From 92c98931f28b2523752ce0155b6a730f890b440e Mon Sep 17 00:00:00 2001
From: Hanno Becker <hanno.becker@arm.com>
Date: Wed, 1 May 2019 17:09:11 +0100
Subject: [PATCH] Fix mpi_bigendian_to_host() on bigendian systems

The previous implementation of mpi_bigendian_to_host() did
a byte-swapping regardless of the endianness of the system.

Fixes #2622.
---
 library/bignum.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/library/bignum.c b/library/bignum.c
index 41946183c..a1822fc6c 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -742,10 +742,15 @@ cleanup:
 static mbedtls_mpi_uint mpi_uint_bigendian_to_host_c( mbedtls_mpi_uint x )
 {
     uint8_t i;
+    unsigned char *x_ptr;
     mbedtls_mpi_uint tmp = 0;
-    /* This works regardless of the endianness. */
-    for( i = 0; i < ciL; i++, x >>= 8 )
-        tmp |= ( x & 0xFF ) << ( ( ciL - 1 - i ) << 3 );
+
+    for( i = 0, x_ptr = (unsigned char*) &x; i < ciL; i++, x_ptr++ )
+    {
+        tmp <<= CHAR_BIT;
+        tmp |= (mbedtls_mpi_uint) *x_ptr;
+    }
+
     return( tmp );
 }