Add explanation for okm_string size
Add explanation for why the size of `okm_string` buffer
is twice as `okm` buffer.
diff --git a/tests/suites/test_suite_hkdf.function b/tests/suites/test_suite_hkdf.function
index e41422a..fc0e242 100644
--- a/tests/suites/test_suite_hkdf.function
+++ b/tests/suites/test_suite_hkdf.function
@@ -19,6 +19,10 @@
unsigned char info[128] = { '\0' };
unsigned char expected_okm[128] = { '\0' };
unsigned char okm[128] = { '\0' };
+ /*
+ * okm_string is the string representation of okm,
+ * so its size is twice as the size of okm.
+ */
unsigned char okm_string[256] = { '\0' };
const mbedtls_md_info_t *md = mbedtls_md_info_from_type( md_alg );