Clean up BN_consttime_swap.

Updated "condition" logic lifted from Theo Buehler's LibreSSL commit https://github.com/libressl-portable/openbsd/commit/517358603b4be76d48a50007a0d414c2072697dd

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/7619)

crypto/bn/bn_lib.c

@@ -737,26 +737,25 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b, int cl, int dl)
     return bn_cmp_words(a, b, cl);
 }
 
-/*
+/*-
  * Constant-time conditional swap of a and b.
- * a and b are swapped if condition is not 0.  The code assumes that at most one bit of condition is set.
- * nwords is the number of words to swap.  The code assumes that at least nwords are allocated in both a and b,
- * and that no more than nwords are used by either a or b.
- * a and b cannot be the same number
+ * a and b are swapped if condition is not 0.
+ * nwords is the number of words to swap.
+ * Assumes that at least nwords are allocated in both a and b.
+ * Assumes that no more than nwords are used by either a or b.
  */
 void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords)
 {
     BN_ULONG t;
     int i;
 
+    if (a == b)
+        return;
+
     bn_wcheck_size(a, nwords);
     bn_wcheck_size(b, nwords);
 
-    assert(a != b);
-    assert((condition & (condition - 1)) == 0);
-    assert(sizeof(BN_ULONG) >= sizeof(int));
-
-    condition = ((condition - 1) >> (BN_BITS2 - 1)) - 1;
+    condition = ((~condition & ((condition - 1))) >> (BN_BITS2 - 1)) - 1;
 
     t = (a->top ^ b->top) & condition;
     a->top ^= t;
@@ -794,42 +793,16 @@ void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords)
     a->flags ^= t;
     b->flags ^= t;
 
-#define BN_CONSTTIME_SWAP(ind) \
-        do { \
-                t = (a->d[ind] ^ b->d[ind]) & condition; \
-                a->d[ind] ^= t; \
-                b->d[ind] ^= t; \
-        } while (0)
-
-    switch (nwords) {
-    default:
-        for (i = 10; i < nwords; i++)
-            BN_CONSTTIME_SWAP(i);
-        /* Fallthrough */
-    case 10:
-        BN_CONSTTIME_SWAP(9);   /* Fallthrough */
-    case 9:
-        BN_CONSTTIME_SWAP(8);   /* Fallthrough */
-    case 8:
-        BN_CONSTTIME_SWAP(7);   /* Fallthrough */
-    case 7:
-        BN_CONSTTIME_SWAP(6);   /* Fallthrough */
-    case 6:
-        BN_CONSTTIME_SWAP(5);   /* Fallthrough */
-    case 5:
-        BN_CONSTTIME_SWAP(4);   /* Fallthrough */
-    case 4:
-        BN_CONSTTIME_SWAP(3);   /* Fallthrough */
-    case 3:
-        BN_CONSTTIME_SWAP(2);   /* Fallthrough */
-    case 2:
-        BN_CONSTTIME_SWAP(1);   /* Fallthrough */
-    case 1:
-        BN_CONSTTIME_SWAP(0);
-    }
-#undef BN_CONSTTIME_SWAP
+    /* conditionally swap the data */
+    for (i = 0; i < nwords; i++) {
+        t = (a->d[i] ^ b->d[i]) & condition;
+        a->d[i] ^= t;
+        b->d[i] ^= t;
+    }
 }
 
+#undef BN_CONSTTIME_SWAP_FLAGS
+
 /* Bits of security, see SP800-57 */
 
 int BN_security_bits(int L, int N)