operations no longer require two distinct BN_CTX structures. This may put
more "strain" on the current BN_CTX implementation (which has a fixed limit
to the number of variables it will hold), but so far this limit is not
triggered by any of the tests pass and I will be changing BN_CTX in the
near future to avoid this problem anyway.

This also changes the default RSA implementation code to use the BN_CTX in
favour of initialising some of its variables locally in each function.

dependency from asn1.

SDIRS.

included.

This is currently *very* experimental and needs to be more fully integrated
with the main verification code.

PR: 845

PR: 847

PR: 849

PR: 849

porting efforts. Also, add Richard's name to the prior change.

- Remove some unnecessary "+1"-like fudges. Sizes should be handled
  exactly, as enlarging size parameters causes needless bloat and may just
  make bugs less likely rather than fixing them: bn_expand() macro,
  bn_expand_internal(), and BN_sqr().
- Deprecate bn_dup_expand() - it's new since 0.9.7, unused, and not that
  useful.
- Remove unnecessary zeroing of unused bytes in bn_expand2().
- Rewrite BN_set_word() - it should be much simpler, the previous
  complexities probably date from old mismatched type issues.
- Add missing bn_check_top() macros in bn_word.c
- Improve some degenerate case handling in BN_[add|sub]_word(), add
  comments, and avoid a bignum expansion if an overflow isn't possible.

functions and macros.

This change has associated tags: LEVITTE_before_const and
LEVITTE_after_const.  Those will be removed when this change has been
properly reviewed.

compilers may complain.

Remove certain redundant BN_zero() initialisations, because BN_CTX_get(),
BN_init(), [etc] already initialise to zero.

Correct error checking in bn_sqr.c, and be less wishy-wash about how/why
the result's 'top' value is set (note also, 'max' is always > 0 at this
point).

bignums are passed in and out of functions and APIs in a consistent form
has highlighted that zero-valued bignums don't need any allocated word
data. The use of BN_set_word() to initialise a bignum to zero causes
needless allocation and gives it a return value that must be checked. This
change converts BN_zero() to a self-contained macro that has no
return/expression value and does not cause any expansion of bignum data.

Note, it would be tempting to rewrite the deprecated version as a
success-valued comma expression, such as;
   #define BN_zero(a) ((a)->top = (a)->neg = 0, 1)
However, this evaluates 'a' twice and would confuse initialisation loops
(eg. while(..) { BN_zero(bn++) } ). As such, the deprecated version
continues to use BN_set_word().

change to work properly; BN_zero() should set 'neg' to zero as well as
'top' to match the behaviour of BN_new().

For reference. Note that both cc and gcc support -Wl flag, but we can't
use -Wl,-[not]all with both drivers, because cc rearranges options
passed through -Wl. We can't use -Wl,-all,libcrypto.a,-notall with cc
either, because it refuses to start with "no input" error.

redefine bn_clear_top2max() to be a NOP in the non-debugging case, and
remove some unnecessary usages in bn_nist.c.

Submitted by: Nils Larsch
Reviewed by: Geoff Thorpe, Ulf Möller

return a "zero" bignum as BN_new() does - so reset 'top'. During
BN_CTX_end(), released bignums should be consistent so enforce this in
debug builds. Also, reduce the number of wasted BN_clear_free() calls from
BN_CTX_end() (typically by 75% or so).

Submitted by: Nils Larsch
Reviewed by: Geoff Thorpe, Ulf Möller