Merge pull request #3652 from d-otte/mbedtls-2.7
Backport 2.7: adjusting size of sliding window array to correct size
diff --git a/ChangeLog.d/adjusting sliding_window_size_PR3592.txt b/ChangeLog.d/adjusting sliding_window_size_PR3592.txt
new file mode 100644
index 0000000..6089565
--- /dev/null
+++ b/ChangeLog.d/adjusting sliding_window_size_PR3592.txt
@@ -0,0 +1,3 @@
+Changes
+ * Reduce stack usage significantly during sliding window exponentiation.
+ Reported in #3591 and fix contributed in #3592 by Daniel Otte.
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 754b50a..a2fa3bd 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -83,12 +83,12 @@
* Maximum window size used for modular exponentiation. Default: 6
* Minimum value: 1. Maximum value: 6.
*
- * Result is an array of ( 2 << MBEDTLS_MPI_WINDOW_SIZE ) MPIs used
+ * Result is an array of ( 2 ** MBEDTLS_MPI_WINDOW_SIZE ) MPIs used
* for the sliding window calculation. (So 64 by default)
*
* Reduction in size, reduces speed.
*/
-#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum windows size used. */
+#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum window size used. */
#endif /* !MBEDTLS_MPI_WINDOW_SIZE */
#if !defined(MBEDTLS_MPI_MAX_SIZE)
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index d8332da..d23a7ef 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -2814,7 +2814,7 @@
*/
/* MPI / BIGNUM options */
-//#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum windows size used. */
+//#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum window size used. */
//#define MBEDTLS_MPI_MAX_SIZE 1024 /**< Maximum number of bytes for usable MPIs. */
/* CTR_DRBG options */
diff --git a/library/bignum.c b/library/bignum.c
index 3135ec4..0e39e3a 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1850,7 +1850,7 @@
size_t i, j, nblimbs;
size_t bufsize, nbits;
mbedtls_mpi_uint ei, mm, state;
- mbedtls_mpi RR, T, W[ 2 << MBEDTLS_MPI_WINDOW_SIZE ], Apos;
+ mbedtls_mpi RR, T, W[ 1 << MBEDTLS_MPI_WINDOW_SIZE ], Apos;
int neg;
if( mbedtls_mpi_cmp_int( N, 0 ) <= 0 || ( N->p[0] & 1 ) == 0 )