Replacing PGP 2.x with GnuPG
This article is based on an earlier PGP 2.x/GnuPG compatability guide (http://www.toehold.com/~kyle/pgp-
compat.html) written by Kyle Hasselbacher (<
kyle@toehold.com
>). Mike Ashley (<
jashley@acm.org
>)
edited and expanded it. Michael Fischer v. Mollard (<
mfvm@gmx.de
>) transformed the HTML source to Doc-
Book SGML and also expanded it further. Some of the details described here came from the
gnupg-devel
and
gnupg-user
mailing lists. The workaround for both signing with and encrypting to an RSA key were taken
from Gero Treuner’s compatability script (http://muppet.faveve.uni-stuttgart.de/~gero/gpg-2comp/changes.html).
Please direct questions, bug reports, or suggesstions to the maintainer, Mike Ashley.
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".
Introduction
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.
Note: Using the extension modules
idea.c
and
rsa.c
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.
Extending GnuPG to support PGP 2.x keys
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
1
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.
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 ftp.funet.fi (tp://ftp.funet.fi/pub/crypt/cryptography/asymmetric/rsa/rsaref2.tar.gz) or at debian.org
( ftp://non-us.debian.org/debian-non-US/dists/stable/non-US/source/rsaref_19930105.orig.tar.gz), that may be
used legally without a charge in the USA for non commercial use. Due to export restrictions this code cannot
1.
The RSA patent expires in September 2000. The IDEA patent expires in 2011.
1
Replacing PGP 2.x with GnuPG
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.
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 license (http://www.ascom.ch/infosec/idea/licensing.html)
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.
To use the extension modules first obtain their source code,
idea.c
and
rsa.c
or
rsaref.c
from the
directory of code contributed (ftp://ftp.gnupg.org/pub/gcrypt/contrib/) to GnuPG. Once you have the code, it
must be compiled. If you use gcc, you would compile it as follows:
alice%
g Ell EyP Eshred Efsg Eo ide ideF
[...]
alice%
g Ell EyP Eshred Efsg Eo rs rsF
[...] # or
alice%
g Ell EyP Eshred Efsg Eo rs rsrefF GusrGliGrsrefF
The last argument
/usr/lib/rsaref.a
must be replaced with the real location of the RSAREF library
on your local machine.
Once compiled, GnuPG must be instructed to load it. This may be done using the option
load-extension
,
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
idea
and
rsa
in your
~/.gnupg
directory, in the options file you may add
load-extension ~/.gnupg/idea
load-extension ~/.gnupg/rsa
If you do not specify an explicit path, GnuPG searches the extension modules in the default GnuPG module
directory, which is
/usr/local/lib/gnupg
. If you compiled GnuPG with a different install prefix using
-
prefix PREFIX
when you configured your GnuPG source, then the module directory is
PREFIX/lib/gnupg
.
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.
Importing PGP 2.x keys
Once the extensions are loaded it is straightforward to import a PGP 2.x key pair using the option
import
.
There are two caveats, however.
•
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.
•
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
allow-non-selfsigned-uid
to force GnuPG to
2
Replacing PGP 2.x with GnuPG
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.
alice%
pgp Ekx lie puliFpgp
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
<
alice@cyb.org>
1024-bit key, Key ID 24E2C409, created 1999/09/18
Key extracted to file ’public.pgp’.
alice%
pgp Ekx lie privteFpgp FpgpGseringFpgp
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
<
alice@cyb.org>
1024-bit key, Key ID 24E2C409, created 1999/09/18
Key extracted to file ’private.pgp’.
alice%
gpg Eimport puliFpgp
gpg: key 24E2C409: public key imported
gpg: Total number processed: 1
gpg:
imported: 1
(RSA: 1)
alice%
gpg Eimport privteFpgp
gpg: key 24E2C409: secret key imported
gpg: Total number processed: 1
gpg:
secret keys read: 1
gpg:
secret keys imported: 1
Using PGP 2.x keys
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. 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.
Encrypting a document to a PGP 2.x user
Encrypting a document uses several command-line options, and the document to be encrypted must be specified
as a file.
alice%
gpg ErfIWWI EipherElgo ide EompressElgo I Eenrypt Ereipient lie seret
gpg:
RSA keys are deprecated; please consider creating a new key and use this key in the future
3
Replacing PGP 2.x with GnuPG
gpg: this cipher algorithm is depreciated; please use a more standard one!
Each of the command-line options are necessary.
•
The option
rfc1991
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 unus-
able by PGP 2.x.
•
The option
cipher-algo
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.
•
PGP 2.x’s compression algorithm motivates how the rest of the command is formed. The option
compress-
algo
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.
Signing a document for a PGP 2.x user
Signing a document is no different than when any other key is used.
alice%
gpg ElolEuser HxPRiPgRHW Esign doument
You need a passphrase to unlock the secret key for
user: "Alice
<
alice@cyb.com>"
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
In this example, the option
local-user
is used to specify which private key to use for signing. Also, the
output file is
document.gpg
. If the signature is to be verified using PGP 2.x, it must be renamed to a filename
with a
.pgp
extension.
Signing and encrypting a document for a PGP 2.x user
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.
There are four steps. The first creates a detached signature
alice%
gpg EdethEsignture Ereipient lie ElolEuser HxPRiPgRHW doument
You need a passphrase to unlock the secret key for
user: "Alice
<
alice@cyb.com>"
1024-bit RSA key, ID 24E2C409, created 1999-09-18
gpg: RSA keys are deprecated; please consider creating a new key and use this
4
Replacing PGP 2.x with GnuPG
key in the future
The second step converts the document to an internal, literal format that is unencrupted.
alice%
gpg Estore Ez H Eoutput doumentFlit doument
The third step combines the detached signature with the literal document. This is what PGP 2.x uses to
verify the signature after decryption.
alice%
t xotesFsig xotesFlit | gpg EnoEoptions EnoEliterl Estore EompressElgo I Eoutput doE
umentFz
gpg: NOTE: -no-literal is not for normal use!
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.
alice%
gpg ErfIWWI EipherElgo ide EnoEliterl Eenrypt Ereipient lie Eoutput doumentFpgp doE
umentFz
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!
The signed and encrypted document can also be ASCII-armored using the usual options.
alice%
gpg ErfIWWI EipherElgo ide EnoEliterl Eenrypt Ereipient lie Eoutput doumentFs E
rmor doumentFz
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!
Decrypting a PGP 2.x document
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.
alice%
gpg seretFpgp
You need a passphrase to unlock the secret key for
user: "Alice
<
alice@cyb.org>"
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]:
Again, the file renaming dialog can be avoided by renaming the input file with a
.gpg
extension. The note
emitted by GnuPG regarding cipher algorithm 1 not found in the preferences may be safely ignored if seen.
5
Replacing PGP 2.x with GnuPG
Verifying a PGP 2.x signature
Verifying a signature made using a PGP 2.x key is straightforward.
alice%
gpg doumentFpgp
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
<
alice@cyb.org>"
The file renaming dialog can be avoided if the document being verified is renamed with a
.gpg
extension
before invoking gpg.
Working with clear-text signatures
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.
Appendix A. GNU Free Documentation License
Version 1.1, March 2000
Copyright (C) 2000 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
0. PREAMBLE
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