Add a wrapper around, and test and build support for, curve25519.

We want to use donna-c64 when we have a GCC with support for
64x64->uint128_t multiplying.  If not, we want to use libnacl if we
can, unless it's giving us the unsafe "ref" implementation.  And if
that isn't going to work, we'd like to use the
portable-and-safe-but-slow 32-bit "donna" implementation.

We might need more library searching for the correct libnacl,
especially once the next libnacl release is out -- it's likely to have
bunches of better curve25519 implementations.

I also define a set of curve25519 wrapper functions, though it really
shouldn't be necessary.

We should eventually make the -donna*.c files get build with
-fomit-frame-pointer, since that can make a difference.

1 files changed, 99 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 9963a97d4..927283435 100644
--- a/configure.ac
+++ b/configure.ac
@@ -36,6 +36,8 @@ AC_ARG_ENABLE(static-zlib,
    AS_HELP_STRING(--enable-static-zlib, Link against a static zlib library. Requires --with-zlib-dir))
 AC_ARG_ENABLE(static-tor,
    AS_HELP_STRING(--enable-static-tor, Create an entirely static Tor binary. Requires --with-openssl-dir and --with-libevent-dir and --with-zlib-dir))
+AC_ARG_ENABLE(curve25519,
+   AS_HELP_STRING(--disable-curve25519, Build Tor with no curve25519 elliptic-curve crypto support))
 
 if test "$enable_static_tor" = "yes"; then
   enable_static_libevent="yes";
@@ -638,6 +640,103 @@ if test "$upnp" = "true"; then
     fi
 fi
 
+dnl ============================================================
+dnl We need an implementation of curve25519.
+
+dnl set these defaults.
+have_a_curve25519=no
+build_curve25519_donna=no
+build_curve25519_donna_c64=no
+use_curve25519_donna=no
+use_curve25519_nacl=no
+CURVE25519_LIBS=
+
+if test x$enable_curve25519 != xno; then
+
+  dnl The best choice is using curve25519-donna-c64, but that requires
+  dnl that we
+  AC_CACHE_CHECK([whether we can use curve25519-donna-c64],
+    tor_cv_can_use_curve25519_donna_c64,
+    [AC_RUN_IFELSE(
+      [AC_LANG_PROGRAM([dnl
+        #include <stdint.h>
+        typedef unsigned uint128_t __attribute__((mode(TI)));
+	], [dnl
+          uint64_t a = ((uint64_t)2000000000) * 1000000000;
+	  uint64_t b = ((uint64_t)1234567890) << 24;
+	  uint128_t c = ((uint128_t)a) * b;
+	  return ((uint64_t)(c>>96)) == 522859 &&
+	         ((uint64_t)(c>>64))&0xffffffffL == 3604448702L &&
+                 ((uint64_t)(c>>32))&0xffffffffL == 2351960064L &&
+                 ((uint64_t)(c))&0xffffffffL == 0;
+        ])],
+	[tor_cv_can_use_curve25519_donna_c64=yes],
+        [tor_cv_can_use_curve25519_donna_c64=no],
+	[AC_COMPILE_IFELSE(
+          [AC_LANG_PROGRAM([dnl
+            #include <stdint.h>
+            typedef unsigned uint128_t __attribute__((mode(TI)));
+            ], [dnl
+              uint64_t a = ((uint64_t)2000000000) * 1000000000;
+	      uint64_t b = ((uint64_t)1234567890) << 24;
+	      uint128_t c = ((uint128_t)a) * b;
+	      return ((uint64_t)(c>>96)) == 522859 &&
+	             ((uint64_t)(c>>64))&0xffffffffL == 3604448702L &&
+                     ((uint64_t)(c>>32))&0xffffffffL == 2351960064L &&
+                     ((uint64_t)(c))&0xffffffffL == 0;
+            ])],
+            [tor_cv_can_use_curve25519_donna_c64=cross],
+	    [tor_cv_can_use_curve25519_donna_c64=no])])])
+
+  AC_CACHE_CHECK([whether we can use curve25519 from nacl],
+    tor_cv_can_use_curve25519_nacl,
+    [tor_saved_LIBS="$LIBS"
+     LIBS="$LIBS -lnacl"
+     AC_LINK_IFELSE(
+       [AC_LANG_PROGRAM([dnl
+         #include <crypto_scalarmult_curve25519.h>
+         #ifdef crypto_scalarmult_curve25519_ref_BYTES
+	 #error Hey, this is the reference implementation!
+	 #endif
+       ], [
+	 unsigned char *a, *b, *c; crypto_scalarmult_curve25519(a,b,c);
+       ])], [tor_cv_can_use_curve25519_nacl=yes],
+       [tor_cv_can_use_curve25519_nacl=no])
+     LIBS="$tor_saved_LIBS" ])
+
+   dnl Okay, now we need to figure out which one to actually use. Fall back
+   dnl to curve25519-donna.c
+
+   if test x$tor_cv_can_use_curve25519_donna_c64 != xno; then
+     build_curve25519_donna_c64=yes
+     use_curve25519_donna=yes
+   elif test x$tor_cv_can_use_curve25519_nacl = xyes; then
+     use_curve25519_nacl=yes
+     CURVE25519_LIBS=-lnacl
+   else
+     build_curve25519_donna=yes
+     use_curve25519_donna=yes
+   fi
+   have_a_curve25519=yes
+fi
+
+if test x$have_a_curve25519 = xyes; then
+  AC_DEFINE(CURVE25519_ENABLED, 1,
+            [Defined if we have a curve25519 implementation])
+fi
+if test x$use_curve25519_donna = xyes; then
+  AC_DEFINE(USE_CURVE25519_DONNA, 1,
+            [Defined if we should use an internal curve25519_donna{,_c64} implementation])
+fi
+if test x$use_curve25519_nacl = xyes; then
+  AC_DEFINE(USE_CURVE25519_NACL, 1,
+            [Defined if we should use a curve25519 from nacl])
+fi
+AM_CONDITIONAL(BUILD_CURVE25519_DONNA, test x$build_curve25519_donna = xyes)
+AM_CONDITIONAL(BUILD_CURVE25519_DONNA_C64, test x$build_curve25519_donna_c64 = xyes)
+AM_CONDITIONAL(CURVE25519_ENABLED, test x$have_a_curve25519 = xyes)
+AC_SUBST(CURVE25519_LIBS)
+
 dnl Make sure to enable support for large off_t if available.
 AC_SYS_LARGEFILE