[crypto/ec] don't assume points are of order group->order

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6535)

2 files changed, 44 insertions, 14 deletions

diff --git a/crypto/ec/ec_mult.c b/crypto/ec/ec_mult.c
index b668e87ff7..c821cb8250 100644
--- a/crypto/ec/ec_mult.c
+++ b/crypto/ec/ec_mult.c
@@ -132,10 +132,11 @@ static int ec_mul_consttime(const EC_GROUP *group, EC_POINT *r,
                             const BIGNUM *scalar, const EC_POINT *point,
                             BN_CTX *ctx)
 {
-    int i, order_bits, group_top, kbit, pbit, Z_is_one;
+    int i, cardinality_bits, group_top, kbit, pbit, Z_is_one;
     EC_POINT *s = NULL;
     BIGNUM *k = NULL;
     BIGNUM *lambda = NULL;
+    BIGNUM *cardinality = NULL;
     BN_CTX *new_ctx = NULL;
     int ret = 0;
 
@@ -144,8 +145,6 @@ static int ec_mul_consttime(const EC_GROUP *group, EC_POINT *r,
 
     BN_CTX_start(ctx);
 
-    order_bits = BN_num_bits(group->order);
-
     s = EC_POINT_new(group);
     if (s == NULL)
         goto err;
@@ -160,18 +159,20 @@ static int ec_mul_consttime(const EC_GROUP *group, EC_POINT *r,
 
     EC_POINT_BN_set_flags(s, BN_FLG_CONSTTIME);
 
+    cardinality = BN_CTX_get(ctx);
     lambda = BN_CTX_get(ctx);
     k = BN_CTX_get(ctx);
-    if (k == NULL)
+    if (k == NULL || !BN_mul(cardinality, group->order, group->cofactor, ctx))
         goto err;
 
     /*
-     * Group orders are often on a word boundary.
+     * Group cardinalities are often on a word boundary.
      * So when we pad the scalar, some timing diff might
      * pop if it needs to be expanded due to carries.
      * So expand ahead of time.
      */
-    group_top = bn_get_top(group->order);
+    cardinality_bits = BN_num_bits(cardinality);
+    group_top = bn_get_top(cardinality);
     if ((bn_wexpand(k, group_top + 1) == NULL)
         || (bn_wexpand(lambda, group_top + 1) == NULL))
         goto err;
@@ -181,25 +182,25 @@ static int ec_mul_consttime(const EC_GROUP *group, EC_POINT *r,
 
     BN_set_flags(k, BN_FLG_CONSTTIME);
 
-    if ((BN_num_bits(k) > order_bits) || (BN_is_negative(k))) {
+    if ((BN_num_bits(k) > cardinality_bits) || (BN_is_negative(k))) {
         /*-
          * this is an unusual input, and we don't guarantee
          * constant-timeness
          */
-        if (!BN_nnmod(k, k, group->order, ctx))
+        if (!BN_nnmod(k, k, cardinality, ctx))
             goto err;
     }
 
-    if (!BN_add(lambda, k, group->order))
+    if (!BN_add(lambda, k, cardinality))
         goto err;
     BN_set_flags(lambda, BN_FLG_CONSTTIME);
-    if (!BN_add(k, lambda, group->order))
+    if (!BN_add(k, lambda, cardinality))
         goto err;
     /*
-     * lambda := scalar + order
-     * k := scalar + 2*order
+     * lambda := scalar + cardinality
+     * k := scalar + 2*cardinality
      */
-    kbit = BN_is_bit_set(lambda, order_bits);
+    kbit = BN_is_bit_set(lambda, cardinality_bits);
     BN_consttime_swap(kbit, k, lambda, group_top + 1);
 
     group_top = bn_get_top(group->field);
@@ -300,7 +301,7 @@ static int ec_mul_consttime(const EC_GROUP *group, EC_POINT *r,
      * This is XOR. pbit tracks the previous bit of k.
      */
 
-    for (i = order_bits - 1; i >= 0; i--) {
+    for (i = cardinality_bits - 1; i >= 0; i--) {
         kbit = BN_is_bit_set(k, i) ^ pbit;
         EC_POINT_CSWAP(kbit, r, s, group_top, Z_is_one);
         if (!EC_POINT_add(group, s, r, s, ctx))
diff --git a/test/recipes/30-test_evp_data/evppkey.txt b/test/recipes/30-test_evp_data/evppkey.txt
index e191b6f933..74351258d1 100644
--- a/test/recipes/30-test_evp_data/evppkey.txt
+++ b/test/recipes/30-test_evp_data/evppkey.txt
@@ -17257,6 +17257,35 @@ PeerKey=KAS-ECC-CDH_B-571_C24-Peer-PUBLIC
 Ctrl=ecdh_cofactor_mode:1
 SharedSecret=02da266a269bdc8d8b2a0c6bb5762f102fc801c8d5394a9271539136bd81d4b69cfbb7525cd0a983fb7f7e9deec583b8f8e574c6184b2d79831ec770649e484dc006fa35b0bffd0b
 
+# for cofactor-order points, ECC CDH (co-factor ECDH) should fail. Test that.
+
+PrivateKey=ALICE_cf_sect283k1
+-----BEGIN PRIVATE KEY-----
+MIGQAgEAMBAGByqGSM49AgEGBSuBBAAQBHkwdwIBAQQkAHtPwRfQZ9pWgSctyHdt
+xt3pd8ESMI3ugVx8MDLkiVB8GkCRoUwDSgAEA+xpY5sDcgM2yYxoWOrzH7WUH+b3
+n68A32kODgcKu8PXRYEKBH8Xzbr974982ZJW1sGrDs+P81sIFH8tdp45Jkr+OtfM
+8uKr
+-----END PRIVATE KEY-----
+
+PublicKey=ALICE_cf_sect283k1_PUB
+-----BEGIN PUBLIC KEY-----
+MF4wEAYHKoZIzj0CAQYFK4EEABADSgAEA+xpY5sDcgM2yYxoWOrzH7WUH+b3n68A
+32kODgcKu8PXRYEKBH8Xzbr974982ZJW1sGrDs+P81sIFH8tdp45Jkr+OtfM8uKr
+-----END PUBLIC KEY-----
+
+PublicKey=BOB_cf_sect283k1_PUB
+-----BEGIN PUBLIC KEY-----
+MF4wEAYHKoZIzj0CAQYFK4EEABADSgAEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
+AAABAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB
+-----END PUBLIC KEY-----
+
+PrivPubKeyPair = ALICE_cf_sect283k1:ALICE_cf_sect283k1_PUB
+
+# ECDH Alice with Bob peer
+Derive=ALICE_cf_sect283k1
+PeerKey=BOB_cf_sect283k1_PUB
+Ctrl=ecdh_cofactor_mode:1
+Result = DERIVE_ERROR
 
 Title = Test keypair mistmatches