Techniques are disclosed relating to securely communicating traffic. In some embodiments, an apparatus includes a secure circuit storing keys usable to encrypt data communications between devices over a network. The secure circuit is configured to store information that defines a set of usage criteria for the keys. The set of usage criteria specifies that a first key is dedicated to encrypting data being communicated from a first device to a second device. The secure circuit is configured to receive a request to encrypt a portion of a message with the first key, the request indicating that the message is being sent from the first device to the second device, and to encrypt the portion of the message with the first key in response to determining that the set of usage criteria permits encryption with the first key for a message being sent from the first device to the second device.