[TYPO3] MD5 on file, into database

[Torben Dam]

I have the following question:

how is it possible to have an MD5 value of a file
stored in the database along with the file name ?

I assume I need to do changes in tca.php file, but I can't
figure out how to make a calculation (basically call a function)
at "save form" time.

What I want to do is:

* Form has an input field for a file name
* when form is saved (list element created), the function
  for calculation of MD5 should be called
* then the MD5 value should be stored along with the file name

Alternatively there can be a button for "make calculation" during
edit, but I assume first method is best.

I looked at TCA documentation but can't find info on
user hooks or function calls.

Can anyone point me in the right direction ?

Thank you.

Torben