1 files changed, 96 insertions, 0 deletions
diff --git a/jni/ed25519/ge_double_scalarmult.c b/jni/ed25519/ge_double_scalarmult.c
new file mode 100644
index 00000000..f8bf4bf7
--- /dev/null
+++ b/jni/ed25519/ge_double_scalarmult.c
@@ -0,0 +1,96 @@
+#include "ge.h"
+
+static void slide(signed char *r,const unsigned char *a)
+{
+  int i;
+  int b;
+  int k;
+
+  for (i = 0;i < 256;++i)
+    r[i] = 1 & (a[i >> 3] >> (i & 7));
+
+  for (i = 0;i < 256;++i)
+    if (r[i]) {
+      for (b = 1;b <= 6 && i + b < 256;++b) {
+        if (r[i + b]) {
+          if (r[i] + (r[i + b] << b) <= 15) {
+            r[i] += r[i + b] << b; r[i + b] = 0;
+          } else if (r[i] - (r[i + b] << b) >= -15) {
+            r[i] -= r[i + b] << b;
+            for (k = i + b;k < 256;++k) {
+              if (!r[k]) {
+                r[k] = 1;
+                break;
+              }
+              r[k] = 0;
+            }
+          } else
+            break;
+        }
+      }
+    }
+
+}
+
+static ge_precomp Bi[8] = {
+#include "base2.h"
+} ;
+
+/*
+r = a * A + b * B
+where a = a[0]+256*a[1]+...+256^31 a[31].
+and b = b[0]+256*b[1]+...+256^31 b[31].
+B is the Ed25519 base point (x,4/5) with x positive.
+*/
+
+void ge_double_scalarmult_vartime(ge_p2 *r,const unsigned char *a,const ge_p3 *A,const unsigned char *b)
+{
+  signed char aslide[256];
+  signed char bslide[256];
+  ge_cached Ai[8]; /* A,3A,5A,7A,9A,11A,13A,15A */
+  ge_p1p1 t;
+  ge_p3 u;
+  ge_p3 A2;
+  int i;
+
+  slide(aslide,a);
+  slide(bslide,b);
+
+  ge_p3_to_cached(&Ai[0],A);
+  ge_p3_dbl(&t,A); ge_p1p1_to_p3(&A2,&t);
+  ge_add(&t,&A2,&Ai[0]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[1],&u);
+  ge_add(&t,&A2,&Ai[1]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[2],&u);
+  ge_add(&t,&A2,&Ai[2]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[3],&u);
+  ge_add(&t,&A2,&Ai[3]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[4],&u);
+  ge_add(&t,&A2,&Ai[4]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[5],&u);
+  ge_add(&t,&A2,&Ai[5]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[6],&u);
+  ge_add(&t,&A2,&Ai[6]); ge_p1p1_to_p3(&u,&t); ge_p3_to_cached(&Ai[7],&u);
+
+  ge_p2_0(r);
+
+  for (i = 255;i >= 0;--i) {
+    if (aslide[i] || bslide[i]) break;
+  }
+
+  for (;i >= 0;--i) {
+    ge_p2_dbl(&t,r);
+
+    if (aslide[i] > 0) {
+      ge_p1p1_to_p3(&u,&t);
+      ge_add(&t,&u,&Ai[aslide[i]/2]);
+    } else if (aslide[i] < 0) {
+      ge_p1p1_to_p3(&u,&t);
+      ge_sub(&t,&u,&Ai[(-aslide[i])/2]);
+    }
+
+    if (bslide[i] > 0) {
+      ge_p1p1_to_p3(&u,&t);
+      ge_madd(&t,&u,&Bi[bslide[i]/2]);
+    } else if (bslide[i] < 0) {
+      ge_p1p1_to_p3(&u,&t);
+      ge_msub(&t,&u,&Bi[(-bslide[i])/2]);
+    }
+
+    ge_p1p1_to_p2(r,&t);
+  }
+}