Privnote vs. SeeOnce

In this post, I review Privnote and compare it withSeeOnce. Both apps claim to generate self-destructing messages. Which one will be the winner?

Ladies and gentlemen, on this corner is Privnote, a really, really simple app to send self-destructing messages to someone. It works like this:

  1. Navigate to The interface contains a single box where you write your message and a big red button to create the note. Nobody asks you for any password or private info of any kind. There is an additional button for some options, such as encrypting it with a password (using AES), changing the default time for self-destruction, or getting an emailed confirmation that the note has been read and destroyed.
  2. Write the note and click the big red button. The box is replaced by a link like this: already selected for you to copy.
  3. Copy the link and paste it into an email or whatever.
  4. When the recipient clicks on the link, Privnote loads on a secure page and displays the original message, plus assurances that the message has been destroyed at the source. Sure enough, reloading the page displays a warning that the message no longer exists.

Privnote is beautifully simple and it seems to work. Can anyone beat it?

On this other corner is SeeOnce, which also claims to generate self-destructing messages in a pretty simple matter. SeeOnce works like this:

  1. Navigate to You are immediately asked to come up with a Password, though you are assured that you are not making an account anywhere. After supplying this, the interface contains a single box with way more buttons than Privnote, though many of them are things like “Select”, “Clear”, “Help”, etc. (Privnote doesn’t have any help, perhaps because it doesn’t need any?). There are no options to set in SeeOnce.
  2.  Write the note and click the highlighted Lock button. Now another dialog pops up, asking you to identify the recipient on a list, or send it as an (insecure) invitation to a still unknown user. This dialog doesn’t appear if you loaded SeeOnce from a link sent by someone else.
  3. After you do this, a piece of text containing a longish random-looking link fills the box. The link may be something like this: and it is already selected as in Privnote.
  4. You can copy and paste the contents (the link alone is enough) into email or texting, or simply click the Email button, which will put the whole thing into your default email. Alternatively, you can click a Hide button, which will convert the stuff into apparently normal text taken from a cover text (a popup asks you for the cover text, if you haven’t loaded one yet), before emailing it.
  5. When the recipient clicks on the link, SeeOnce loads on a secure page and asks for a Password. After supplying this, the original message is revealed. Reloading the link and re-typing the Password leads to a message stating that “unlocking has failed” (SeeOnce needs to exchange two messages between the same parties before this happens right away, otherwise the link does not fail immediately but rather after writing a reply).

A little more complicated than Privnote, but still quite manageable. Now, the devil is in the details, as they say. We need to look at what’s inside as well as the features and the overall simplicity of the process. Price is not much of an issue since both apps are free, but availability on different platforms might be.

Price. winner: SeeOnce

Both apps are free, but Privnote has ads. This is not only uncool, but poses a security risk since the ads could potentially inject malicious code into the page, compromising everything. SeeOnce, on the contrary, stays true to the open source ethos and contains no ads. SeeOnce can do this because it doesn’t rely on servers for its operation and therefore expenses are insignificant.

Simplicity. winner: Privnote

It’s hard to be simpler than Privnote: you click a link, enter a message, copy a link; on the receiving side, just click a link and you’re done. SeeOnce is almost there, but it does ask you to come up with a Password, which is extra work and requires the user to exercise his/her memory, never a good thing (we’ll see later that this isn’t as bad as it looks). On the other hand, emailing can be done without cut and paste by just clicking an Email button. Still, Privnote wins this one.

Features. winner: SeeOnce

Privnote does have a few extra settings, such as the ability to encrypt the message with a chosen password rather than the default 54-bit key (but then, how do you send the password to the recipient in a secure manner?), whereas SeeOnce encryption is always under user control (and this is why it asks you for a Password before it does anything). Privnote also has the ability to send a read receipt, which SeeOnce lacks (we see why below). Still, SeeOnce wins this one because it has a comprehensive Help system (to its credit, Privnote hardly needs one) and the ability to hide its output as innocent text, which can be life-saving in places where encryption is frowned upon. SeeOnce also has the ability to switch to secure real-time chat if the correspondents find themselves emailing one another every few minutes.

