The last contribution to #Signal's server software was on April 22, 2020. Due to the sudden commit drop to literally 0 it is no longer possible to seriously call current Signal versions fully open-source.
That has no effect on the messenger's end-to-end encryption or its overall security but it does compromise the trust in the Signal Foundation, especially when considering that they did not give any reason for not disclosing the source code.
The German tech magazine Golem shares my opinion:
"Signal-Server nicht mehr Open Source"
("Signal's server not open-source anymore")
@datenschutzratgeber they used commits but they didn‘t pushed them. That’s why it looks like that. Seems not very uncommon to me. They did not push, because they developed a new payment feature. Doesn’t seem weird to me. I would have done the same, to not give governments the chance to cancel the project before it’s ready.
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