty Good Privacy", "Phil's Pretty Good Software", and the "Pretty
     Good" label for computer software and hardware products are all
     trademarks of Philip Zimmermann and Phil's Pretty Good Software.  PGP
     is (c) Copyright Philip R. Zimmermann, 1990-1992.  Philip Zimmermann
     also holds the copyright for the PGP User's Manual, as well as any
     foreign language translations of the manual or the software.

     The author assumes no liability for damages resulting from the use of
     this software, even if the damage results from defects in this
     software, and makes no representations concerning the merchantability
     of this software or its suitability for any specific purpose.  It is
     provided "as is" without express or implied warranty of any kind.


     Patent Rights on the Algorithms
     -------------------------------

     When I first released PGP, I half-expected to encounter some form of
     legal harassment from the Government.  Indeed, there has been legal
     harrassment, but it hasn't come from the Government-- it has come
     from a private corporation.

     The RSA public key cryptosystem was developed at MIT with Federal
     funding from grants from the National Science Foundation and the
     Navy.  It is patented by MIT (U.S. patent #4,405,829, issued 20 Sep
     1983).  A company in California called Public Key Partners (PKP) holds
     the exclusive commercial license to sell and sub-license the RSA
     public key cryptosystem.  The author of this software implementation
     of the RSA algorithm is providing this implementation for educational
     use only.  Licensing this algorithm from PKP is the responsibility of
     you, the user, not Philip Zimmermann, the author of this software
     implementation.  The author assumes no liability for any patent
     infringement that may result from the unlicensed use by the user of
     the underlying RSA algorithm used in this software.  Foreign users
     should note that the RSA patent does not apply outside the US, and
     there is no RSA patent in any other country.  Federal agencies may
     use it because the Government paid for the development of RSA.

     Unfortunately, PKP is not offering any licensing of their RSA patent
     to end users of PGP.  This essentially makes PGP contraband in the
     USA.  Jim Bidzos, president of PKP, threatened to take legal action
     against me unless I stop distributing PGP, until they can devise a
     licensing scheme for it.  I agreed to this, since PGP is already in
     wide circulation and waiting a while for a licensing arrangement from
     PKP seemed reasonable.  Mr. Bidzos assured me (he even used the word
     "promise") several times since the initial 5 June 91 release of PGP
     that they were working on a licensing scheme for PGP.  Apparently, my
     release of PGP helped provide the impetus for them to offer some sort

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 34


     of a freeware-style license for noncommercial use of the RSA
     algorithm.  However, in December 1991 Mr. Bidzos said he had no plans
     to ever license the RSA algorithm to PGP users, and denied ever
     implying that he would.  Meanwhile, I have continued to refrain from
     distributing PGP, although I've recently updated the PGP User's
     Guide, and have provided the design guidance for these new revisions
     of PGP.  Ironically, all this legal controversy from PKP has imparted
     a forbidden flavor to PGP that has only served to amplify its
     universal popularity.

     I wrote my PGP software from scratch, with my own implementation of
     the RSA algorithm.  I didn't steal any software from PKP.  Before
     publishing PGP, I got a formal written legal opinion from a patent
     attorney with extensive experience in software patents.  I'm
     convinced that publishing PGP the way I did does not violate patent
     law.  However, it is a well known axiom in the US legal system that
     regardless of the law, he with the most money and lawyers prevails,
     if not by actually winning then by crushing the little guy with legal
     expenses.

     Not only did PKP acquire the exclusive patent rights for the RSA
     cryptosystem, which was developed with your tax dollars, but they
     also somehow acquired the exclusive rights to three other patents
     covering rival public key schemes invented by others, also developed
     with your tax dollars.  This essentially gives one company a legal
     lock in the USA on nearly all practical public key cryptosystems.
     They even appear to be claiming patent rights on the very concept of
     public key cryptography, regardless of what clever new original
     algorithms are independently invented by others.  And you thought
     patent law was designed to encourage innovation!  PKP does not
     actually develop any software-- they don't even have an engineering
     department-- they are essentially a litigation company.

