TCN Protocol

Covid Watch's TCN Protocol received significant news coverage^[12] and was followed by similar decentralized protocols in early April 2020 like DP-3T, PACT,^[13] and Google/Apple Exposure Notification framework. Covid Watch then helped other groups like the TCN Coalition and MIT SafePaths^[14] implement the TCN Protocol within their open source projects to further the development of decentralized technology and foster global interoperability of contact tracing and exposure alerting apps, a key aspect of achieving widespread adoption.^[15] Covid Watch volunteers and nonprofit staff also built a fully open source mobile app for sending anonymous exposure alerts first using the TCN Protocol^[16] and later using the very similar Google/Apple Exposure Notification Framework (ENF).^[17][18]


The protocol, like BlueTrace and the Google / Apple contact tracing project, use Bluetooth Low Energy to track and log encounters with other users.^[19][20][21] The major distinction between TCN and protocols like BlueTrace is the fact the central reporting server never has access to contact logs nor is it responsible for processing and informing clients of contact.^[22][23] Because contact logs are never transmitted to third parties, it has major privacy benefits over approaches like the one used in BlueTrace.^[24][25][26] This approach however, by its very nature, does not allow for human-in-the-loop reporting, potentially leading to false positives if the reports are not verified by public health agencies.^{[19]: p. 6}


The TCN protocol received notoriety as one of the first widely released digital contact tracing protocols^[15][27][28] alongside BlueTrace,^[29] the Exposure Notification framework, and the Pan-European Privacy-Preserving Proximity Tracing (PEPP-PT) project.^[30][31] It also stood out for its incorporation of blockchain technology,^[32] and its influence over the Google/Apple project.^{[33][20][34][35][36]}

Overview[edit]

The TCN protocol works off the basis of Temporary Contact Numbers (TCN), semi-random identifiers derived from a seed.^[37] When two clients encounter each other, a unique TCN is generated, exchanged, and then locally stored in a contact log.^[38] Then, once a user tests positive for infection, a report is sent to a central server. Each client on the network then collects the reports from the server and independently checks their local contact logs for a TCN contained in the report. If a matching TCN is found, then the user has come in close contact with an infected patient, and is warned by the client. Since each device locally verifies contact logs, and thus contact logs are never transmitted to third parties, the central reporting server cannot by itself ascertain the identity or contact log of any client in the network. This is in contrast to competing protocols like BlueTrace, where the central reporting server receives and processes client contact logs.^[39]