new module: md5 with substitution

[nick8606]

should it be done in "m00000_sxx" or "m00000_mxx"?

In both.

will it be ok if after "replacement" the new w has possibly 400 or 500 characters, instead of just 6 to 10?

sizeof(w)==256, too short for your task. It's needed to use new local array for data after replacing.

can md5_update_vector perform multiple blocks at once?

3rd parameter of md5_update_vector() is data size. All this size is processed.