Bug 328383 – <dev-libs/libtommath-0.42.0: weakness in mp_prime_next_prime()

• Product: Gentoo Security
• Status: RESOLVED FIXED

[Description Mark Karpeles 2010-07-15 12:08:18 UTC]

Function mp_prime_next_prime() (bn_mp_prime_next_prime.c) will find the next prime number based on a given prime number.

It will increment by a given step and test each returned number to see if those are prime. The "t" parameter defines the number of Miller-Rabin trials done which each prime contained in table ltm_prime_tab, starting at 0.

However the prime testing code has a bug and will test t times the same prime from ltm_prime_tab, resulting in potentially weaker prime testing.

bn_mp_prime_next_prime.c line 146

mp_set(&b, ltm_prime_tab[t]);
should be:
mp_set(&b, ltm_prime_tab[x]);

(the offset in ltm_prime_tab is supposed to be x, the incrementing value from the for loop, as in bn_mp_prime_is_prime.c line 63, testing the same prime again and again has no effect)

Reproducible: Always

Steps to Reproduce:

Actual Results:  
Potentially weaker prime (in worst case with t=1 - which is the effective case here - mp_prime_miller_rabin has 25% chances of being wrong as explained in bn_mp_prime_miller_rabin.c on line 22).
Also CPU is wasted doing the same operation again and again for nothing

Expected Results:  
Strong primes

Package dev-libs/libtomcrypt uses libtommath but doesn't seem to be calling mp_prime_next_prime() anymore (grep returns nothing)

[Comment 1 Mark Karpeles 2010-07-15 13:49:15 UTC]

Created attachment 238887 [details, diff]
Fix for mp_prime_next_prime() bug when checking generated prime

[Comment 2 Mark Karpeles 2010-07-15 14:25:02 UTC]

Update: mp_prime_next_prime() is used in net-misc/dropbear-0.52-r1

File dropbear-0.52/libtommath/bn_mp_prime_next_prime.c has been confirmed with the same bug.

I will contact upstream for dropbear to notify about this bug and let them fix their bundled libtommath version.

mp_prime_next_prime() called at:
./dropbear-0.52/gendss.c:83 (t=18)
./dropbear-0.52/genrsa.c:110 (t=8)

[Comment 3 Mark Karpeles 2010-07-15 15:13:47 UTC]

The dropbear issue has been moved to bug 328409

[Comment 4 Mark Karpeles 2010-07-23 14:57:09 UTC]

Upstream has released libtommath-0.42.0 which fixes this issue.

http://www.libtom.org/?page=download&newsitems=5&whatfile=ltm

Download from:
http://www.libtom.org/files/ltm-0.42.0.tar.bz2
http://www.libtom.org/files/ltm-0.42.0.tar.bz2.sig

Note that upstream site which was down since ~2008 is now back.

[Comment 5 Tim Harder 2011-06-10 01:01:33 UTC]

I added libtommath-0.42.0 to CVS which fixes the issue.

[Comment 6 Tim Sammut (RETIRED) 2011-06-12 18:31:57 UTC]

(In reply to comment #5)
> I added libtommath-0.42.0 to CVS which fixes the issue.

Great, thank you, Tim.

Arches, please test and mark stable:
=dev-libs/libtommath-0.42.0
Target keywords : "amd64 arm hppa ppc ppc64 x86"

[Comment 7 Agostino Sarubbo 2011-06-12 18:43:45 UTC]

amd64 ok.


Anyway for be picky, in src_test, when a binary for test are generated, "-O?" is not respected.
e.g.

cc -march=native -O2 -g0 -I./ -Wall -W -Wshadow -Wsign-compare -O3 -funroll-loops -fomit-frame-pointer demo/demo.o libtommath.a -o test

But it does not block obviously ;)

[Comment 8 Paweł Hajdan, Jr. (RETIRED) 2011-06-13 10:03:12 UTC]

x86 stable

[Comment 9 Ian Delaney (RETIRED) 2011-06-13 21:29:59 UTC]

amd64:

ok

[Comment 10 Jeroen Roovers (RETIRED) 2011-06-14 03:24:09 UTC]

Stable for HPPA.

[Comment 11 Markus Meier 2011-06-14 19:52:49 UTC]

arm stable

[Comment 12 Markos Chandras (RETIRED) 2011-06-18 07:49:51 UTC]

amd64 done. Thanks Agostino and Ian

[Comment 13 Kacper Kowalik (Xarthisius) (RETIRED) 2011-06-19 12:03:43 UTC]

ppc/ppc64 stable, last arch done

[Comment 14 Tim Sammut (RETIRED) 2011-06-20 03:31:35 UTC]

Thanks, everyone. GLSA Vote: no.

[Comment 15 Pierre-Yves Rofes (RETIRED) 2011-10-08 21:41:42 UTC]

no too, and closing.