     Public key cryptography is destined to become a crucial technology in
     the protection of our civil liberties and privacy in our increasingly
     connected society.  Why should the Government try to limit access to
     this key technology, when a single monopoly can do it for them?

     It appears certain that there will be future releases of PGP,
     regardless of the outcome of licensing problems with Public Key
     Partners.  If PKP does not license PGP, then future releases of PGP
     might not come from me.  There are countless fans of PGP outside the
     US, and many of them are software engineers who want to improve PGP
     and promote it, regardless of what I do.  The second release of PGP
     was a joint effort of an international team of software engineers,
     implementing enhancements to the original PGP with design guidance
     from me.  It is being released by Peter Gutmann in New Zealand, out
     of reach of US patent law.  It is being released only in Europe and
     New Zealand, but it may spontaneously spread to the USA without any
     help from me or the PGP development team.

     The IDEA(tm) conventional block cipher used by PGP is covered by a
     patent in Europe, held by ETH and a Swiss company called Ascom-Tech
     AG.  The patent number is PCT/CH91/00117.  International patents are
     pending.  IDEA(tm) is a trademark of Ascom-Tech AG.  There is no
     license fee required for noncommercial use of IDEA.  Ascom Tech AG

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 35


     has granted permission for PGP to use the IDEA cipher, and places no
     restrictions on using PGP for any purpose, including commercial use.
     You may not extract the IDEA cipher from PGP and put it in another
     commercial product without a license.  Commercial users of IDEA may
     obtain licensing details from Dieter Profos, Ascom Tech AG, Solothurn
     Lab, Postfach 151, 4502 Solothurn, Switzerland, Tel [phone],
     Fax [phone].

     The ZIP compression routines in PGP come from freeware source code,
     with the author's permission.  I'm not aware of any patents on the
     ZIP algorithm, but you're welcome to check into that question
     yourself.  If there are any obscure patent claims that apply to ZIP,
     then sorry, you'll have to take care of the patent licensing, not me.

     All this patent stuff reminds me of a Peanuts cartoon I saw in the
     newspaper where Lucy showed Charlie Brown a fallen autumn leaf and
     said "This is the first leaf to fall this year."  Charlie Brown said,
     "How do you know that?  Leaves have been falling for weeks."  Lucy
     replied, "I had this one notarized."


     Licensing and Distribution
     --------------------------

     In the USA PKP controls, through US patent law, the licensing of the
     RSA algorithm.  But I have no objection to anyone freely using or
     distributing my PGP software, without payment of fees to me.  You must
     keep the copyright notices on PGP and keep this documentation with
     it.  However, if you live in the USA, this may not satisfy any legal
     obligations you may have to PKP for using the RSA algorithm as
     mentioned above.

     In fact, if you live in the USA, and you are not a Federal agency,
     you shouldn't actually run PGP on your computer, because Public Key
     Partners wants to forbid you from running my software.  PGP is
     contraband.

     Of course, I can't give any assurances, but my guess is that it seems
     unlikely that PKP would waste their time pursuing PGP end users for
     patent infringement.  There are just too many PGP users to go after.
     And why would they single you out?  But I certainly wouldn't want to
     imply that you do anything improper-- if PKP were offering licenses,
     I would urge you to obtain one.  But since they aren't, well, I guess
     you should just refrain from using PGP if you live in the USA.

     PGP is not shareware, it's freeware.  Forbidden freeware.  Published
     as a community service.  If I sold PGP for money, then I would get
     sued by PKP for using their RSA algorithm.  More importantly, giving
     PGP away for free will encourage far more people to use it, which
     hopefully will have a greater social impact.  This could lead to
     widespread awareness and use of the RSA public key cryptosystem,
     which will probably make more money for PKP in the long run.  If only
     they could see that.

     All the source code for PGP is available for free under the "Copyleft"
     General Public License from the Free Software Foundation (FSF).  A

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 36


     copy of the FSF General Public License is included in the source
     release package of PGP.

     Regardless of and perhaps contrary to some provisions of the FSF
     General Public License, the following terms apply:

     1)  Written discussions of PGP in magazines or books may include
         fragments of PGP source code and documentation, without
         restrictions.

