commit a4af1c47d2d9bf4cb62f8df2f12ebe2ed7fbde89
author Hanno Becker <hanno.becker@arm.com> Tue Apr 18 09:07:45 2017 +0100
committer Hanno Becker <hanno.becker@arm.com> Tue Apr 18 09:07:45 2017 +0100
tree 929793cd2d8240d50b261531a4f6656081fb12fd
parent b65c2be5f174468a178b6d9456b20ca969d4a3d6

Correct sign in modular exponentiation algorithm.

The modular exponentiation function  handled the sign incorrectly. This commit fixes this and a test case which should have caught it.

library/bignum.c

diff --git a/library/bignum.c b/library/bignum.c
index 8b9082c..efc1a51 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1790,7 +1790,7 @@
      */
     MBEDTLS_MPI_CHK( mpi_montred( X, N, mm, &T ) );
 
-    if( neg )
+    if( neg && E->n != 0 && ( E->p[0] & 1 ) != 0 )
     {
         X->s = -1;
         MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( X, N, X ) );