/usr/share/doc/gnupg-doc/pgp2x/pgp2x.html

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CLASS="ARTICLE"
><DIV
CLASS="ARTICLE"
><DIV
CLASS="TITLEPAGE"
><H1
CLASS="TITLE"
><A
NAME="AEN2"
>Replacing PGP 2.x with GnuPG</A
></H1
><P
CLASS="COPYRIGHT"
>Copyright &copy; 1999 Kyle Hasselbacher</P
><HR></DIV
><BLOCKQUOTE
CLASS="ABSTRACT"
><DIV
CLASS="ABSTRACT"
><P
></P
><A
NAME="AEN7"
></A
><P
>This article is based on an earlier
<A
HREF="http://www.toehold.com/~kyle/pgp-compat.html"
TARGET="_top"
>PGP 2.x/GnuPG 
compatability guide</A
> written by Kyle Hasselbacher 
(<CODE
CLASS="EMAIL"
>&#60;<A
HREF="mailto:kyle@toehold.com"
>kyle@toehold.com</A
>&#62;</CODE
>).
Mike Ashley (<CODE
CLASS="EMAIL"
>&#60;<A
HREF="mailto:jashley@acm.org"
>jashley@acm.org</A
>&#62;</CODE
>) edited and expanded it.
Michael Fischer v. Mollard (<CODE
CLASS="EMAIL"
>&#60;<A
HREF="mailto:mfvm@gmx.de"
>mfvm@gmx.de</A
>&#62;</CODE
>) transformed the 
HTML source to DocBook SGML and also expanded it further.
Some of the details described here came from the 
<TT
CLASS="LITERAL"
>gnupg-devel</TT
> and <TT
CLASS="LITERAL"
>gnupg-user</TT
> mailing lists.
The workaround for both signing with and encrypting to an RSA key were
taken from 
<A
HREF="http://muppet.faveve.uni-stuttgart.de/~gero/gpg-2comp/changes.html"
TARGET="_top"
>Gero 
Treuner's compatability script</A
>.
Please direct questions, bug reports, or suggesstions to
the maintainer, Mike Ashley.</P
><P
>Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".</P
><P
></P
></DIV
></BLOCKQUOTE
><DIV
CLASS="SECT1"
><HR><H2
CLASS="SECT1"
><A
NAME="AEN17"
>Introduction</A
></H2
><P
>This document describes how to communicate with people still using
old versions of PGP 2.x
GnuPG can be used as a nearly complete replacement for
PGP 2.x.
You may encrypt and decrypt PGP 2.x messages using imported
old keys, but you cannot generate PGP 2.x keys.
This document demonstrates how to extend the standard distribution of
GnuPG to support PGP 2.x keys as well as what options must be used to
ensure inter-operation with PGP 2.x users.  
It also warns of anomalies
with the interoperation of PGP 2.x and GnuPG.  </P
><P
><SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Note:</I
></SPAN
> Using the extension modules
<TT
CLASS="FILENAME"
>idea.c</TT
> and <TT
CLASS="FILENAME"
>rsa.c</TT
> without
licensing the patented algorithms they implement may be illegal.
I do not recommend you use these modules.  
If you have PGP 2.x keys, I suggest you revoke them in favor of new keys and
encourage correspondents who use PGP 2.x keys to do the same.</P
></DIV
><DIV
CLASS="SECT1"
><HR><H2
CLASS="SECT1"
><A
NAME="AEN24"
>Extending GnuPG to support PGP 2.x keys</A
></H2
><P
>The standard distribution of GnuPG does not support PGP 2.x keys since