     2)  Although the FSF General Public License allows non-proprietary
         derivative products, it prohibits proprietary derivative products.
         Despite this, I may grant you a special license if you want to
         derive a proprietary commercial product from some of PGP's parts.
         There may or may not be a fee depending on what kind of a deal you
         plan to pursue with PKP.  Retaining my copyright notice and
         attribution might suffice in some cases.  Give me a call and we'll
         discuss it.  I'm real easy to please.

     Feel free to disseminate the complete PGP release package as widely
     as possible.  Give it to all your friends.  If you have access to any
     electronic Bulletin Boards Systems, please upload the complete PGP
     executable object release package to as many BBS's as possible.  You
     may disseminate the PGP source release package too, if you've got
     it.  The PGP version 2.1 executable object release package for MSDOS
     contains the PGP executable software, documentation, sample key rings
     including my own public key, and signatures for the software and this
     manual, all in one PKZIP compressed file called PGP21.ZIP.  The PGP
     source release package for MSDOS contains all the C source files in
     one PKZIP compressed file called PGP21SRC.ZIP.

     You may obtain free copies or updates to PGP from thousands of BBS's
     worldwide or from other public sources such as Internet FTP sites.
     Don't ask me for a copy directly from me, since I'd rather avoid
     further legal problems with PKP at this time.  I might be able to
     tell you where you can get it, however.

     After all this work I have to admit I wouldn't mind getting some fan
     mail for PGP, to gauge its popularity.  Let me know what you think
     about it and how many of your friends use it.  Bug reports and
     suggestions for enhancing PGP are welcome, too.  Perhaps a future PGP
     release will reflect your suggestions.

     This project has not been funded and the project has nearly eaten me
     alive.  This means you can't count on a reply to your mail, unless
     you only need a short written reply and you include a stamped
     self-addressed envelope.  But I do reply to E-mail.  Please keep it in
     English, as my foreign language skills are weak.  If you call and I'm
     not in, it's best to just try again later.  I usually don't return
     long distance phone calls, unless you leave a message that I can call
     you collect.  If you need any significant amount of my time, I am
     available on a paid consulting basis, and I do return those calls.

     The most inconvenient mail I get is for some well-intentioned person
     to send me a few dollars asking me for a copy of PGP.  I can't send
     it to them because of the legal threats from PKP (or worse--

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 37


     sometimes these requests are from foreign countries, and I would be
     risking violating cryptographic export control laws).  Even if there
     were no legal hassles involved in sending PGP to them, they usually
     don't send enough money to make it worth my time ($50 might be worth
     my time if I were actually selling this stuff).  I'm just not set up
     as a low cost low volume mail order business.  I can't just ignore
     the request and keep the money, because they probably regard the
     money as a fee for me to fulfill their request.  If I return the
     money, I might have to get in my car and drive down to the post
     office and buy some postage stamps, because these requests rarely
     include a stamped self-addressed envelope.  And I have to take the
     time to write a polite reply that I can't do it.  If I postpone the
     reply and set the letter down on my desk, it might be buried within
     minutes and won't see the light of day again for months.  Multiply
     these minor inconveniences by the number of requests I get, and you
     can see the problem.  Isn't it enough that the software is free?  It
     would be nicer if people could try to get PGP from any of the myriad
     other sources.  If you don't have a modem, ask a friend to get it for
     you.  If you can't find it yourself, I don't mind answering a quick
     phone call.

     If anyone wants to volunteer to improve PGP, please let me know.  It
     could certainly use some more work.  Some features were deferred to
     get it out the door.  A number of PGP users have since donated their
     time to port PGP to Unix on Sun SPARCstations, to Ultrix, to VAX/VMS,
     to OS/2, to the Amiga, and to the Atari ST.  Perhaps you can help
     port it to some new environments.  But please let me know if you plan
     to port PGP, to avoid duplication of effort, and to avoid starting
     with an obsolete version of the source code.

