Add comment in entropy.c
@@ -366,6 +366,11 @@ int mbedtls_entropy_func( void *data, unsigned char *output, size_t len )
memset( buf, 0, MBEDTLS_ENTROPY_BLOCK_SIZE );
#if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
+ /*
+ * Note that at this stage it is assumed that the accumulator was started
+ * in a previous call to entropy_update(). If this is not guaranteed, the
+ * code below will fail.
+ */
if( ( ret = mbedtls_sha512_finish_ext( &ctx->accumulator, buf ) ) != 0 )
goto exit;
