-AC_SEARCH_LIBS(__gmpz_init, gmp, , [AC_MSG_ERROR([GNU MP not found, see http://gmplib.org/])])
+AC_ARG_WITH(openssl, AS_HELP_STRING([--with-openssl], [Use OpenSSL instead of GMP (beware of the OpenSSL license)]), [with_openssl=$enableval], [with_openssl=no])
+AM_CONDITIONAL(WITH_OPENSSL, [test x"$with_openssl" != xno])
+
+AS_IF([test x"$with_openssl" != xno],
+ [AC_SEARCH_LIBS(BN_init, ssl, , [AC_MSG_ERROR([OpenSSL not found, see http://www.openssl.org/, or try --without-openssl])])],
+ [AC_SEARCH_LIBS(__gmpz_init, gmp, , [AC_MSG_ERROR([GNU MP not found, see http://gmplib.org/, or try --with-openssl])])])