Add options to use blank external dependencies

This commit makes external dependencies unnecessary when
`WITH_BLANK_EXTERNAL_DEPENDENCIES` is specified.
It is useful in some restricted environments (e.g. webassembly)
or in some specific applications (e.g. deterministic signature
generation).

Author: contrun

README.md

@@ -693,7 +693,8 @@ compiler is not gcc/clang compatible, the user can modify the CFLAGS as well as
 for 8-bit MCUs to 64-bit CPUs. If the toolchain does not have a [`stdint.h`](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/stdint.h.html)
 header, it is still possible to compile libecc by exporting LIBECC_NOSTDLIB=1: in this case, the code will try to
 guess and fit to native C types or throw an error so that the user can adapt [src/words/types.h](src/words/types.h) to its specific case.
-* The library core is platform independent. However, when the platform is not recognized (i.e. everything aside UNIX/Windows/Mac OS),
+* The library core is platform independent. However, when the platform is not recognized (i.e. everything aside UNIX/Windows/Mac OS)
+and when not compiled with `WITH_BLANK_EXTERNAL_DEPENDENCIES` specified,
 an error is thrown at compilation time asking the user to provide implementations for **external dependencies**
 in [src/external_deps/](src/external_deps), namely:
   * The printing helper in [src/external_deps/print.c](src/external_deps/print.c). This helper serves output debugging purposes.
@@ -703,6 +704,10 @@ in [src/external_deps/](src/external_deps), namely:
   schemes. One should notice that a **good random source** is **crucial** for the security of Elliptic Curve based signature schemes,
   so great care must be taken when implementing this.
 
+If `WITH_BLANK_EXTERNAL_DEPENDENCIES` is specified in compiling time, blank implementation for these helpers are provided.
+As the random helper does not really generate random bits, this may be dangerous. They are provided
+because it is useful in some restricted environments or in some specific applications (e.g. deterministic signature generation).
+
 Some other external dependencies could arise depending on the compilation chain and/or the platform. Such an example is the
 implementation of the gcc and clang stack protection option, usually expecting the user to provide stack canaries generation
 (with random values) and failover behavior.
@@ -890,7 +895,8 @@ other compilers (`-c` flag to generate object files, `-o` flag to define output
 [partially implemented](http://sdcc.sourceforge.net/mediawiki/index.php/Standard_compliance).
 * The compiler has "exotic" targets such as the Zilog Z80 MCU.
 
-We suppose that the user has also provided the **external dependencies** for print, random and time
+Unless compiling with `WITH_BLANK_EXTERNAL_DEPENDENCIES` specified (which can be dangerous),
+we suppose that the user has also provided the **external dependencies** for print, random and time
 functions (otherwise explicit errors will be thrown by #error directives).
 
 We will show how overloading the Makefile flags can be of use in this case. Say that we want

src/external_deps/print.c

@@ -26,6 +26,10 @@ void ext_printf(const char *format, ...)
 	vprintf(format, arglist);
 	va_end(arglist);
 }
+
+#elif defined(WITH_BLANK_EXTERNAL_DEPENDENCIES)
+void ext_printf(const char *_format, ...) {}
+
 #else
 #error "print.c: you have to implement ext_printf"
 #endif

src/external_deps/rand.c

@@ -106,6 +106,14 @@ int get_random(unsigned char *buf, u16 len)
 	return ret;
 }
 
+#elif defined(WITH_BLANK_EXTERNAL_DEPENDENCIES)
+int get_random(unsigned char *buf, u16 len) {
+  for (int i = 0; i < len; i++) {
+    buf[i] = 0;
+  }
+  return 0;
+}
+
 /* No platform detected, the user must provide an implementation! */
 #else
 /* WARNING: when providing/implementing the get_random function, one must:

src/external_deps/time.c

@@ -66,6 +66,12 @@ int get_ms_time(u64 *time)
 	return ret;
 }
 
+#elif defined(WITH_BLANK_EXTERNAL_DEPENDENCIES)
+int get_ms_time(u64 *time) {
+  *time = 0;
+  return 0;
+}
+
 /* No platform detected, the used must provide an implementation! */
 #else
 #error "time.c: you have to implement get_ms_time()"