DocSecrets is an add-on for Google Docs that hides and encrypts sensitive information such as passwords or an idea for a patent.
All secrets are encrypted using AES in your browser. This means that inserted secrets are never sent or stored in unencrypted form. Likely, your pass-phrase is never sent or stored anywhere. Only you can decrypt your secrets so pick a passphrase that you can remember. All data is stored under your document as properties.
DocSecrets supports document sharing and enables you to censor parts of your document. If you reveal your passphrase to your collaborators they to can view and edit secrets.