From c81c5889e9b3cc4474ece038fa62983ca03b358e Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Thu, 4 Jun 2020 19:14:58 +0200
Subject: [PATCH] Separate out low-level mpi_safe_cond_assign
Separate out a version of mpi_safe_cond_assign that works on
equal-sized limb arrays, without worrying about allocation sizes or
signs.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
---
library/bignum.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/library/bignum.c b/library/bignum.c
index 7f9183f8e..c513525e7 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -242,6 +242,22 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
memcpy( Y, &T, sizeof( mbedtls_mpi ) );
}
+/*
+ * Conditionally assign dest = src, without leaking information
+ * about whether the assignment was made or not.
+ * dest and src must be arrays of limbs of size n.
+ * assign must be 0 or 1.
+ */
+static void mpi_safe_cond_assign( size_t n,
+ mbedtls_mpi_uint *dest,
+ const mbedtls_mpi_uint *src,
+ unsigned char assign )
+{
+ size_t i;
+ for( i = 0; i < n; i++ )
+ dest[i] = dest[i] * ( 1 - assign ) + src[i] * assign;
+}
+
/*
* Conditionally assign X = Y, without leaking information
* about whether the assignment was made or not.
@@ -261,10 +277,9 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
X->s = X->s * ( 1 - assign ) + Y->s * assign;
- for( i = 0; i < Y->n; i++ )
- X->p[i] = X->p[i] * ( 1 - assign ) + Y->p[i] * assign;
+ mpi_safe_cond_assign( Y->n, X->p, Y->p, assign );
- for( ; i < X->n; i++ )
+ for( i = Y->n; i < X->n; i++ )
X->p[i] *= ( 1 - assign );
cleanup: