Read access is a tree of symmetric keys where a folder key decrypts its children; a file key reveals nothing about siblings; write access is a separate tree of signing keys; sharing for reading is handing over a key and the data stays where it is.

It is the access-control idea the whole system is named for.

Read access is a tree of symmetric keys. Each folder's key decrypts the keys of its children.

Hold the key to a folder, and you can read everything beneath it. Peergos calls this the cryptree.

Hold the key to one file, and that is all you get. Not its siblings. Not even their names.

Write access is a second tree, of signing key pairs. The two trees are separate. Reading never implies writing.

To share a folder for reading, you hand over its key. The data stays where it is.

Check yourself. You hold the key to one file. What do you learn about the folder it is in?