bn/bn_{div|shift}.c: introduce fixed-top interfaces.

[openssl.git] / crypto / bn / bn_shift.c

diff --git a/crypto/bn/bn_shift.c b/crypto/bn/bn_shift.c
index 15d4b321ba2684f74b5a1d87015a5d3f523c639f..b7a1e0ff9ae3c20b7ed93ad81c03e82a22198a1c 100644 (file)
--- a/crypto/bn/bn_shift.c
+++ b/crypto/bn/bn_shift.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1995-2018 The OpenSSL Project Authors. All Rights Reserved.
  *
  * Licensed under the OpenSSL license (the "License").  You may not use
  * this file except in compliance with the License.  You can obtain a copy
@@ -7,6 +7,7 @@
  * https://www.openssl.org/source/license.html
  */
 
+#include <assert.h>
 #include "internal/cryptlib.h"
 #include "bn_lcl.h"
 
@@ -82,40 +83,70 @@ int BN_rshift1(BIGNUM *r, const BIGNUM *a)
 
 int BN_lshift(BIGNUM *r, const BIGNUM *a, int n)
 {
-    int i, nw, lb, rb;
-    BN_ULONG *t, *f;
-    BN_ULONG l;
-
-    bn_check_top(r);
-    bn_check_top(a);
+    int ret;
 
     if (n < 0) {
         BNerr(BN_F_BN_LSHIFT, BN_R_INVALID_SHIFT);
         return 0;
     }
 
+    ret = bn_lshift_fixed_top(r, a, n);
+
+    bn_correct_top(r);
+    bn_check_top(r);
+
+    return ret;
+}
+
+/*
+ * In respect to shift factor the execution time is invariant of
+ * |n % BN_BITS2|, but not |n / BN_BITS2|. Or in other words pre-condition
+ * for constant-time-ness is |n < BN_BITS2| or |n / BN_BITS2| being
+ * non-secret.
+ */
+int bn_lshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n)
+{
+    int i, nw;
+    unsigned int lb, rb;
+    BN_ULONG *t, *f;
+    BN_ULONG l, m, rmask = 0;
+
+    assert(n >= 0);
+
+    bn_check_top(r);
+    bn_check_top(a);
+
     nw = n / BN_BITS2;
     if (bn_wexpand(r, a->top + nw + 1) == NULL)
         return 0;
-    r->neg = a->neg;
-    lb = n % BN_BITS2;
-    rb = BN_BITS2 - lb;
-    f = a->d;
-    t = r->d;
-    t[a->top + nw] = 0;
-    if (lb == 0)
-        for (i = a->top - 1; i >= 0; i--)
-            t[nw + i] = f[i];
-    else
-        for (i = a->top - 1; i >= 0; i--) {
-            l = f[i];
-            t[nw + i + 1] |= (l >> rb) & BN_MASK2;
-            t[nw + i] = (l << lb) & BN_MASK2;
+
+    if (a->top != 0) {
+        lb = (unsigned int)n % BN_BITS2;
+        rb = BN_BITS2 - lb;
+        rb %= BN_BITS2;            /* say no to undefined behaviour */
+        rmask = (BN_ULONG)0 - rb;  /* rmask = 0 - (rb != 0) */
+        rmask |= rmask >> 8;
+        f = &(a->d[0]);
+        t = &(r->d[nw]);
+        l = f[a->top - 1];
+        t[a->top] = (l >> rb) & rmask;
+        for (i = a->top - 1; i > 0; i--) {
+            m = l << lb;
+            l = f[i - 1];
+            t[i] = (m | ((l >> rb) & rmask)) & BN_MASK2;
         }
-    memset(t, 0, sizeof(*t) * nw);
+        t[0] = (l << lb) & BN_MASK2;
+    } else {
+        /* shouldn't happen, but formally required */
+        r->d[nw] = 0;
+    }
+    if (nw != 0)
+        memset(r->d, 0, sizeof(*t) * nw);
+
+    r->neg = a->neg;
     r->top = a->top + nw + 1;
-    bn_correct_top(r);
-    bn_check_top(r);
+    r->flags |= BN_FLG_FIXED_TOP;
+
     return 1;
 }
 
@@ -173,3 +204,54 @@ int BN_rshift(BIGNUM *r, const BIGNUM *a, int n)
     bn_check_top(r);
     return 1;
 }
+
+/*
+ * In respect to shift factor the execution time is invariant of
+ * |n % BN_BITS2|, but not |n / BN_BITS2|. Or in other words pre-condition
+ * for constant-time-ness for sufficiently[!] zero-padded inputs is
+ * |n < BN_BITS2| or |n / BN_BITS2| being non-secret.
+ */
+int bn_rshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n)
+{
+    int i, top, nw;
+    unsigned int lb, rb;
+    BN_ULONG *t, *f;
+    BN_ULONG l, m, mask;
+
+    bn_check_top(r);
+    bn_check_top(a);
+
+    assert(n >= 0);
+
+    nw = n / BN_BITS2;
+    if (nw >= a->top) {
+        /* shouldn't happen, but formally required */
+        BN_zero(r);
+        return 1;
+    }
+
+    rb = (unsigned int)n % BN_BITS2;
+    lb = BN_BITS2 - rb;
+    lb %= BN_BITS2;            /* say no to undefined behaviour */
+    mask = (BN_ULONG)0 - lb;   /* mask = 0 - (lb != 0) */
+    mask |= mask >> 8;
+    top = a->top - nw;
+    if (r != a && bn_wexpand(r, top) == NULL)
+        return 0;
+
+    t = &(r->d[0]);
+    f = &(a->d[nw]);
+    l = f[0];
+    for (i = 0; i < top - 1; i++) {
+        m = f[i + 1];
+        t[i] = (l >> rb) | ((m << lb) & mask);
+        l = m;
+    }
+    t[i] = l >> rb;
+
+    r->neg = a->neg;
+    r->top = top;
+    r->flags |= BN_FLG_FIXED_TOP;
+
+    return 1;
+}