From 70ad8397259f96f53104929b0c416f6617280b69 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Wed, 21 Mar 2018 16:28:41 +0100
Subject: [PATCH 1/3] Clarify the behavior of bignum resize and copy functions

---
 include/mbedtls/bignum.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 3bf02a7ee..31383b1eb 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -204,6 +204,8 @@ void mbedtls_mpi_free( mbedtls_mpi *X );
 /**
  * \brief          Enlarge to the specified number of limbs
  *
+ *                 This function does nothing if the MPI is already large enough.
+ *
  * \param X        MPI to grow
  * \param nblimbs  The target number of limbs
  *
@@ -215,19 +217,23 @@ int mbedtls_mpi_grow( mbedtls_mpi *X, size_t nblimbs );
 /**
  * \brief          Resize down, keeping at least the specified number of limbs
  *
+ *                 If \c X is smaller than \c nblimbs, it is resized up
+ *                 instead.
+ *
  * \param X        MPI to shrink
  * \param nblimbs  The minimum number of limbs to keep
  *
  * \return         0 if successful,
  *                 MBEDTLS_ERR_MPI_ALLOC_FAILED if memory allocation failed
+ *                 (this can only happen when resizing up).
  */
 int mbedtls_mpi_shrink( mbedtls_mpi *X, size_t nblimbs );
 
 /**
  * \brief          Copy the contents of Y into X
  *
- * \param X        Destination MPI
- * \param Y        Source MPI
+ * \param X        Destination MPI. It is enlarged if necessary.
+ * \param Y        Source MPI.
  *
  * \return         0 if successful,
  *                 MBEDTLS_ERR_MPI_ALLOC_FAILED if memory allocation failed

From 4e4be7cf623803865d4259f6c3a11414b6565221 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Wed, 21 Mar 2018 16:29:03 +0100
Subject: [PATCH 2/3] Optimize unnecessary zeorizing in mbedtls_mpi_copy

Based on a contribution by Alexey Skalozub
(https://github.com/ARMmbed/mbedtls/pull/405).
---
 ChangeLog        |  6 ++++++
 library/bignum.c | 12 +++++++++---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 8db021591..25f52c804 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.x.x branch released xxxx-xx-xx
+
+Changes
+   * Optimize unnecessary zeroing in mbedtls_mpi_copy. Based on a contribution
+     by Alexey Skalozub.
+
 = mbed TLS 2.7.0 branch released 2018-02-03
 
 Security
diff --git a/library/bignum.c b/library/bignum.c
index d27c130bc..e57e6af17 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -184,7 +184,7 @@ int mbedtls_mpi_shrink( mbedtls_mpi *X, size_t nblimbs )
  */
 int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
 {
-    int ret;
+    int ret = 0;
     size_t i;
 
     if( X == Y )
@@ -203,9 +203,15 @@ int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
 
     X->s = Y->s;
 
-    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, i ) );
+    if( X->n < i )
+    {
+        MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, i ) );
+    }
+    else
+    {
+        memset( X->p + i, 0, ( X->n - i ) * ciL );
+    }
 
-    memset( X->p, 0, X->n * ciL );
     memcpy( X->p, Y->p, i * ciL );
 
 cleanup:

From 092bf3dd3898e67b9dff2bb5d4f2aa289a827135 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Sun, 1 Apr 2018 12:43:48 +0200
Subject: [PATCH 3/3] Add original PR reference

---
 ChangeLog | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 25f52c804..8a3f0af04 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -4,7 +4,7 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 Changes
    * Optimize unnecessary zeroing in mbedtls_mpi_copy. Based on a contribution
-     by Alexey Skalozub.
+     by Alexey Skalozub in #405.
 
 = mbed TLS 2.7.0 branch released 2018-02-03