simple-ots - One-Time Secret

With simple-ots you keep sensitive information out of your messages. simple-ots enables you to create messages which are retrievable via a one-time link.

Once the link has been opened the message will be destroyed by the system, thus making sure, that you know as soon as a third person intercepted the one-time link.

How it works

The data is stored encrypted inside a MySQL table. When posting a message the system will create a random key $key and initialization vector $iv for encryption. The message will be encrypted using the $key and $iv and encrypt().

The encrypted message together with $iv and a create_hash($key) are stored inside the database and the user is presented an url containing the database id of the message and the key for decoding the message.

For reading the message the system will load the table row with the given id and compare_hash() the given key with the hash inside the database to verify that it is the right key to decrypt() the message. If the key is correct, the row will be deleted from the table and the decrypted message will be displayed.

Installation

All you need is your favorite webserver with PHP support and PHP PDO support. The webserver should be setup with good SSL settings, otherwise the setup will not make any sense ;)

Create a directory with write access for the user running the PHP module. Make sure the path is reachable by PHP, but not being served by the webserver! Or protect the database by other means, otherwise people are able to download the database.

The script (poorly) uses the PHP PDO driver, so you should be able to use MySQL as well or any other supported DB. The table will be named ots_table.

Attack Vectors

• Brute force the secret for a specific id. Speed would depend on the machine handling the requests and/or the database, but using the interface would result in the message's destruction and the original recipient knows that the message has been compromised.
• Capturing the link would be possible, but again using the interface would result in the message's destruction.
• With access to the database one would be able to brute force the key more directly, but every row has it's own key.
• Since the rows are only deleted it might be possible to reconstruct an already destroyed message from the hdd, in case no other data has taken the allocated space, yet.
• There is no MAC and encrypted data could therefore be altered without being noticed, if an attacker has access to the database. The interface itself does not allow updates.
• SQL-Injection seems to be unlikely, since the message given by the user is run through the encrypt() function first and the id and key to request a message are run through PDO::prepare().

Thanks to

Jack posting the details on how encryption should be done: http://stackoverflow.com/questions/10916284/how-to-encrypt-decrypt-data-in-php#answer-10945097