Commiting files Uploads parts requires Sha Digest via API Curl

I have successfully uploaded files in parts and and its successful. I can get part id, offsets etc

Now I want to commit the uploaded files parts using the curl API Request below

>
curl -X POST \
https://upload.box.com/api/2.0/files/upload_sessions/F971964745A5CD0C001BBE4E58196BFD/commit \
-H 'authorization: Bearer ACCESS_TOKEN' \
-H 'content-type: application/json' \
-H 'digest: sha=DhIcpd61HctMIGTMu7tyY54Da2U=' \
-d '{
"parts":[
{"part_id": "BFDF5379","offset": 0,"size": 8388608},
{"part_id": "E8A3ED8E","offset": 8388608,"size": 1611392}
],
"attributes":{"content_modified_at":"2017-04-08T00:58:08Z"}
}'

>

Here is my question: Which digest is required here again.

Let assume that my files was uploaded in 4 parts. Is it base64 encoded Sha1 digest of each file partsize for the file already uploaded via upload Parts.

Where do I get the Sha1 digest stated in the API above. Again will this Sha1 be base64encode again.

Please I need an explanations. Thanks