Merge branch 'asn_bug22006_final_squashed'

changes/bug22006

@@ -0,0 +1,4 @@
+  o Minor features (ed25519):
+    - Add validation function that checks for torsion components in ed25119
+      public keys. Currently unused but will be used by prop224 client-side
+      code. Addresses ticket #22006. Math help by Ian Goldberg.

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;
+}
+

src/common/crypto_ed25519.h

@@ -127,6 +127,8 @@ void ed25519_pubkey_copy(ed25519_public_key_t *dest,
 void ed25519_set_impl_params(int use_donna);
 void ed25519_init(void);
 
+int ed25519_validate_pubkey(const ed25519_public_key_t *pubkey);
+
 #ifdef TOR_UNIT_TESTS
 void crypto_ed25519_testing_force_impl(const char *name);
 void crypto_ed25519_testing_restore_impl(void);

src/ext/ed25519/donna/ed25519_donna_tor.h

@@ -30,4 +30,9 @@ int ed25519_donna_blind_public_key(unsigned char *out, const unsigned char *inp,
 int ed25519_donna_pubkey_from_curve25519_pubkey(unsigned char *out,
   const unsigned char *inp, int signbit);
 
+
+int
+ed25519_donna_scalarmult_with_group_order(unsigned char *out,
+                                          const unsigned char *pubkey);
+
 #endif

src/ext/ed25519/donna/ed25519_tor.c

@@ -340,5 +340,32 @@ ed25519_donna_pubkey_from_curve25519_pubkey(unsigned char *out,
   return 0;
 }
 
+/* Do the scalar multiplication of <b>pubkey</b> with the group order
+ * <b>modm_m</b>.  Place the result in <b>out</b> which must be at least 32
+ * bytes long. */
+int
+ed25519_donna_scalarmult_with_group_order(unsigned char *out,
+                                          const unsigned char *pubkey)
+{
+  static const bignum256modm ALIGN(16) zero = { 0 };
+  unsigned char pkcopy[32];
+  ge25519 ALIGN(16) Point, Result;
+
+  /* No "ge25519_unpack", negate the public key and unpack it back.
+   * See ed25519_donna_blind_public_key() */
+  memcpy(pkcopy, pubkey, 32);
+  pkcopy[31] ^= (1<<7);
+  if (!ge25519_unpack_negative_vartime(&Point, pkcopy)) {
+    return -1; /* error: bail out */
+  }
+
+  /* There is no regular scalarmult function so we have to do:
+   * Result = l*P + 0*B */
+  ge25519_double_scalarmult_vartime(&Result, &Point, modm_m, zero);
+  ge25519_pack(out, &Result);
+
+  return 0;
+}
+
 #include "test-internals.c"
 

src/ext/ed25519/ref10/blinding.c

@@ -74,3 +74,40 @@ int ed25519_ref10_blind_public_key(unsigned char *out,
 
   return 0;
 }
+
+/* This is the group order encoded in a format that
+ * ge_double_scalarmult_vartime() understands. The group order m is:
+ * m = 	 2^252 +  27742317777372353535851937790883648493 =
+ *       0x1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed
+ */
+static const uint8_t modm_m[32] = {0xed,0xd3,0xf5,0x5c,0x1a,0x63,0x12,0x58,
+                                   0xd6,0x9c,0xf7,0xa2,0xde,0xf9,0xde,0x14,
+                                   0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
+                                   0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x10};
+
+/* Do the scalar multiplication of <b>pubkey</b> with the group order
+ * <b>modm_m</b>.  Place the result in <b>out</b> which must be at least 32
+ * bytes long. */
+int
+ed25519_ref10_scalarmult_with_group_order(unsigned char *out,
+                                          const unsigned char *pubkey)
+{
+  unsigned char pkcopy[32];
+  unsigned char zero[32] = {0};
+  ge_p3 Point;
+  ge_p2 Result;
+
+  /* All this is done to fit 'pubkey' in 'Point' so that it can be used by
+   * ed25519 ref code. Same thing as in blinding function */
+  memcpy(pkcopy, pubkey, 32);
+  pkcopy[31] ^= (1<<7);
+  if (ge_frombytes_negate_vartime(&Point, pkcopy) != 0) {
+    return -1; /* error: bail out */
+  }
+
+  /* There isn't a regular scalarmult -- we have to do r = l*P + 0*B */
+  ge_double_scalarmult_vartime(&Result, modm_m, &Point, zero);
+  ge_tobytes(out, &Result);
+
+  return 0;
+}

src/ext/ed25519/ref10/ed25519_ref10.h

@@ -27,4 +27,8 @@ int ed25519_ref10_blind_public_key(unsigned char *out,
                               const unsigned char *inp,
                               const unsigned char *param);
 
