The best alternatives to Secure Send are FusionAuth, GrapheneOS, and blockslack.io. If these 3 options don't work for you, we've listed a few more alternatives below.
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Universal-2 by AssemblyAI— Speech-to-Text API for conversational data
FusionAuth is the customer authentication and authorization platform that make developers' lives awesome. You'll get all the features your app needs like user registration, SSO, MFA, and more, plus a customizable, scalable solution you can run on any computer, anywhere in the world.
GrapheneOS is an Android-based, security-hardened, privacy focused, free and open-source, mobile operating system for selected smartphones. It was known as Android Hardening until March 2019, and is focused on privacy and security. It is compatible with several Google Pixel smartphones.
For permissionless adhoc chat between small groups of people. Powered by Blockstack; all communication is encrypted and the service is fully decentralized.