Carte (from carte-de-visite) is a cryptographic token that allows client to authenticate on any node besides the home node. Since the client does not have access to the private signing key (giving such access would weaken the security), it would need to ask the home node to sign every request that needs authentication or to perform authentication on every node individually. Carte is a compromise between security and performance, it gives acceptable user experience, minimizing damage if some cartes are leaked.
After authentication on the home node, the client requests a set of cartes. Each carte is a BASE64-encoded array of bytes that is a concatenation of CarteFingerprint and its signature.
carte = fingerprint + signature
The fingerprint contains the name of the home node, the IP address of the client, beginning and end timestamps of the carte’s life and a random salt. Binding to time and IP address minimizes ability of an attacker to use a carte that was intercepted somehow. For the same reason it is recommended to use cartes with a short lifetime (several minutes). If a client needs to work longer, it can request several cartes in a single request that correspond to successive periods of time. When one carte expires, the client throws it off and uses the next one in the sequence.
If client wants to post a content (comment, reaction etc.) to a node, carte authentication is not enough, because the content needs its own signature. To give better user experience, the client may use carte to post a temporary content without a signature and then ask the home node to post its signed version. If during a short period of time the signed content do not arrive, the node erases the temporary version.