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Joined: 12 May 2004
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PostPosted: Fri Dec 02, 2016 10:26 am    Post subject: [ GLSA 201612-01 ] GnuPG Reply with quote

Gentoo Linux Security Advisory

Title: GnuPG: RNG output is predictable (GLSA 201612-01)
Severity: normal
Exploitable: local
Date: December 02, 2016
Bug(s): #591536
ID: 201612-01

Synopsis

Due to a design flaw, the output of GnuPG's Random Number Generator
(RNG) is predictable.


Background

The GNU Privacy Guard, GnuPG, is a free replacement for the PGP suite of
cryptographic software.


Affected Packages

Package: app-crypt/gnupg
Vulnerable: < 1.4.21
Unaffected: >= 1.4.21
Architectures: All supported architectures


Description

A long standing bug (since 1998) in Libgcrypt (see “GLSA 201610-04”
below) and GnuPG allows an attacker to predict the output from the
standard RNG. Please review the “Entropy Loss and Output Predictability
in the Libgcrypt PRNG” paper below for a deep technical analysis.


Impact

An attacker who obtains 580 bytes of the random number from the standard
RNG can trivially predict the next 20 bytes of output.
This flaw does not affect the default generation of keys, because
running gpg for key creation creates at most 2 keys from the pool. For a
single 4096 bit RSA key, 512 bytes of random are required and thus for
the second key (encryption subkey), 20 bytes could be predicted from the
the first key.
However, the security of an OpenPGP key depends on the primary key
(which was generated first) and thus the 20 predictable bytes should not
be a problem. For the default key length of 2048 bit nothing will be
predictable.


Workaround

There is no known workaround at this time.

Resolution

All GnuPG 1 users should upgrade to the latest version:
Code:
# emerge --sync
      # emerge --ask --oneshot --verbose ">=app-crypt/gnupg-1.4.21"
   


References

CVE-2016-6313

Entropy Loss and Output Predictability in the Libgcrypt PRNG

GLSA 201610-04
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