Availability. winner: tie

Both apps are available from secure links on a regular browser, though SeeOnce can run offline and Privnote cannot. SeeOnce is also available as a Chrome extension and in the Android store. So SeeOnce has an edge here, but I’m going to call it a tie since sending emails requires Internet and most likely a browser.

Security. winner: SeeOnce

Ah, here’s the biggie. Both apps stem from radically different approaches to achieve the same goal. Privnote is fundamentally server-based (except its encryption option, which appears to be client-based), whereas SeeOnce is strictly client-based (after the web server delivers the code, that is). Let’s see what’s underneath each one:

  • In Privnote, the message (encrypted with a symmetric key, which is sent in plaintext with the link but the server does not see) is sent to a server, where it is stored. Clicking on the sender-generated link first instructs the server to send the encrypted message the recipient’s machine, where it is decrypted with the key contained in the link. The Privnote server will deliver the data if this is the first time this particular link has been clicked by anyone, and the other optional conditions, such as expiration date, have been met. Then the server deletes the stored data, or so we are told, so that a repeated request using the same link cannot be met. Still, Privnote can tell the difference between an expired link and one that was never issued, which leads me to think that some memory of having stored the message remains for a while.
  • In SeeOnce, the message is locally encrypted with the result of combining a public key, which was received in a previous message from the same correspondent, and a random private key that is stored locally and is overwritten when a new message for this correspondent is encrypted. The underlying protocol is fairly complex but transparent to the user. SeeOnce never communicates with servers, so the reason why a message “self-destructs” (actually, no longer can be decrypted) is that at least one of the keys has been overwritten and cannot be obtained anywhere else, even if someone has been copying every message exchanged. This is also why SeeOnce cannot produce a read receipt: it was a different program that actually sent the message; the SeeOnce server never knew about the sender or any of his/her data.

There are three reasons why the approach followed by SeeOnce is much more secure:

  1. The first one is that Privnote displays the decrypting key in plaintext (or an equivalent, given that the client-side code can be viewed at any time) as part of the link. It needs to do this because it does not ask for any information about the recipient before preparing the link, so anyone should be able to follow the link. If the link is sent by email, for instance (and remember we are encrypting the message because we believe email to be insecure), the link can be nabbed by someone else, who then can read the message, instead of the intended recipient. Getting some control over who can actually read the message would require some sort of recipient authentication, a password at the minimum, which is what SeeOnce does.
  2. Whenever data is stored in a server, the user loses control over it. Privnote can say they have destroyed the message until they are blue in the face, but they cannot prove it. If a government agency serves them a request to make a copy, they might be doing it without the users’ knowledge. A hacker can break in and look at the data. The server itself may be saving the data as part of a scheduled backup. Now, Privnote states that this data is encrypted with a key that is not sent to the server, but since that key is included at the end of the link sent by email (otherwise the recipient would never be able to decrypt the message), if the link is compromised as we saw above, then the agency or hacker can decrypt the message. The only protection against complete loss of security is user-controlled symmetric local encryption with a separate key, which Privnote offers as an option, but then the user has the problem of transmitting the key. SeeOnce stores data only locally, and so this is much less of a problem. Stored data is encrypted by the user Password (is it beginning to look like this wasn’t such a hassle after all?) and can optionally be wiped or moved to a different machine. Anything transmitted is encrypted with a public-key algorithm, so that key transmission is never an issue.
  3. Code executing on a server is invisible to the user. Therefore, a Privnote user has no way of making sure that the code is honest and free from errors. In Privnote, this means the code that supposedly is keeping track of how many times a particular link has been followed, and which deletes the data on the server. On the other hand, the complete SeeOnce code is visible to the user by just typing ctrl-U on the browser. It is long and complex, to be sure, but it hides nothing. The program itself has a mechanism to ensure that the code has not been tampered with by a third party, fully documented in the Help pages.

Both programs have features to recommend them but in the end it comes down to a personal choice: do you value ease of use above anything else, or is it security what you value the most in a security product? Perhaps the only way to tell is to take both for a spin and decide for yourself.

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