Recs.
Updated
SpecsUpdate
Pros
Pro Truly open and secure, comes from the Open Whisper Systems Team
Backed by people nut about privacy, including
- Edward Snowden, Whistleblower and privacy advocate
- Laura Poitras, Oscar winning filmmaker and journalist
- Bruce Schneier, internationally renowned security technologist
- Matt Green, Cryptographer, Johns Hopkins University
Pro Free as in both free beer and freedom, not as in "The customer is the product"
Pay nothing - The development team is supported by community donations and grants. There are no advertisements, and it doesn't cost anything to use.
Pro Free and open source
Signal is free and open source software, enabling anyone to verify its security by auditing the code. It's the only private messenger that uses open source, peer-reviewed cryptographic protocols to keep your messages safe.
Cons
Con Centralized architecture
Signal's server architecture has been partially decentralized since December 2013, when it was announced that the messaging protocol that is used in Signal had successfully been integrated into the Android-based open-source operating system CyanogenMod. As of CyanogenMod 11.0, the client logic is contained in a system app called WhisperPush. According to Open Whisper Systems, "the Cyanogen team runs their own Signal messaging server for WhisperPush clients, which federates with Open Whisper Systems' Signal server, so that both clients can exchange messages with each-other seamlessly".
The WhisperPush source code is available under the GPLv3 license. In January 2016, however, the CyanogenMod team announced that they will be discontinuing WhisperPush on February 1, and recommended that its users switch to Signal. After this, Signal's server architecture will be entirely centralized.
Con Single device
Signal can only be registered to one mobile device at a time. But you can link Signal to Signal Desktop.
Con Account and messages are lost when you lose your phone
There is no transfer of your account and messages when you lose access to your phone, for example when it gets stolen or malfunctions. Other platforms have some way of backing up your account and messages, but Signal doesn't have that.
Con Cannot work w\o Google services
Signal relies on Google services for push notifications so although it is encrypted and open source it needs proprietary component to work. That means you can't use Signal on some custom Android ROM (preferably rooted for full control) without Google apps installed.
Con Code being used in Whatsapp and other parties undermines trust
It's unlikely FB would add encryption it does not have access to.
Con If SMS/MMS imported, other apps can't read SMS
For those using Join, Pushbullet, or similar apps to send and receive SMS/MMS from web or desktop, putting your SMS in Signal somehow breaks this functionally. The dev is aware but plans to remedy the situation have not been named a priority.