Exp mod: clarify preprocessing
Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 0f63c31..4cebd95 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1654,7 +1654,14 @@
X->s = 1;
/*
- * Make sure that A has exactly as many limbs as N.
+ * Make sure that X is in a form that is safe for consumption by
+ * the core functions.
+ *
+ * - The core functions will not touch the limbs of X above N->n. The
+ * result will be correct if those limbs are 0, which the mod call
+ * ensures.
+ * - Also, X must have at least as many limbs as N for the calls to the
+ * core functions.
*/
if (mbedtls_mpi_cmp_mpi(X, N) >= 0) {
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(X, X, N));