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Signal vs Zalo: catalog facts
Signal	Zalo
Free Free; developed by a nonprofit funded by grants and donations	Free Free to download and use for messaging and calls.
Android, iOS, Windows, macOS, Linux	Windows, macOS, Android, iOS
End-to-end encryption on every message and call using the open source Signal Protocol No ads, affiliate marketers or tracking	Widely used in Vietnam, so most contacts there already have it Messaging and calling in one app
Requires a phone number that can receive an SMS or phone call to register Desktop app requires Signal to be installed on a phone first	Official site is mostly in Vietnamese Closed source, run on the developer's servers
Downloadable app	Downloadable app, Hosted service
Open source	Closed source
License: AGPL-3.0	License not stated
Account needed	Not stated if an account is needed
Needs internet	Not stated if it works offline
Official website Full listing and sources Checked September 20, 2026	Official website Full listing and sources Checked September 24, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.