ed25519: Add func that checks for torsion component in pubkeys.

See https://lists.torproject.org/pipermail/tor-dev/2017-April/012213.html .

1 files changed, 50 insertions, 0 deletions

diff --git a/src/common/crypto_ed25519.c b/src/common/crypto_ed25519.c
index 188e18c710..6ebb2a78b9 100644
--- a/src/common/crypto_ed25519.c
+++ b/src/common/crypto_ed25519.c
@@ -28,6 +28,7 @@
 #include "crypto_format.h"
 #include "torlog.h"
 #include "util.h"
+#include "util_format.h"
 
 #include "ed25519/ref10/ed25519_ref10.h"
 #include "ed25519/donna/ed25519_donna_tor.h"
@@ -57,6 +58,9 @@ typedef struct {
 
   int (*pubkey_from_curve25519_pubkey)(unsigned char *, const unsigned char *,
                                        int);
+
+  int (*ed25519_scalarmult_with_group_order)(unsigned char *,
+                                             const unsigned char *);
 } ed25519_impl_t;
 
 /** The Ref10 Ed25519 implementation. This one is pure C and lightly
@@ -77,6 +81,7 @@ static const ed25519_impl_t impl_ref10 = {
   ed25519_ref10_blind_public_key,
 
   ed25519_ref10_pubkey_from_curve25519_pubkey,
+  ed25519_ref10_scalarmult_with_group_order,
 };
 
 /** The Ref10 Ed25519 implementation. This one is heavily optimized, but still
@@ -97,6 +102,7 @@ static const ed25519_impl_t impl_donna = {
   ed25519_donna_blind_public_key,
 
   ed25519_donna_pubkey_from_curve25519_pubkey,
+  ed25519_donna_scalarmult_with_group_order,
 };
 
 /** Which Ed25519 implementation are we using?  NULL if we haven't decided
@@ -754,3 +760,47 @@ ed25519_init(void)
   pick_ed25519_impl();
 }
 
+/* Return true if <b>point</b> is the identity element of the ed25519 group. */
+static int
+ed25519_point_is_identity_element(const uint8_t *point)
+{
+  /* The identity element in ed25159 is the point with coordinates (0,1). */
+  static const uint8_t ed25519_identity[32] = {
+    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 };
+  tor_assert(sizeof(ed25519_identity) == ED25519_PUBKEY_LEN);
+  return tor_memeq(point, ed25519_identity, sizeof(ed25519_identity));
+}
+
+/** Validate <b>pubkey</b> to ensure that it has no torsion component.
+ *  Return 0 if <b>pubkey</b> is valid, else return -1. */
+int
+ed25519_validate_pubkey(const ed25519_public_key_t *pubkey)
+{
+  uint8_t result[32] = {9};
+
+  /* First check that we were not given the identity element */
+  if (ed25519_point_is_identity_element(pubkey->pubkey)) {
+    log_warn(LD_CRYPTO, "ed25519 pubkey is the identity\n");
+    return -1;
+  }
+
+  /* For any point on the curve, doing l*point should give the identity element
+   * (where l is the group order). Do the computation and check that the
+   * identity element is returned. */
+  if (get_ed_impl()->ed25519_scalarmult_with_group_order(result,
+                                                         pubkey->pubkey) < 0) {
+    log_warn(LD_CRYPTO, "ed25519 group order scalarmult failed\n");
+    return -1;
+  }
+
+  if (!ed25519_point_is_identity_element(result)) {
+    log_warn(LD_CRYPTO, "ed25519 validation failed\n");
+    return -1;
+  }
+
+  return 0;
+}
+