For odd moduli use a input-independent modular inverse algorithm.

Also adds a (not const time) implementation of almost Montgomery reduction.

2 files changed, 20 insertions, 20 deletions

diff --git a/src/lib/math/mp/mp_asm.cpp b/src/lib/math/mp/mp_asm.cpp
index 6d60d7e77..cfbb027d7 100644
--- a/src/lib/math/mp/mp_asm.cpp
+++ b/src/lib/math/mp/mp_asm.cpp
@@ -24,9 +24,6 @@ void bigint_cnd_swap(word cnd, word x[], word y[], size_t size)
    {
    const word mask = CT::expand_mask(cnd);
 
-   CT::poison(x, size);
-   CT::poison(y, size);
-
    for(size_t i = 0; i != size; ++i)
       {
       word a = x[i];
@@ -34,9 +31,6 @@ void bigint_cnd_swap(word cnd, word x[], word y[], size_t size)
       x[i] = CT::select(mask, b, a);
       y[i] = CT::select(mask, a, b);
       }
-
-   CT::unpoison(x, size);
-   CT::unpoison(y, size);
    }
 
 /*
@@ -47,9 +41,6 @@ word bigint_cnd_add(word cnd, word x[], const word y[], size_t size)
    {
    const word mask = CT::expand_mask(cnd);
 
-   CT::poison(x, size);
-   CT::poison(y, size);
-
    word carry = 0;
    for(size_t i = 0; i != size; ++i)
       {
@@ -61,10 +52,6 @@ word bigint_cnd_add(word cnd, word x[], const word y[], size_t size)
       x[i] = CT::select(mask, z, x[i]);
       }
 
-   CT::unpoison(x, size);
-   CT::unpoison(y, size);
-   CT::unpoison(carry);
-
    return carry & mask;
    }
 
@@ -76,9 +63,6 @@ word bigint_cnd_sub(word cnd, word x[], const word y[], size_t size)
    {
    const word mask = CT::expand_mask(cnd);
 
-   CT::poison(x, size);
-   CT::poison(y, size);
-
    word carry = 0;
    for(size_t i = 0; i != size; ++i)
       {
@@ -86,13 +70,21 @@ word bigint_cnd_sub(word cnd, word x[], const word y[], size_t size)
       x[i] = CT::select(mask, z, x[i]);
       }
 
-   CT::unpoison(x, size);
-   CT::unpoison(y, size);
-   CT::unpoison(carry);
-
    return carry & mask;
    }
 
+void bigint_cnd_abs(word cnd, word x[], size_t size)
+   {
+   const word mask = CT::expand_mask(cnd);
+
+   word carry = mask & 1;
+   for(size_t i = 0; i != size; ++i)
+      {
+      const word z = word_add(~x[i], 0, &carry);
+      x[i] = CT::select(mask, z, x[i]);
+      }
+   }
+
 /*
 * Two Operand Addition, No Carry
 */
diff --git a/src/lib/math/mp/mp_core.h b/src/lib/math/mp/mp_core.h
index 86bc920cf..73f13742c 100644
--- a/src/lib/math/mp/mp_core.h
+++ b/src/lib/math/mp/mp_core.h
@@ -40,6 +40,14 @@ word bigint_cnd_add(word cnd, word x[], const word y[], size_t size);
 BOTAN_DLL
 word bigint_cnd_sub(word cnd, word x[], const word y[], size_t size);
 
+/*
+* 2s complement absolute value
+* If cond > 0 sets x to ~x + 1
+* Runs in constant time
+*/
+BOTAN_DLL
+void bigint_cnd_abs(word cnd, word x[], size_t size);
+
 /**
 * Two operand addition
 * @param x the first operand (and output)