From c190574cee382ace1ee0f8fdacc136f1797287e6 Mon Sep 17 00:00:00 2001
From: Frank Denis <github@pureftpd.org>
Date: Fri, 17 Nov 2017 10:43:30 +0100
Subject: [PATCH] x25519-ref10: reject low order points before the
 multiplication

---
 .../curve25519/ref10/x25519_ref10.c           | 37 +++++++++++++++++++
 test/default/scalarmult.c                     |  2 -
 test/default/scalarmult.exp                   |  1 -
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/src/libsodium/crypto_scalarmult/curve25519/ref10/x25519_ref10.c b/src/libsodium/crypto_scalarmult/curve25519/ref10/x25519_ref10.c
index f80f28c9..89fefbd6 100644
--- a/src/libsodium/crypto_scalarmult/curve25519/ref10/x25519_ref10.c
+++ b/src/libsodium/crypto_scalarmult/curve25519/ref10/x25519_ref10.c
@@ -3,10 +3,44 @@
 #include <stdint.h>
 
 #include "../scalarmult_curve25519.h"
+#include "export.h"
 #include "private/ed25519_ref10.h"
 #include "utils.h"
 #include "x25519_ref10.h"
 
+static int
+has_small_order(const unsigned char s[32])
+{
+    CRYPTO_ALIGN(16)
+    static const unsigned char blacklist[][32] = {
+        { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },
+        { 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },
+        { 0xe0, 0xeb, 0x7a, 0x7c, 0x3b, 0x41, 0xb8, 0xae, 0x16, 0x56, 0xe3, 0xfa, 0xf1, 0x9f, 0xc4, 0x6a, 0xda, 0x09, 0x8d, 0xeb, 0x9c, 0x32, 0xb1, 0xfd, 0x86, 0x62, 0x05, 0x16, 0x5f, 0x49, 0xb8, 0x00 },
+        { 0x5f, 0x9c, 0x95, 0xbc, 0xa3, 0x50, 0x8c, 0x24, 0xb1, 0xd0, 0xb1, 0x55, 0x9c, 0x83, 0xef, 0x5b, 0x04, 0x44, 0x5c, 0xc4, 0x58, 0x1c, 0x8e, 0x86, 0xd8, 0x22, 0x4e, 0xdd, 0xd0, 0x9f, 0x11, 0x57 },
+        { 0xec, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f },
+        { 0xed, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f },
+        { 0xee, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f },
+        { 0xcd, 0xeb, 0x7a, 0x7c, 0x3b, 0x41, 0xb8, 0xae, 0x16, 0x56, 0xe3, 0xfa, 0xf1, 0x9f, 0xc4, 0x6a, 0xda, 0x09, 0x8d, 0xeb, 0x9c, 0x32, 0xb1, 0xfd, 0x86, 0x62, 0x05, 0x16, 0x5f, 0x49, 0xb8, 0x80 },
+        { 0x4c, 0x9c, 0x95, 0xbc, 0xa3, 0x50, 0x8c, 0x24, 0xb1, 0xd0, 0xb1, 0x55, 0x9c, 0x83, 0xef, 0x5b, 0x04, 0x44, 0x5c, 0xc4, 0x58, 0x1c, 0x8e, 0x86, 0xd8, 0x22, 0x4e, 0xdd, 0xd0, 0x9f, 0x11, 0xd7 },
+        { 0xd9, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff },
+        { 0xda, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff },
+        { 0xdb, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }
+    };
+    size_t        i, j;
+    unsigned char c;
+
+    for (i = 0; i < sizeof blacklist / sizeof blacklist[0]; i++) {
+        c = 0;
+        for (j = 0; j < 32; j++) {
+            c |= s[j] ^ blacklist[i][j];
+        }
+        if (c == 0) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
 static int
 crypto_scalarmult_curve25519_ref10(unsigned char *q,
                                    const unsigned char *n,
@@ -25,6 +59,9 @@ crypto_scalarmult_curve25519_ref10(unsigned char *q,
     unsigned int   swap;
     unsigned int   b;
 
+    if (has_small_order(p)) {
+        return -1;
+    }
     for (i = 0; i < 32; i++) {
         t[i] = n[i];
     }
diff --git a/test/default/scalarmult.c b/test/default/scalarmult.c
index 3dfb2e83..0e055380 100644
--- a/test/default/scalarmult.c
+++ b/test/default/scalarmult.c
@@ -54,8 +54,6 @@ main(void)
 
     ret = crypto_scalarmult(k, bobsk, small_order_p);
     assert(ret == -1);
-    sodium_bin2hex(hex, sizeof hex, k, crypto_scalarmult_BYTES);
-    printf("%s\n", hex);
 
     sodium_free(bobpk);
     sodium_free(alicepk);
diff --git a/test/default/scalarmult.exp b/test/default/scalarmult.exp
index 4847bb04..1a38d891 100644
--- a/test/default/scalarmult.exp
+++ b/test/default/scalarmult.exp
@@ -2,4 +2,3 @@
 de9edb7d7b7dc1b4d35b61c2ece435373f8343c85b78674dadfc7e146f882b4f
 4a5d9d5ba4ce2de1728e3bf480350f25e07e21c947d19e3376f09b3c1e161742
 4a5d9d5ba4ce2de1728e3bf480350f25e07e21c947d19e3376f09b3c1e161742
-0000000000000000000000000000000000000000000000000000000000000000