This article is based on an earlier PGP 2.x/GnuPG compatability guide
(http://www.toehold.com/~kyle/pgpcompat. 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).
lease 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 interoperation 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 patented1 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 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% gcc -Wall -O2 -shared -fPIC -o idea idea.c
[...]
alice% gcc -Wall -O2 -shared -fPIC -o rsa rsa.c
[...] # or
alice% gcc -Wall -O2 -shared -fPIC -o rsa rsaref.c /usr/lib/rsaref.a
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 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 -kx alice public.pgp
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 -kx alice private.pgp .pgp/secring.pgp
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 -import public.pgp
gpg: key 24E2C409: public key imported
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
alice%gpg -import private.pgp
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 -rfc1991 -cipher-algo idea -compress-algo 1 -encrypt -recipient alice secret
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!
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 unusable 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 compressalgo 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 -local-user 0x24E2C409 -sign document
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 -detach-signature -recipient alice -local-user 0x24E2C409 document
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
The second step converts the document to an internal, literal format that is unencrupted.
alice% gpg -store -z 0 -output document.lit document
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% cat Notes.sig Notes.lit | gpg -no-options -no-literal -store -compress-algo 1 -output document.
z
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 -rfc1991 -cipher-algo idea -no-literal -encrypt -recipient alice -output document.pgp document.
z
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 -rfc1991 -cipher-algo idea -no-literal -encrypt -recipient alice -output document.asc -
armor document.z
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 secret.pgp
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.
Verifying a PGP 2.x signature
Verifying a signature made using a PGP 2.x key is straightforward.
alice% gpg document.pgp
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
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