PGP 2.x uses IDEA as its symmetric cipher and RSA for its public key
cipher. 
These algorithms are patented<A
NAME="AEN27"
HREF="#FTN.AEN27"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
and may only be used under certain restrictions. 
It is a GNU policy not to use patented algorithms, since patents on
algorithms are a contradiction to the spirit of free software.
Employing these algorithms limits your freedom to use GnuPG as you wish.</P
><P
>It may or may not be legal to use RSA and IDEA without licensing
these algorithms.
RSA is only patented in the United States, so it is legal to build RSA
versions outside of the United States. 
The extension module for GnuPG is such a version and it may be 
legally used outside the United States, but it is illegal to use it 
in the United States.  
In the United States there exists a reference implementation for RSA 
called RSAREF, available at
<A
HREF="tp://ftp.funet.fi/pub/crypt/cryptography/asymmetric/rsa/rsaref2.tar.gz"
TARGET="_top"
>ftp.funet.fi</A
> or at 
<A
HREF=" ftp://non-us.debian.org/debian-non-US/dists/stable/non-US/source/rsaref_19930105.orig.tar.gz"
TARGET="_top"
>debian.org</A
>, 
that may be used legally without a charge in the USA for non commercial 
use.
Due to export restrictions this code cannot be exported, so there
are two ways to integrate RSA into GnuPG: one for the USA and Canada, and
one for the rest of the world.</P
><P
>The situation for IDEA is simpler.
IDEA is patented in Europe and in the USA, and a patent for Japan is 
pending. 
The patent holder, Ascom, grants a non-commerical
<A
HREF="http://www.ascom.ch/infosec/idea/licensing.html"
TARGET="_top"
>license</A
>
for no charge, but the definition of non-commercial use is rather strict
You need to buy a license from Ascom if you want to use IDEA commercially.</P
><P
>To use the extension modules first obtain their source code,
<TT
CLASS="FILENAME"
>idea.c</TT
> and <TT
CLASS="FILENAME"
>rsa.c</TT
> or
<TT
CLASS="FILENAME"
>rsaref.c</TT
> from the directory of code 
<A
HREF="ftp://ftp.gnupg.org/pub/gcrypt/contrib/"
TARGET="_top"
>contributed</A
>
to GnuPG. 
Once you have the code, it must be compiled.
If you use <SPAN
CLASS="APPLICATION"
>gcc</SPAN
>, you would compile it as follows: </P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gcc -Wall -O2 -shared -fPIC -o idea idea.c</KBD
>  
[...]  
<SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gcc -Wall -O2 -shared -fPIC -o rsa rsa.c</KBD
>  
[...] # or
<SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gcc -Wall -O2 -shared -fPIC -o rsa rsaref.c /usr/lib/rsaref.a</KBD
>
 </PRE
></TD
></TR
></TABLE
><P
>The last argument <TT
CLASS="FILENAME"
>/usr/lib/rsaref.a</TT
> must be
replaced with the real location of the RSAREF library on your local machine.</P
><P
>Once compiled, GnuPG must be instructed to load it. 