+int
+ed25519_ref10_scalarmult_with_group_order(unsigned char *out,
+                                          const unsigned char *pubkey);
+
 #endif

src/or/dirserv.c

@@ -703,10 +703,22 @@ dirserv_add_descriptor(routerinfo_t *ri, const char **msg, const char *source)
   /* Do keypinning again ... this time, to add the pin if appropriate */
   int keypin_status;
   if (ri->cache_info.signing_key_cert) {
+    ed25519_public_key_t *pkey = &ri->cache_info.signing_key_cert->signing_key;
+    /* First let's validate this pubkey before pinning it */
+    if (ed25519_validate_pubkey(pkey) < 0) {
+      log_warn(LD_DIRSERV, "Received bad key from %s (source %s)",
+               router_describe(ri), source);
+      control_event_or_authdir_new_descriptor("REJECTED",
+                                         ri->cache_info.signed_descriptor_body,
+                                         desclen, *msg);
+      routerinfo_free(ri);
+      return ROUTER_AUTHDIR_REJECTS;
+    }
+
+    /* Now pin it! */
     keypin_status = keypin_check_and_add(
       (const uint8_t*)ri->cache_info.identity_digest,
-      ri->cache_info.signing_key_cert->signing_key.pubkey,
-      ! key_pinning);
+      pkey->pubkey, ! key_pinning);
   } else {
     keypin_status = keypin_check_lone_rsa(
       (const uint8_t*)ri->cache_info.identity_digest);

src/test/ed25519_exts_ref.py

@@ -69,6 +69,11 @@ def signatureWithESK(m,h,pk):
 def newSK():
     return os.urandom(32)
 
+def random_scalar(entropy_f): # 0..L-1 inclusive
+    # reduce the bias to a safe level by generating 256 extra bits
+    oversized = int(binascii.hexlify(entropy_f(32+32)), 16)
+    return oversized % ell
+
 # ------------------------------------------------------------
 
 MSG = "This is extremely silly. But it is also incredibly serious business!"
@@ -126,6 +131,31 @@ class SelfTest(unittest.TestCase):
 
         self._testSignatures(besk, bpk)
 
+    def testIdentity(self):
+        # Base point:
+        # B is the unique point (x, 4/5) \in E for which x is positive
+        By = 4 * inv(5)
+        Bx = xrecover(By)
+        B = [Bx % q,By % q]
+
+        # Get identity E by doing: E = l*B, where l is the group order
+        identity = scalarmult(B, ell)
+
+        # Get identity E by doing: E = l*A, where A is a random point
+        sk = newSK()
+        pk = decodepoint(publickey(sk))
+        identity2 = scalarmult(pk, ell)
+
+        # Check that identities match
+        assert(identity == identity2)
+        # Check that identity is the point (0,1)
+        assert(identity == [0L,1L])
+
+        # Check identity element: a*E = E, where a is a random scalar
+        scalar = random_scalar(os.urandom)
+        result = scalarmult(identity, scalar)
+        assert(result == identity == identity2)
+
 # ------------------------------------------------------------
 
 # From pprint.pprint([ binascii.b2a_hex(os.urandom(32)) for _ in xrange(8) ])

src/test/test_crypto.c

@@ -2170,6 +2170,9 @@ test_crypto_ed25519_simple(void *arg)
   tt_int_op(0, OP_EQ, ed25519_public_key_generate(&pub1, &sec1));
   tt_int_op(0, OP_EQ, ed25519_public_key_generate(&pub2, &sec1));
 
