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Category:   Application (Security)  >   GnuPG (Gnu Privacy Guard) Vendors:
(Conectiva Issues Fix) GnuPG ElGamal Signature Flaw May Disclose Type 20 ElGamal Private Keys to Remote Users
SecurityTracker Alert ID:  1008426
SecurityTracker URL:
CVE Reference:   CVE-2003-0971   (Links to External Site)
Date:  Dec 9 2003
Impact:   Disclosure of authentication information
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 1.0.2 and later versions
Description:   A vulnerability was reported in GnuPG in the creation of ElGamal keys for digital signature. Keys used for signing can be compromised.

It is reported that Phong Nguyen discovered a flaw that allows a remote user to determine your private key within a few seconds.

In version 1.0.2 (January 2000), the flaw was introduced when the GnuPG code was modified to improve the efficiency of encryption using ElGamal keys. A common factor was used for encrypting and signing. As a result, a remtote user with access to a target user's signature can conduct a cryptographic attack to determine the target user's private key. This flaw reportedly affects only ElGamal sign+encrypt keys (type 20) in GnuPG version 1.0.2 and later.

The vendor reports that ElGamal encrypt-only keys (type 16) are not affected. Also, the DSA keys and RSA keys are not vulnerable, according to the report.

Impact:   A remote user can determine the ElGamal private key.
Solution:   Conectiva has released a fix.

Vendor URL: (Links to External Site)
Cause:   State error
Underlying OS:  Linux (Conectiva)
Underlying OS Comments:  8, 9

Message History:   This archive entry is a follow-up to the message listed below.
Nov 27 2003 GnuPG ElGamal Signature Flaw May Disclose Type 20 ElGamal Private Keys to Remote Users

 Source Message Contents

Subject:  [conectiva-updates] [CLA-2003:798] Conectiva Security Announcement - gnupg

Hash: SHA1

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PACKAGE   : gnupg
SUMMARY   : GnuPG's ElGamal signing keys vulnerability
DATE      : 2003-12-09 16:21:00
ID        : CLA-2003:798
RELEASES  : 8, 9

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 GnuPG[1] is an OpenPGP-compliant tool for secure communication used
 to, for example, sign emails, encrypt, decrypt and verify (signed)
 Phong Nguyen discovered[2] a vulnerability (CAN-2003-0971[3]) in the
 way GnuPG deals with type 20 ElGamal sign+encrypt keys which allows
 an attacker to recover the corresponding private key from a
 signature. Other keys, such as ElGamal type 16 used only for
 encryption, are not affected.
 This is a serious vulnerability with immediate impact: all ElGamal
 type 20 keys should be revoked and considered to be compromised, as
 well as data signed or encrypted with such a key.
 Please note that:
 - by default, GnuPG does not generate this type of key;
 - in order to create an ElGamal type 20 key, one must (in recent
 versions of GnuPG) add the --expert command line option, and even
 then a warning is given saying that this key should not be used. In
 older versions, only the warning is given.
 To identify the vulnerable type 20 ElGamal keys, look for the "G"
 identifier in a key listing such as the example below:
 pub  2048G/xxxxxxxx 2001-01-05 John Doe <>
 Other keys (including the ones identified by "g", lower case) are not
 The packages provided with this update have a patch[4] created by
 David Shaw which disables the creation of these keys and no longer
 allows them to be used to create signatures.

 It is recommended that all GnuPG users upgrade their packages.
 Additionally, users who have ever created ElGamal type 20
 sign+encrypt keys should revoke them and consider them to be
 compromised, as well as data signed or encrypted with such a key.


 The apt tool can be used to perform RPM packages upgrades:

 - run:                 apt-get update
 - after that, execute: apt-get upgrade

 Detailed instructions reagarding the use of apt and upgrade examples 
 can be found at

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All packages are signed with Conectiva's GPG key. The key and instructions
on how to import it can be found at
Instructions on how to check the signatures of the RPM packages can be
found at

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All our advisories and generic update instructions can be viewed at

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Copyright (c) 2003 Conectiva Inc.

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Version: GnuPG v1.0.6 (GNU/Linux)
Comment: For info see



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