Sync / End-to-End Encryption covers two linked features of note-taking software: sync, which keeps the same notes current across a person's devices, and end-to-end encryption of that sync, which leaves the content readable only on those devices.
📝Obsidian's documentation describes Obsidian Sync as an add-on service that lets users privately sync notes across devices. When creating a remote vault, users choose between end-to-end encryption, the default, under which Obsidian can never access the data, and standard encryption, whose key Obsidian manages on company servers, where it can be used to decrypt the data. The choice applies only to the remote vault; Obsidian does not encrypt the local vault.
Obsidian states that it cannot recover a lost password or the encrypted data, and that some metadata, including the mapping between encrypted file paths and content, is not end-to-end encrypted.
Definitions of end-to-end encryption vary in scope. The NIST glossary, citing SP 800-12 Rev. 1, defines it as communications encryption in which data is encrypted when passed through a network while routing information remains visible. Definition of End-to-end Encryption, a June 2022 IETF Internet-Draft (a working document, not a standard), describes communication between two or more parties without any third party having access to its content, and holds that decryption at an intermediary such as a service provider means the property is absent.
The 2019 essay that named 📝Local-First Software notes that local-first apps can use end-to-end encryption so that servers storing a user's files hold only encrypted data.