A file upload proceeds in the following steps

1. Check filename is valid and free

2. Create a transaction file with a plan for the upload

3. For every section of the file which is up to 5 MiB:

   • Encrypt the 5 MiB file section with a random symmetric key
   • Split the cipher text into 128 KiB fragments
   • Create a FileAccess cryptree node with merkle links to all the resulting fragments and an encrypted link to the next section (even if there isn't a next section)
   • Add the FileAccess to the champ of the writing key pair under a random 32 byte label

4. Add a cryptree link from the parent directory to the file

5. Delete the transaction file

A modification, such as uploading a file, can be done through any Peergos server as the writes are proxied through an ipfs p2p stream to the owner's storage ipfs node.