This may be done using the option <CODE
CLASS="OPTION"
>load-extension</CODE
>, 
either on the command line or in the options file although typically it
is done in the options file.
For example, if you have put <TT
CLASS="FILENAME"
>idea</TT
>
and <TT
CLASS="FILENAME"
>rsa</TT
> in your <TT
CLASS="FILENAME"
>~/.gnupg</TT
>
directory, in the options file you may add</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
>load-extension ~/.gnupg/idea  
load-extension ~/.gnupg/rsa      </PRE
></TD
></TR
></TABLE
><P
>If you do not specify an explicit path, GnuPG searches the
extension modules in the default GnuPG module directory, which is
<TT
CLASS="FILENAME"
>/usr/local/lib/gnupg</TT
>. 
If you compiled GnuPG with a different install prefix using
<CODE
CLASS="OPTION"
>--prefix PREFIX</CODE
> when you configured
your GnuPG source, then the module directory is 
<TT
CLASS="FILENAME"
>PREFIX/lib/gnupg</TT
>.
Copy the two files `rsa' and `idea' into the module directory described
above.
Make sure everyone can read these files.
You do not have to make these files executable as these files are not
programs but shared modules.</P
></DIV
><DIV
CLASS="SECT1"
><HR><H2
CLASS="SECT1"
><A
NAME="AEN59"
>Importing PGP 2.x keys</A
></H2
><P
>Once the extensions are loaded it is straightforward to import a PGP 2.x
key pair using the option <CODE
CLASS="OPTION"
>import</CODE
>. 
There are two caveats, however.</P
><P
><P
></P
><UL
><LI
><P
>You must not export a private key from PGP 2.x as an ASCII-armored file.  
Because PGP 2.x predates the OpenPGP specification,
the armored message header PGP 2.x uses is not compliant with OpenPGP.
Because private key export is rare, GnuPG does not check for the
case when the ASCII-armored message is a private key.</P
></LI
><LI
><P
>GnuPG expects imported public keys to be self-signed
by the corresponding private key.
This is a prudent  precaution, and
both GnuPG and newer versions of PGP self-sign public keys when they
are first created.  
This is not done by PGP 2.x, however.
To solve this, you can first self-sign the public key before exporting
it from PGP 2.x.  
Alternatively, you can use the option
<CODE
CLASS="OPTION"
>allow-non-selfsigned-uid</CODE
> to force GnuPG to take the
key anyway.
It is recommended that you self-sign the key either
before exporting it or after you have imported it, though, since using
a non-self-signed key is a security risk.</P
></LI
></UL
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
><KBD
CLASS="USERINPUT"
> pgp -kx alice public.pgp</KBD
> 
Pretty Good Privacy(tm) 2.6.2 - Public-key encryption for the masses. 
[...]
Extracting from key ring: '/u/alice/.pgp/pubring.pgp', userid "alice". 
Key for user ID: Alice &lt;alice@cyb.org&#62; 
1024-bit key, Key ID 24E2C409, created 1999/09/18

