Constant-time GCD function.

[oweals/openssl.git] / crypto / bn / bn_mod.c

diff --git a/crypto/bn/bn_mod.c b/crypto/bn/bn_mod.c
index 13b583f76c9f49a89612a1d98649f4c34e0d1d21..f7d2e2650ed8b6ecac38b05428ffafca1f7091e8 100644 (file)
--- a/crypto/bn/bn_mod.c
+++ b/crypto/bn/bn_mod.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 1998-2016 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1998-2018 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
@@ -8,7 +8,7 @@
  */
 
 #include "internal/cryptlib.h"
-#include "bn_lcl.h"
+#include "bn_local.h"
 
 int BN_nnmod(BIGNUM *r, const BIGNUM *m, const BIGNUM *d, BN_CTX *ctx)
 {
@@ -35,18 +35,74 @@ int BN_mod_add(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
 
 /*
  * BN_mod_add variant that may be used if both a and b are non-negative and
- * less than m
+ * less than m. The original algorithm was
+ *
+ *    if (!BN_uadd(r, a, b))
+ *       return 0;
+ *    if (BN_ucmp(r, m) >= 0)
+ *       return BN_usub(r, r, m);
+ *
+ * which is replaced with addition, subtracting modulus, and conditional
+ * move depending on whether or not subtraction borrowed.
  */
-int BN_mod_add_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
-                     const BIGNUM *m)
+int bn_mod_add_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
+                         const BIGNUM *m)
 {
-    if (!BN_uadd(r, a, b))
+    size_t i, ai, bi, mtop = m->top;
+    BN_ULONG storage[1024 / BN_BITS2];
+    BN_ULONG carry, temp, mask, *rp, *tp = storage;
+    const BN_ULONG *ap, *bp;
+
+    if (bn_wexpand(r, mtop) == NULL)
         return 0;
-    if (BN_ucmp(r, m) >= 0)
-        return BN_usub(r, r, m);
+
+    if (mtop > sizeof(storage) / sizeof(storage[0])
+        && (tp = OPENSSL_malloc(mtop * sizeof(BN_ULONG))) == NULL)
+        return 0;
+
+    ap = a->d != NULL ? a->d : tp;
+    bp = b->d != NULL ? b->d : tp;
+
+    for (i = 0, ai = 0, bi = 0, carry = 0; i < mtop;) {
+        mask = (BN_ULONG)0 - ((i - a->top) >> (8 * sizeof(i) - 1));
+        temp = ((ap[ai] & mask) + carry) & BN_MASK2;
+        carry = (temp < carry);
+
+        mask = (BN_ULONG)0 - ((i - b->top) >> (8 * sizeof(i) - 1));
+        tp[i] = ((bp[bi] & mask) + temp) & BN_MASK2;
+        carry += (tp[i] < temp);
+
+        i++;
+        ai += (i - a->dmax) >> (8 * sizeof(i) - 1);
+        bi += (i - b->dmax) >> (8 * sizeof(i) - 1);
+    }
+    rp = r->d;
+    carry -= bn_sub_words(rp, tp, m->d, mtop);
+    for (i = 0; i < mtop; i++) {
+        rp[i] = (carry & tp[i]) | (~carry & rp[i]);
+        ((volatile BN_ULONG *)tp)[i] = 0;
+    }
+    r->top = mtop;
+    r->flags |= BN_FLG_FIXED_TOP;
+    r->neg = 0;
+
+    if (tp != storage)
+        OPENSSL_free(tp);
+
     return 1;
 }
 
+int BN_mod_add_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
+                     const BIGNUM *m)
+{
+    int ret = bn_mod_add_fixed_top(r, a, b, m);
+
+    if (ret)
+        bn_correct_top(r);
+
+    return ret;
+}
+
 int BN_mod_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
                BN_CTX *ctx)
 {
@@ -55,6 +111,70 @@ int BN_mod_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
     return BN_nnmod(r, r, m, ctx);
 }
 
+/*
+ * BN_mod_sub variant that may be used if both a and b are non-negative,
+ * a is less than m, while b is of same bit width as m. It's implemented
+ * as subtraction followed by two conditional additions.
+ *
+ * 0 <= a < m
+ * 0 <= b < 2^w < 2*m
+ *
+ * after subtraction
+ *
+ * -2*m < r = a - b < m
+ *
+ * Thus it takes up to two conditional additions to make |r| positive.
+ */
+int bn_mod_sub_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
+                         const BIGNUM *m)
+{
+    size_t i, ai, bi, mtop = m->top;
+    BN_ULONG borrow, carry, ta, tb, mask, *rp;
+    const BN_ULONG *ap, *bp;
+
+    if (bn_wexpand(r, mtop) == NULL)
+        return 0;
+
+    rp = r->d;
+    ap = a->d != NULL ? a->d : rp;
+    bp = b->d != NULL ? b->d : rp;
+
+    for (i = 0, ai = 0, bi = 0, borrow = 0; i < mtop;) {
+        mask = (BN_ULONG)0 - ((i - a->top) >> (8 * sizeof(i) - 1));
+        ta = ap[ai] & mask;
+
+        mask = (BN_ULONG)0 - ((i - b->top) >> (8 * sizeof(i) - 1));
+        tb = bp[bi] & mask;
+        rp[i] = ta - tb - borrow;
+        if (ta != tb)
+            borrow = (ta < tb);
+
+        i++;
+        ai += (i - a->dmax) >> (8 * sizeof(i) - 1);
+        bi += (i - b->dmax) >> (8 * sizeof(i) - 1);
+    }
+    ap = m->d;
+    for (i = 0, mask = 0 - borrow, carry = 0; i < mtop; i++) {
+        ta = ((ap[i] & mask) + carry) & BN_MASK2;
+        carry = (ta < carry);
+        rp[i] = (rp[i] + ta) & BN_MASK2;
+        carry += (rp[i] < ta);
+    }
+    borrow -= carry;
+    for (i = 0, mask = 0 - borrow, carry = 0; i < mtop; i++) {
+        ta = ((ap[i] & mask) + carry) & BN_MASK2;
+        carry = (ta < carry);
+        rp[i] = (rp[i] + ta) & BN_MASK2;
+        carry += (rp[i] < ta);
+    }
+
+    r->top = mtop;
+    r->flags |= BN_FLG_FIXED_TOP;
+    r->neg = 0;
+
+    return 1;
+}
+
 /*
  * BN_mod_sub variant that may be used if both a and b are non-negative and
  * less than m
@@ -96,7 +216,7 @@ int BN_mod_mul(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,
     ret = 1;
  err:
     BN_CTX_end(ctx);
-    return (ret);
+    return ret;
 }
 
 int BN_mod_sqr(BIGNUM *r, const BIGNUM *a, const BIGNUM *m, BN_CTX *ctx)