&gt; I tend to repeat the &#x27;central server&#x27; and &#x27;a phone number is not an address and not public information, it certainly is no identity&#x27; criticism.<p>Your phone number is <i>not</i> your identity in Signal. If you change SIM cards then your friends won&#x27;t notice and the app works as before.<p>Signal is based on asym encryption - your private key effectively encodes your identity in combination with your public key.<p>&gt; And only if the first client is Android?<p>There is no point in dwelling on that. Android and its syblings are the only <i>potentially</i> secure popular smartphone OSs. Apple and Microsoft OSs are fundamentally flawed in that regard.