gpg: Add build and runtime support for larger RSA keys

* configure.ac: Added --enable-large-secmem option.
* g10/options.h: Add opt.flags.large_rsa.
* g10/gpg.c: Contingent on configure option: adjust secmem size,
add gpg --enable-large-rsa, bound to opt.flags.large_rsa.
* g10/keygen.c: Adjust max RSA size based on opt.flags.large_rsa
* doc/gpg.texi: Document --enable-large-rsa.

--

This is a cherry-pick of 534e2876ac from
STABLE-BRANCH-1-4 against master

Some older implementations built and used RSA keys up to 16Kib, but
the larger secret keys now fail when used by more recent GnuPG, due to
secure memory limitations.

Building with ./configure --enable-large-secmem will make gpg
capable of working with those secret keys, as well as permitting the
use of a new gpg option --enable-large-rsa, which let gpg generate RSA
keys up to 8Kib when used with --batch --gen-key.

Debian-bug-id: 739424

Minor edits by wk.

GnuPG-bug-id: 1732

configure.ac

@@ -107,6 +107,7 @@ card_support=yes
 use_ccid_driver=yes
 dirmngr_auto_start=yes
 use_tls_library=no
+large_secmem=no
 
 GNUPG_BUILD_PROGRAM(gpg, yes)
 GNUPG_BUILD_PROGRAM(gpgsm, yes)
@@ -223,6 +224,20 @@ AC_ARG_ENABLE(selinux-support,
 AC_MSG_RESULT($selinux_support)
 
 
+AC_MSG_CHECKING([whether to allocate extra secure memory])
+AC_ARG_ENABLE(large-secmem,
+              AC_HELP_STRING([--enable-large-secmem],
+                             [allocate extra secure memory]),
+              large_secmem=$enableval, large_secmem=no)
+AC_MSG_RESULT($large_secmem)
+if test "$large_secmem" = yes ; then
+   SECMEM_BUFFER_SIZE=65536
+else
+   SECMEM_BUFFER_SIZE=32768
+fi
+AC_DEFINE_UNQUOTED(SECMEM_BUFFER_SIZE,$SECMEM_BUFFER_SIZE,
+                   [Size of secure memory buffer])
+
 AC_MSG_CHECKING([whether to enable trust models])
 AC_ARG_ENABLE(trust-models,
               AC_HELP_STRING([--disable-trust-models],