     Because so many foreign language translations of PGP have been
     produced, most of them are not distributed with the regular PGP
     release package because it would require too much disk space.
     Separate language translation "kits" are available from a number of
     independent sources, and are sometimes available separately from the
     same distribution centers that carry the regular PGP release
     software.  These kits include translated versions of the file
     LANGUAGE.TXT, PGP.HLP, and the PGP User's Guide.  If you want to
     produce a translation for your own native language, contact me first
     to get the latest information and standard guidelines, and to find out
     if it's been translated to your language already.

     Future versions of PGP may have to change the data formats for
     messages, signatures, keys and key rings, in order to provide
     important new features.  This may cause backward compatibility
     problems with this version of PGP.  Future releases may provide
     conversion utilities to convert old keys, but you may have to dispose
     of old messages created with the old PGP.

     If you have access to the Internet, watch for announcements of new
     releases of PGP on the Internet newsgroups "sci.crypt" and PGP's own
     newsgroup, "alt.security.pgp".  There is also an interest group
     mailing list called info-pgp, which is intended for users without
     access to the "alt.security.pgp" newsgroup.  Info-pgp is moderated by
     Hugh Miller, and you may subscribe to it by writing him a letter at
     [e-mail].  Include your name and Internet

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 38


     address.  If you want to know where to get PGP, Hugh can send you a
     list of Internet FTP sites and BBS phone numbers.  Hugh may also be
     reached at [e-mail].



     Export Controls
     ---------------

     The Government has made it illegal in many cases to export good
     cryptographic technology, and that may include PGP.  They regard this
     kind of software as munitions.  This is determined by volatile State
     Department policies, not fixed laws.  I will not export this software
     out of the US or Canada in cases when it is illegal to do so under US
     State Department policies, and I assume no responsibility for other
     people exporting it on their own.

     If you live outside the US or Canada, I advise you not to violate US
     State Department policies by getting PGP from a US source.  Since
     thousands of domestic users got it after its initial publication, it
     somehow leaked out of the US and spread itself widely abroad, like
     dandelion seeds blowing in the wind.  If PGP has already found its
     way into your country, then I don't think you're violating US export
     law if you pick it up from a source outside of the US.  And there are
     no import restrictions on bringing cryptographic technology into the
     USA.

     Some foreign governments impose serious penalties on anyone inside
     their country using encrypted communications.  In some countries they
     might even shoot you for that.




     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 39



     Recommended Introductory Readings
     =================================

     1)  Dorothy Denning, "Cryptography and Data Security", Addison-Wesley,
         Reading, MA 1982
     2)  Dorothy Denning, "Protecting Public Keys and Signature Keys",
         IEEE Computer, Feb 1983
     3)  Martin E. Hellman, "The Mathematics of Public-Key Cryptography,"
         Scientific American, August 1979

     Other Readings
     ==============

     4)  Ronald Rivest, "The MD5 Message Digest Algorithm", MIT Laboratory
         for Computer Science, 1991
     5)  Xuejia Lai, "On the Design and Security of Block Ciphers",
         Institute for Signal and Information Processing, ETH-Zentrum,
         Zurich, Switzerland, 1992
     6)  Xuejia Lai, James L. Massey, Sean Murphy, "Markov Ciphers and
         Differential Cryptanalysis", Advances in Cryptology- EUROCRYPT'91
     7)  Philip Zimmermann, "A Proposed Standard Format for RSA
         Cryptosystems", Advances in Computer Security, Vol III, edited by
         Rein Turn, Artech House, 1988



     To Contact the Author
     =====================

     Philip Zimmermann may be reached at:

     Boulder Software Engineering
     3021 Eleventh Street
     Boulder, Colorado 80304  USA
     Phone [phone] (voice or FAX)  (10:00am - 7:00pm Mountain Time)
     Internet:  [e-mail]




     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 40



     Where to Get PGP
     ================

     The following describes how to get the freeware public key
     cryptographic software PGP (Pretty Good Privacy) from an anonymous
     FTP site on Internet, or from other sources.

     PGP has sophisticated key management, an RSA/conventional hybrid
     encryption scheme, message digests for digital signatures, data
     compression before encryption, and good ergonomic design.  PGP is
     well featured and fast, and has excellent user documentation.  Source
     code is free.

