OnionShare is an open source tool for securely and anonymously sending and receiving files using Tor onion services. It works by starting a web server directly on your computer and making it accessible as an unguessable Tor web address that others can load in Tor Browserto download files from you, or upload files to you. It doesn’t require setting up a separate server, using a third party file-sharing service, or even logging into an account.
Unlike services like email, Google Drive, DropBox, WeTransfer, or nearly any other way people typically send files to each other, when you use OnionShare you don’t give any companies access to the files that you’re sharing. So long as you share the unguessable web address in a secure way (like pasting it in an encrypted messaging app), no one but you and the person you’re sharing with can access the files.
- A user-friendly drag-and-drop graphical user interface that works in Windows, Mac OS X, and Linux
- Ability to share multiple files and folders at once
- Support for multiple people downloading files at once
- Automatically copies the unguessable URL to your clipboard
- Shows you the progress of file transfers
- When file is done transferring, automatically closes OnionShare to reduce the attack surface
- Localized into several languages, and supports international unicode filenames
What it protects against
- Third parties don’t have access to files being shared. The files are hosted directly on the sender’s computer and don’t get uploaded to any server. Instead, the sender’s computer becomes the server. Traditional ways of sending files, like in an email or using a cloud hosting service, require trusting the service with access to the files being shared.
- Network eavesdroppers can’t spy on files in transit. Because connections between Tor onion services and Tor Browser are end-to-end encrypted, no network attackers can eavesdrop on the shared files while the recipient is downloading them. If the eavesdropper is positioned on the sender’s end, the recipient’s end, or is a malicious Tor node, they will only see Tor traffic. If the eavesdropper is a malicious rendezvous node used to connect the recipient’s Tor client with the sender’s onion service, the traffic will be encrypted using the onion service key.
- Anonymity of sender and recipient are protected by Tor. OnionShare and Tor Browser protect the anonymity of the users. As long as the sender anonymously communicates the OnionShare URL with the recipient, the recipient and eavesdroppers can’t learn the identity of the sender.
- If an attacker enumerates the onion service, the shared files remain safe. There have been attacks against the Tor network that can enumerate onion services. If someone discovers the .onion address of an OnionShare onion service, they still cannot download the shared files without knowing the slug. The slug is generated by choosing two random words from a list of 6800 words, meaning there are 6800^2, or about 46 million possible slugs. But they can only make 20 wrong guesses before OnionShare stops the server, preventing brute force attacks against the slug. The OnionShare server also checks request URIs using a constant time string comparison function, so timing attacks can’t be used to help guess the slug.
What it doesn’t protect against
- Communicating the OnionShare URL might not be secure. The sender is responsible for securely communicating the OnionShare URL with the recipient. If they send it insecurely (such as through an email message, and their email is being monitored by an attacker), the eavesdropper will learn that they’re sending files with OnionShare. If the attacker loads the URL in Tor Browser before the legitimate recipient gets to it, they can download the files being shared. If this risk fits the sender’s threat model, they must find a more secure way to communicate the URL, such as in an encrypted email, chat, or voice call. This isn’t necessary in cases where the files being shared aren’t secret.
- Communicating the OnionShare URL might not be anonymous. While OnionShare and Tor Browser allow for anonymously sending files, if the sender wishes to remain anonymous they must take extra steps to ensure this while communicating the OnionShare URL. For example, they might need to use Tor to create a new anonymous email or chat account, and only access it over Tor, to use for sharing the URL. This isn’t necessary in cases where there’s no need to protect anonymity, such as coworkers who know each other sharing work documents.
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