Key extracted to file 'public.pgp'. 

<SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>pgp -kx alice private.pgp .pgp/secring.pgp</KBD
>
Pretty Good Privacy(tm) 2.6.2 - Public-key encryption for the masses.
[...] 

Extracting from key ring: '.pgp/secring.pgp', userid "alice".
Key for user ID: Alice &lt;alice@cyb.org&#62; 
1024-bit key, Key ID 24E2C409, created 1999/09/18 

Key extracted to file 'private.pgp'.

<SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
><SPAN
CLASS="APPLICATION"
>gpg</SPAN
> --import public.pgp</KBD
>
gpg: key 24E2C409: public key imported 
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1) 

<SAMP
CLASS="PROMPT"
>alice%</SAMP
><KBD
CLASS="USERINPUT"
><SPAN
CLASS="APPLICATION"
>gpg</SPAN
> --import private.pgp</KBD
> 
gpg: key 24E2C409: secret key imported 
gpg: Total number processed: 1 
gpg:       secret keys read: 1 
gpg:   secret keys imported: 1</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECT1"
><HR><H2
CLASS="SECT1"
><A
NAME="AEN81"
>Using PGP 2.x keys</A
></H2
><P
>An imported public key can be used to encrypt documents to a PGP 2.x
private key holder and check signatures made using a PGP 2.x private
key. 
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>It is important to realize that it is impossible to use a new OpenPGP
key to communicate with an PGP 2.x user, so you must import an old style
key if you want to communicate with a PGP 2.x user.</I
></SPAN
> </P
><DIV
CLASS="SECT2"
><HR><H3
CLASS="SECT2"
><A
NAME="AEN85"
>Encrypting a document to a PGP 2.x user</A
></H3
><P
>Encrypting a document uses several command-line options, and the
document to be encrypted must be specified as a file. </P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
><SPAN
CLASS="APPLICATION"
>gpg</SPAN
> --rfc1991 --cipher-algo idea --compress-algo 1 --encrypt --recipient alice secret</KBD
> 
gpg:
RSA keys are deprecated; please consider creating a new key and use this key in the future 
gpg: this cipher algorithm is depreciated; please use a more standard one!</PRE
></TD
></TR
></TABLE
><P
>Each of the command-line options are necessary.</P
><P
></P
><UL
><LI
><P
>The option <CODE
CLASS="OPTION"
>rfc1991</CODE
> is used to force
GnuPG to be more compliant with RFC 1991, which is the old PGP
specification implemented by PGP 2.x. 
If it is omitted, the output from GnuPG will be malformed and unusable 
by PGP 2.x.</P
></LI
><LI
><P
>The option <CODE
CLASS="OPTION"
>cipher-algo</CODE
> specifies the
symmetric cipher with which the document is to be encrypted. 
In the special case of encrypting to a PGP 2.x public key, the cipher specified
must be IDEA. 
If it is omitted, the document will usually be encrypted
using 3DES, an algorithm unsupported by PGP 2.x.</P
></LI
><LI
><P
>PGP 2.x's compression algorithm motivates how the rest of
the command is formed. 
The option <CODE
CLASS="OPTION"
>compress-algo</CODE
>
specifies that GnuPG must use the old zlib compression algorithm
used by PGP 2.x. 
Despite this, GnuPG uses partial length headers when
encrypting a stream of unknown size, and this is unsupported by PGP 2.x.
The document to be encrypted must therefore be in a file so that
GnuPG knows the total size of the document to be encrypted before
starting. 
So unfortunately, you cannot use pipes when using PGP 2.x keys.</P
></LI
></UL
></DIV
><DIV
CLASS="SECT2"
><HR><H3
CLASS="SECT2"
><A
NAME="AEN103"
>Signing a document for a PGP 2.x user</A
></H3
><P
>Signing a document is no different than when any other key is used.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
><SPAN
CLASS="APPLICATION"
>gpg</SPAN
> --local-user 0x24E2C409 --sign document</KBD
> 
You need a passphrase to unlock the secret key for 
user: "Alice &lt;alice@cyb.com&#62;" 
1024-bit RSA key, ID 24E2C409, created 1999-09-18 

gpg: RSA keys are deprecated; please consider creating a new key and use this 
key in the future </PRE
></TD
></TR
></TABLE
><P
>In this example, the option <CODE
CLASS="OPTION"
>local-user</CODE
> is used to
specify which private key to use for signing. 
Also, the output file is <TT
CLASS="FILENAME"
>document.gpg</TT
>. 
If the signature is to be verified
using PGP 2.x, it must be renamed to a filename with a
<TT
CLASS="FILENAME"
>.pgp</TT
> extension.</P
></DIV
><DIV
CLASS="SECT2"
><HR><H3
CLASS="SECT2"
><A
NAME="AEN114"
>Signing and encrypting a document for a PGP 2.x user</A
></H3
><P
>GnuPG does not have native support for both signing a document with
an RSA key and encrypting it to an RSA key.
GnuPG can be used in a workaround, however, that requires a few steps
to implement.
The process involves creating a detached signature and then using it
to build an encrypted file that can be decrypted and verified using
PGP 2.x.</P
><P
>There are four steps.
The first creates a detached signature</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gpg --detach-signature --recipient alice --local-user 0x24E2C409 document</KBD
> 

You need a passphrase to unlock the secret key for
user: "Alice &lt;alice@cyb.com&#62;"
1024-bit RSA key, ID 24E2C409, created 1999-09-18

