README for Bundle Security Protocol usage

The standard ciphersuites require, amongst other things, 
an implementation of sha-256 message digest algorithm.

The DTN reference code uses OpenSSL for cryptographic
and related functions. Unfortunately, some versions of
OpenSSL do not include sha-256.

The "configure" process checks for the availability of
sha-256 and provides an error if it is not found.

If your system's OpenSSL does not have sha-256 then you 
can either upgrade it or build and use a local  version 
of OpenSSL. OpenSSL can be obtained from
http://www.openssl.org

OpenSSL 0.9.8 version include sha-256 by default. If your
system uses version 0.9.7 and you do not wish to upgrade
then you can enable sha-256 in later versions of 0.9.7,
such as 0.9.7l and 0.9.7m. To enable sha-256, specify "fips"
when running "Configure".

If you wish to leave you system installation untouched and
build against a local version, then configure dtn using
./configure --with-bsp --with-openssl=/path/to/openssl

Mac OS X note: for Mac OS X users ONLY. If you build dtn
against a local OpenSSL using "--with-openssl=/path/to/openssl"
you MUST also specify with it LDFLAGS="-Wl,-search_paths_first". 
The configuration for OS X users would then be 
./configure --with-bsp --with-openssl=/path/to/openssl LDFLAGS="-Wl,-search_paths_first"
Note that the quotes are required for the LDFLAGS argument.