+  tt_int_op(ed25519_validate_pubkey(&pub1), OP_EQ, 0);
+  tt_int_op(ed25519_validate_pubkey(&pub2), OP_EQ, 0);
+
   tt_mem_op(pub1.pubkey, OP_EQ, pub2.pubkey, sizeof(pub1.pubkey));
   tt_assert(ed25519_pubkey_eq(&pub1, &pub2));
   tt_assert(ed25519_pubkey_eq(&pub1, &pub1));
@@ -2832,6 +2835,68 @@ crypto_rand_check_failure_mode_predict(void)
 
 #undef FAILURE_MODE_BUFFER_SIZE
 
+/** Test that our ed25519 validation function rejects evil public keys and
+ *  accepts good ones. */
+static void
+test_crypto_ed25519_validation(void *arg)
+{
+  (void) arg;
+
+  int retval;
+  ed25519_public_key_t pub1;
+
+  /* See https://lists.torproject.org/pipermail/tor-dev/2017-April/012230.html
+     for a list of points with torsion components in ed25519. */
+
+  { /* Point with torsion component (order 8l) */
+    const char badkey[] =
+      "300ef2e64e588e1df55b48e4da0416ffb64cc85d5b00af6463d5cc6c2b1c185e";
+    retval = base16_decode((char*)pub1.pubkey, sizeof(pub1.pubkey),
+                           badkey, strlen(badkey));
+    tt_int_op(retval, OP_EQ, sizeof(pub1.pubkey));
+    tt_int_op(ed25519_validate_pubkey(&pub1), OP_EQ, -1);
+  }
+
+  { /* Point with torsion component (order 4l) */
+    const char badkey[] =
+      "f43e3a046db8749164c6e69b193f1e942c7452e7d888736f40b98093d814d5e7";
+    retval = base16_decode((char*)pub1.pubkey, sizeof(pub1.pubkey),
+                           badkey, strlen(badkey));
+    tt_int_op(retval, OP_EQ, sizeof(pub1.pubkey));
+    tt_int_op(ed25519_validate_pubkey(&pub1), OP_EQ, -1);
+  }
+
+  { /* Point with torsion component (order 2l) */
+    const char badkey[] =
+      "c9fff3af0471c28e33e98c2043e44f779d0427b1e37c521a6bddc011ed1869af";
+    retval = base16_decode((char*)pub1.pubkey, sizeof(pub1.pubkey),
+                           badkey, strlen(badkey));
+    tt_int_op(retval, OP_EQ, sizeof(pub1.pubkey));
+    tt_int_op(ed25519_validate_pubkey(&pub1), OP_EQ, -1);
+  }
+
+  { /* This point is not even on the curve */
+    const char badkey[] =
+      "e19c65de75c68cf3b7643ea732ba9eb1a3d20d6d57ba223c2ece1df66feb5af0";
+    retval = base16_decode((char*)pub1.pubkey, sizeof(pub1.pubkey),
+                           badkey, strlen(badkey));
+    tt_int_op(retval, OP_EQ, sizeof(pub1.pubkey));
+    tt_int_op(ed25519_validate_pubkey(&pub1), OP_EQ, -1);
+  }
+
+  { /* This one is a good key */
+    const char goodkey[] =
+      "4ba2e44760dff4c559ef3c38768c1c14a8a54740c782c8d70803e9d6e3ad8794";
+    retval = base16_decode((char*)pub1.pubkey, sizeof(pub1.pubkey),
+                           goodkey, strlen(goodkey));
+    tt_int_op(retval, OP_EQ, sizeof(pub1.pubkey));
+    tt_int_op(ed25519_validate_pubkey(&pub1), OP_EQ, 0);
+  }
+
+
+ done: ;
+}
+
 static void
 test_crypto_failure_modes(void *arg)
 {
@@ -2918,6 +2983,7 @@ struct testcase_t crypto_tests[] = {
   ED25519_TEST(convert, 0),
   ED25519_TEST(blinding, 0),
   ED25519_TEST(testvectors, 0),
+  ED25519_TEST(validation, 0),
   { "ed25519_storage", test_crypto_ed25519_storage, 0, NULL, NULL },
   { "siphash", test_crypto_siphash, 0, NULL, NULL },
   { "failure_modes", test_crypto_failure_modes, TT_FORK, NULL, NULL },