TRC Signing Ceremony

The TRC contains policy information about an ISD and acts as a distribution mechanism for the trust anchors of that TRC. It enables securing the control plane interactions, and thus is an integral part of the SCION infrastructure.

In the signing ceremony, the voters of the ISD meet in person to sign an agreed upon TRC. As part of the ceremony, the public keys of all voters are exchanged.

There are two types of signing ceremonies: the ceremony to sign the base TRC and the ceremony to sign a non-base TRC.

The base TRC builds the anchor point for a TRC update chain. All voters need to take part in this ceremony. The very first TRC for an ISD number, the initial TRC, is a special case of the base TRC where the ISD number is chosen. Future base TRCs are only created in trust resets, which are a disaster recovery procedure for catastrophic scenarios where the air-gapped high security keys of multiple voting members of the ISD have been simultaneously compromised. The likelihood of such a compromise is extremely low if keys are adequately stored.

A non-base TRC is the result of a TRC update. Only a quorum of voters need to partake in a non-base TRC signing ceremony.

Ceremony Participants

A signing ceremony typically includes participants from various organizations. For example, a signing ceremony might include something like the following:

  • Ceremony Administrator:
    • Giraffe Example Organization
  • Voting AS Representatives:
    • Elephant Example Organization
    • Gorilla Example Organization
    • Zebra Example Organization
  • Witnesses

The Ceremony Administrator is in charge of moderating the whole signing process, and walks all participants through the steps they need to execute and acts as the information sharing hub between them.

The Voting AS Representative is capable of creating voting signatures on the TRC. This means the voting representative is in possession of a device with the private keys of the respective certificates in the TRC.

The Witness is any person that participates in the ceremony as a passive entity. They observe the ceremony execution.

Ceremony Preparations

Prior to the ceremony, participants decide on the physical location of the ceremony, the devices that will be used during the ceremony and the policy of the ISD.

Location

The location must provide electricity and enough power sockets for each participant. Furthermore, it should provide a monitor (or projector) that allows the ceremony administrator to screen cast.

Devices

Each party brings their own device that is provisioned with the required material, as described below.

  • USB Flash Drive: For exchanging data, a USB flash drive is used. It should be formatted as FAT32 before starting the ceremony. The drive can either be provided by ceremony administrator, or, if preferable, by any of the voting representatives.
  • Ceremony Administrator’s Device: The ceremony administrator should bring a machine that is capable of creating and verifying a TRC. Furthermore, it needs to be able to compute the SHA-512 digest of files.
  • Voting Representative’s Device: The voting representative should bring a machine that is capable of signing and verifying TRCs. Thus, the machine needs to have access to all the voting private keys. Furthermore, it needs to be able to compute the SHA-512 digest of the files. The exact binaries that are required are described in a separate document.

Policy

The voting entities need to agree on the ISD policy, before the ceremony can be executed. Specifically, they need to agree on:

  • Validity of the TRC
  • Grace period (except for base TRCs)
  • Voting Quorum
  • Core ASes
  • Authoritative ASes
  • Description
  • List of CP Root Certificates

When these values are agreed upon, a quorum of voters needs to execute the signing ceremony. The set of needed keys depends on whether a base TRC, or a TRC update is signed. For the base TRC, all voting entities need to be present with both their sensitive and regular voting key.

Ceremony Process

The ceremony process is structured in multiple rounds of data sharing. The ceremony administrator leads the interaction and instructs each participant with what to do.

Phase 1: Certificate Exchange

All entities share the certificates they want to be part of the TRC with ceremony administrator, who aggregates and bundles them. The bundle is then shared with all voters.

The ceremony administrator displays the SHA-512 digest of each bundled certificate on the monitor. Each voting representative verifies that the certificates they contributed have the same hash as what is displayed on the monitor. Further, all voting representatives confirm that the bundled certificates on their machine all have matching hashes.

Phase 2: TRC Payload Creation

The ceremony administrator generates the TRC based on the bundled certificates and the agreed upon ISD policy. The result is displayed on the monitor along with a SHA-512 digest. The TRC is distributed to all voting representatives. All of them must verify that the hash matches.

Phase 3: TRC Signing

Each voting representative attaches a signature for each of their new voting certificate to the TRC. When signing a non-base TRC, the voting representatives further cast a vote with the voting key present in the last TRC.

Phase 4: TRC Assembly

All voting representatives share the signed TRC with the ceremony administrator, who aggregates them in a single signed TRC file. The signed TRC is validated by inspecting its contents on the monitor and verifying the signatures based on the exchanged certificates in phase 1.

The ceremony administrator then shares the signed TRC with all participants. Each of them must then inspect it once more, and verify it based on the certificates exchanged in phase 1.

At this point, the ceremony is concluded. All participants have the signed TRC, and can use it to distribute the trust anchors for their ISD.

Security Model

For this ceremony, we assume that all parties are trustworthy. Issues encountered during the ceremony are assumed to be caused by honest mistakes, and not by malicious intent. To counter mistakes, we include hash comparison checks, such that every participant is sure that they operate on the same data.

Furthermore, the private keys of each participants never leave their machine. The ceremony administrator does not have to be entrusted with private keys.