Hello,

The OONI team published a new research report which shares OONI data and analysis on the *blocking of the Signal Private Messenger App in Iran, China, Cuba, and Uzbekistan*.

Read the report here: https://ooni.org/post/2021-how-signal-private-messenger-blocked-around-the-w...

In April 2021, we released a new OONI Probe test for measuring the blocking of the Signal Private Messenger App (https://ooni.org/nettest/signal). Since then, OONI community members tested Signal worldwide every day, contributing measurements as open data: https://explorer.ooni.org/search?until=2021-10-23&since=2021-09-23&t...

We analyzed all Signal measurements collected from around the world *between April 2021 to September 2021*. During this period, we found that the testing of Signal primarily presented signs of blocking in Iran, China, Cuba, and Uzbekistan.

*# Summary of Findings*

*## Iran*

In Iran, Signal was *tested on 32 AS networks*, consistently presenting signs of blocking on most networks.

The block primarily appears to be performed at a DNS level by returning the IP address 10.10.34.35, which is commonly used to serve the Iranian blockpage.

*## China*

In China, Signal was *tested on 28 AS networks*, consistently presenting signs of blocking on most networks. Signal blocking is implemented at the DNS level, returning IP addresses inside of random IP ranges.

China appears to block Signal by means of *bidirectional DNS injection*, as suggested by the fact that DNS queries from outside the country also result in injected responses, even though they target a publicly routed IP in China that does not run a DNS server.

*## Cuba*

In Cuba, Signal was *temporarily blocked in mid-July 2021* amid anti-government protests.

OONI measurements show that the TLS handshake to the Signal backend service failed consistently with a connection_reset error, likely indicating the use of Deep Packet Inspection (DPI) technology targeting the SNI field of the TLS handshake.

*## Uzbekistan*

In Uzbekistan, the testing of Signal presented *signs of blocking on 8 AS networks*.

When attempting to perform a TLS handshake to all Signal backend services, the connection constantly times out after writing the first data packet to the network. This indicates that the blocking of Signal might be implemented through the use of Deep Packet Inspection (DPI) technology configured to selectively drop packets.

*# Circumvention*

Currently, *circumvention is enabled by default* for Signal users in *Iran, Egypt, Oman, Qatar & the UAE*. In our view, this should probably be enabled for Signal users in Uzbekistan, China, and Cuba too.

Signal users in other countries can:

* On iOS -> Enable the “Censorship Circumvention” setting * On Android -> Use Signal TLS proxies: https://support.signal.org/hc/en-us/articles/360056052052-Proxy-Support

You can help Android Signal users circumvent blocking (if they have a phone number which is not from Iran, Egypt, Oman, Qatar & the UAE) by:

1. Running a Signal TLS proxy: https://github.com/signalapp/Signal-TLS-Proxy 2. Sharing the mobile deep link: https://signal.tube/#<your_host_name>

*# Ongoing measurement*

You can also help monitor the blocking of Signal by running OONI Probe ( https://ooni.org/install/) and contributing measurements ( https://explorer.ooni.org/search?until=2021-10-23&since=2021-09-23&t...) as open data.

We thank OONI Probe users who contributed measurements, supporting this study.

We also thank Signal for their tireless efforts in bringing end-to-end encryption to the masses worldwide.

Please help share this study: https://twitter.com/OpenObservatory/status/1451482874264309763

Thanks for reading!

Best,

OONI team.