1 files changed, 52 insertions, 0 deletions

diff --git a/src/lib/libcrypto/bn/bn_lib.c b/src/lib/libcrypto/bn/bn_lib.c
index 7a5676de69..5461e6ee7d 100644
--- a/src/lib/libcrypto/bn/bn_lib.c
+++ b/src/lib/libcrypto/bn/bn_lib.c
@@ -824,3 +824,55 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b,
                 }
         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
+ */
+void BN_consttime_swap(BN_ULONG condition, BIGNUM *a, BIGNUM *b, int nwords)
+        {
+        BN_ULONG t;
+        int i;
+
+        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;
+
+        t = (a->top^b->top) & condition;
+        a->top ^= t;
+        b->top ^= 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
+}