From 58525180fbda2e8e84e28297193a565c04025ce5 Mon Sep 17 00:00:00 2001
From: Janos Follath <janos.follath@arm.com>
Date: Mon, 28 Oct 2019 12:12:15 +0000
Subject: [PATCH] Make mbedtls_mpi_lt_mpi_ct more portable

The code relied on the assumptions that CHAR_BIT is 8 and that unsigned
does not have padding bits.

In the Bignum module we already assume that the sign of an MPI is either
-1 or 1. Using this, we eliminate the above mentioned dependency.
---
 library/bignum.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/library/bignum.c b/library/bignum.c
index daf0cfd9d..39e718dcf 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1123,12 +1123,11 @@ int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
         return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
 
     /*
-     * Get sign bits of the signs.
+     * Set sign_N to 1 if N >= 0, 0 if N < 0.
+     * We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
      */
-    sign_X = X->s;
-    sign_X = sign_X >> ( sizeof( unsigned ) * 8 - 1 );
-    sign_Y = Y->s;
-    sign_Y = sign_Y >> ( sizeof( unsigned ) * 8 - 1 );
+    sign_X = ( X->s & 2 ) >> 1;
+    sign_Y = ( Y->s & 2 ) >> 1;
 
     /*
      * If the signs are different, then the positive operand is the bigger.