gpg: RSA keys are deprecated; please consider creating a new key and use this
key in the future</PRE
></TD
></TR
></TABLE
><P
>The second step converts the document to an internal, literal format that is
unencrupted.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gpg --store -z 0 --output document.lit document</KBD
> </PRE
></TD
></TR
></TABLE
><P
>The third step combines the detached signature with the literal document.
This is what PGP 2.x uses to verify the signature after decryption.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>cat Notes.sig Notes.lit | gpg --no-options --no-literal --store --compress-algo 1 --output document.z</KBD
> 
gpg: NOTE: --no-literal is not for normal use!</PRE
></TD
></TR
></TABLE
><P
>The fourth and final step is to use GnuPG to encrypt the combined
signature and plaintext to yield an signed and encrypted document
that can be decrypted and verified using PGP 2.x.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gpg --rfc1991 --cipher-algo idea --no-literal --encrypt --recipient alice --output document.pgp document.z</KBD
> 
gpg: NOTE: --no-literal is not for normal use!
gpg: RSA keys are deprecated; please consider creating a new key and use this
key in the future
gpg: this cipher algorithm is depreciated; please use a more standard one!</PRE
></TD
></TR
></TABLE
><P
>The signed and encrypted document can also be ASCII-armored using the usual
options.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
>gpg --rfc1991 --cipher-algo idea --no-literal --encrypt --recipient alice --output document.asc --armor document.z</KBD
> 
gpg: NOTE: --no-literal is not for normal use!
gpg: RSA keys are deprecated; please consider creating a new key and use this
key in the future
gpg: this cipher algorithm is depreciated; please use a more standard one!</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECT2"
><HR><H3
CLASS="SECT2"
><A
NAME="AEN137"
>Decrypting a PGP 2.x document</A
></H3
><P
>An imported private key may be used to decrypt documents encrypted to
the key as well as make signatures using the key. Decrypting a message
is no more difficult than when any other key is used.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
><SPAN
CLASS="APPLICATION"
>gpg</SPAN
> secret.pgp</KBD
> 

You need a passphrase to unlock the secret key for 
user: "Alice &lt;alice@cyb.org&#62;" 
1024-bit RSA key, ID 24E2C409, created 1999-09-18

gpg: NOTE: cipher algorithm 1 not found in preferences 
gpg: secret.pgp: unknown suffix 
Enter new filename [secret]: </PRE
></TD
></TR
></TABLE
><P
>Again, the file renaming dialog can be avoided by renaming the input
file with a <TT
CLASS="FILENAME"
>.gpg</TT
> extension. 
The note emitted by GnuPG regarding cipher algorithm 1 not found in
the preferences may be safely ignored if seen.</P
></DIV
><DIV
CLASS="SECT2"
><HR><H3
CLASS="SECT2"
><A
NAME="AEN146"
>Verifying a PGP 2.x signature</A
></H3
><P
>Verifying a signature made using a PGP 2.x key is straightforward.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>alice%</SAMP
> <KBD
CLASS="USERINPUT"
><SPAN
CLASS="APPLICATION"
>gpg</SPAN
> document.pgp</KBD
> 
gpg: document.pgp: unknown suffix
Enter new filename [document]:  
File `document' exists. Overwrite (y/N)? y 
gpg: old style (PGP 2.x) signature
gpg: Signature made Sat Sep 18 17:55:30 1999 EST using RSA key ID 24E2C409 
gpg: Good signature from "Alice &lt;alice@cyb.org&#62;"</PRE
></TD
></TR
></TABLE
><P
>The file renaming dialog can be avoided if the document being
verified is renamed with a <TT
CLASS="FILENAME"
>.gpg</TT
> extension before
invoking <SPAN
CLASS="APPLICATION"
>gpg</SPAN
>.</P
></DIV
></DIV
><DIV
CLASS="SECT1"
><HR><H2
CLASS="SECT1"
><A
NAME="CLEARTEXT"
>Working with clear-text signatures</A
></H2
><P
>As of GnuPG release 1.0, there are outstanding issues with respect
to passing clear-text signed documents between GnuPG and all versions
of PGP.
The difficulties appear to be due to implementation deviations from the
OpenPGP specification.
With respect to PGP 2.x, signatures made by PGP 2.x can be  verified using
GnuPG and signatures made with GnuPG can be verified using PGP 2.x.
The document output from the verification will in both cases differ,
however, from the original document.
These differences are limited to whitespace, so it should not affect
human readability of signed documents.
If it is important to maintain complete integrity of the document,
you should avoid clear-text signatures.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN27"
HREF="#AEN27"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The RSA patent expires in September 2000.
The IDEA patent expires in 2011.</P
></TD
></TR
></TABLE
></BODY
></HTML
>
gnupg-doc 2003.04.06+dak1-1ubuntu1 / usr / share / doc / gnupg-doc / pgp2x / pgp2x.html
