Signal is a free and open source messaging app for encrypted voice calls and instant messages.
Signal provides security by encrypting communications in transit (end-to-end encryption) and by ensuring that past communications are secure even if encryption keys are stolen (forward secrecy). Several applications, such as WhatsApp, use the Signal protocol for encryption.
The OONI Probe Signal test is designed to measure the reachability of the Signal messaging app within a tested network.
To this end, this test checks whether it’s possible to establish a TLS connection (while validating the TLS certificate against the custom Signal CA root certificate) and send an HTTP GET request to the Signal server backends from the vantage point of the user.
If the test successfully performs an HTTPS request to the tested Signal endpoints, the Signal app is considered reachable from the tested network.
However, if connections to any of the tested Signal endpoints fail, Signal might be unreachable or blocked.
Read the Signal test specification.