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Date: Tue, 16 Feb 2016 20:45:32 +0300
From: Solar Designer <solar@...nwall.com>
To: musl@...ts.openwall.com
Subject: Re: list of security features in musl
On Thu, Feb 11, 2016 at 08:11:19PM +0100, Szabolcs Nagy wrote:
> - about 'security feature lists':
> the fedora project lists 'sha256 based passwd hash' in glibc
> as a security feature[0], that implementation is
> - a denial of service attack vector (computation depends on
> key length more than the admin controlled round count).
> - arch dependent(!), one can craft a passwd entry such that
> only 32bit machines can log in.
What do you mean here? 32-bit overflow/wraparound with very high
rounds= specification?
> - unbounded alloca(!) use was fixed in 2012, long after
> fedora added support for it (the reference implementation
> in the spec still has the problem, among other issues[1]).
> - uses arbitrary sized realloc for the global crypt state
> even though 100 bytes would be enough (checks salt len
> after reallocation).
> - not standard conform c code: aligned attribute, alloca,
> section attribute, undefined behaviour: (tmp - (char *) 0).
> - meant to be used outside the libc, but secrets are cleared
> with memset which can be optimized away.
> (i think there are other issues in this sha256-crypt.c, but
> the point is: implementation details matter so security check
> lists should be taken with a grain of salt.)
>
> [0]: https://fedoraproject.org/wiki/Security_Features#Glibc_Enhancements
> [1]: http://www.openwall.com/lists/musl/2012/08/19/9
Another issue is that SHA-crypt leaks 8 bits via timing (total execution
time, not just cache-timing), for no good reason at all (not a tradeoff):
"18. repeast the following 16+A[0] times, where A[0] represents the first
byte in digest A interpreted as an 8-bit unsigned value
add the salt to digest DS"
For comparison, bcrypt is not cache-timing-safe, but that's a tradeoff.
Alexander
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