     PGP uses the RSA cryptosystem which is claimed by a US patent held by
     a company called Public Key Partners.  PGP users outside the US take
     note that there is no RSA patent outside the US.  Also, bear in mind
     that there are US and Canadian export laws prohibiting anyone inside
     the US and Canada from exporting cryptographic software like this.
     If you live outside the US, you're probably not violating US export
     law if you pick it up from a source outside of the US.

     What follows is a small sample of places that allegedly have PGP, as
     of 3 December 1992.  This information is not guaranteed to be
     correct.

     There are two compressed archive files in the PGP 2.1 MSDOS release.
     You must get pgp21.zip which contains the MSDOS binary executable and
     the PGP User's Guide, and you can optionally get pgp21src.zip which
     contains the source files.  These files can be decompressed with the
     MSDOS shareware archive decompression utility PKUNZIP.EXE.  For Unix
     users who lack an implementation of UNZIP, the source code can also
     be found in the compressed tar file pgp21src.tar.Z.

     A reminder:  Set mode to binary or image when doing an FTP transfer.
     Here are a few Internet sites that have PGP via anonymous FTP:

     Finland:    nic.funet.fi  (128.214.6.100)
                 Directory: /pub/unix/security/crypt/

     Italy:      ghost.dsi.unimi.it  (149.132.2.1)
                 Directory: /pub/security/

     USA:        pencil.cs.missouri.edu  (128.206.100.207)
                 Directory: /pub/crypt/

     For those lacking FTP connectivity to the net, nic.funet.fi also
     offers the files via email.  Send the following mail message to
     [e-mail]:

         ENCODER uuencode
         SEND pub/unix/security/crypt/pgp21src.zip
         SEND pub/unix/security/crypt/pgp21.zip

     This will deposit the two zipfiles, as (about) 15 batched messages in
     your mailbox within about 24 hours.  Save and uudecode.

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 41



     In the US, PGP may be found on God knows how many BBS systems, far
     too many to list here.  Still, if you don't have any local BBS phone
     numbers handy, here are some BBS's you might try.

     The GRAPEVINE BBS in Little Rock Arkansas has set up a special
     account for people to download PGP for free.  The SYSOP is Jim Wenzel,
     at [e-mail].  The following phone numbers are
     applicable and should be dialed in the order presented (i.e., the
     first one is the highest speed line):  [phone], (501)
     753-8121, [phone].  When asked to login use the following
     information:

         name: PGP USER        ('PGP' is 1st name, 'USER' is 2nd name)
         password: PGP

     The Northern Lights BBS in Troy, NY, has PGP for free download.  It
     is run by Daniel Ray.  Call [phone] at 300-2400 bps 8N1.  Then
     login directly to the pgp account as follows:

         tnllogin: pgp
         Password: key

     In Colorado, try this single-line BBS:  303 443-8292.  It is often
     busy, so keep trying.  Log in with your own name.

     PGP is also widely available on Fidonet, a large informal network of
     PC-based bulletin board systems interconnected via modems.  Check
     your local bulletin board systems.  It is available on many foreign
     and domestic Fidonet BBS sites.

     In New Zealand, try this (supposedly free) dial-up BBS system:
        Kappa Crucis:  [phone], -3725, -3324, -8424, -3094, -3393

     For information on PGP implementations on the Apple Macintosh,
     contact Eric Hughes, at [e-mail], phone (510)
     849-4729.  For information on PGP on the Commodore Amiga and
     Atari ST, or any other questions about where to get PGP for any
     other platform, contact Hugh Miller at [e-mail].

     Here are a few people and their email addresses or phone numbers you
     can contact in some countries to get information on local PGP
     availability:

     Peter Gutmann                 Hugh Kennedy
     [e-mail]         [e-mail]
     New Zealand                   Germany

     Branko Lankester              Miguel Angel Gallardo
     [e-mail]           [e-mail]
     [phone]                (341) 474 38 09
     The Netherlands               Spain

     Hugh Miller                   Eric Hughes
     [e-mail]     [e-mail]
     [phone]                [phone]

     PGPDOC2.TXT            Friday, December  4, 1992 3:02 pm            Page 42


     USA                           USA

     Jean-loup Gailly
     [e-mail]
     France