# REVOCATION

The REVOCATION API consists of two parts, to query and to issue revocations.

## Querying for revoked keys

`GNUNET_REVOCATION_query` is used to check if a given ECDSA public key has been revoked. The given callback will be invoked with the result of the check. The query can be canceled using `GNUNET_REVOCATION_query_cancel` on the return value.

## Preparing revocations

It is often desirable to create a revocation record ahead-of-time and store it in an off-line location to be used later in an emergency. This is particularly true for GNUnet revocations, where performing the revocation operation itself is computationally expensive and thus is likely to take some time. Thus, if users want the ability to perform revocations quickly in an emergency, they must pre-compute the revocation message. The revocation API enables this with two functions that are used to compute the revocation message, but not trigger the actual revocation operation.

`GNUNET_REVOCATION_check_pow` should be used to calculate the proof-of-work required in the revocation message. This function takes the public key, the required number of bits for the proof of work (which in GNUnet is a network-wide constant) and finally a proof-of-work number as arguments. The function then checks if the given proof-of-work number is a valid proof of work for the given public key. Clients preparing a revocation are expected to call this function repeatedly (typically with a monotonically increasing sequence of numbers of the proof-of-work number) until a given number satisfies the check. That number should then be saved for later use in the revocation operation.

`GNUNET_REVOCATION_sign_revocation` is used to generate the signature that is required in a revocation message. It takes the private key that (possibly in the future) is to be revoked and returns the signature. The signature can again be saved to disk for later use, which will then allow performing a revocation even without access to the private key.

## Issuing revocations

Given a ECDSA public key, the signature from `GNUNET_REVOCATION_sign` and the proof-of-work, `GNUNET_REVOCATION_revoke` can be used to perform the actual revocation. The given callback is called upon completion of the operation. `GNUNET_REVOCATION_revoke_cancel` can be used to stop the library from calling the continuation; however, in that case it is undefined whether or not the revocation operation will be executed.

### The REVOCATION Client-Service Protocol

The REVOCATION protocol consists of four simple messages.

A `QueryMessage` containing a public ECDSA key is used to check if a particular key has been revoked. The service responds with a `QueryResponseMessage` which simply contains a bit that says if the given public key is still valid, or if it has been revoked.

The second possible interaction is for a client to revoke a key by passing a `RevokeMessage` to the service. The `RevokeMessage` contains the ECDSA public key to be revoked, a signature by the corresponding private key and the proof-of-work. The service responds with a `RevocationResponseMessage` which can be used to indicate that the `RevokeMessage` was invalid (e.g. the proof of work is incorrect), or otherwise to indicate that the revocation has been processed successfully.

### The REVOCATION Peer-to-Peer Protocol

Revocation uses two disjoint ways to spread revocation information among peers. First of all, P2P gossip exchanged via CORE-level neighbours is used to quickly spread revocations to all connected peers. Second, whenever two peers (that both support revocations) connect, the SET service is used to compute the union of the respective revocation sets.

In both cases, the exchanged messages are `RevokeMessage`s which contain the public key that is being revoked, a matching ECDSA signature, and a proof-of-work. Whenever a peer learns about a new revocation this way, it first validates the signature and the proof-of-work, then stores it to disk (typically to a file $GNUNET_DATA_HOME/revocation.dat) and finally spreads the information to all directly connected neighbours.

For computing the union using the SET service, the peer with the smaller hashed peer identity will connect (as a "client" in the two-party set protocol) to the other peer after one second (to reduce traffic spikes on connect) and initiate the computation of the set union. All revocation services use a common hash to identify the SET operation over revocation sets.

The current implementation accepts revocation set union operations from all peers at any time; however, well-behaved peers should only initiate this operation once after establishing a connection to a peer with a larger hashed peer identity.