SimpleX Chat is a free messenger for text, voice messages, files and calls, with apps for Android, iPhone, Windows, Mac and Linux. It is built by SimpleX Chat Ltd, a company registered in London, which also develops the network the app runs on.

SimpleX Chat does not give users an identifier of any kind: no phone number, no username and not even a random ID. Each conversation runs through its own pair of random addresses on the servers, so the servers have no account to link your conversations to. Contacts and groups are stored on your device, not in a server database, and anyone can run their own server.

The price of this design is convenience. You cannot look people up, so every contact has to be added by an invitation link or QR code. One profile cannot be used on two phones, and the company counts about 480,000 people using the app every month.

Cryptography & Protocol

SimpleX runs its own messenger protocol, the SimpleX Messaging Protocol (SMP), with three layers of encryption: TLS between the app and the server, an end-to-end layer inside SMP, and the Double Ratchet algorithm on top. The Double Ratchet gives forward secrecy and post-compromise security, so a stolen key does not expose earlier messages, and the conversation recovers its secrecy after a break-in.

Since version 5.6, released in March 2024, SimpleX adds post-quantum key agreement (Streamlined NTRU Prime) to every ratchet step. It started as an option; SimpleX said it would become the default for all direct chats in the next version. In groups it is limited to small groups of about 10 to 20 members.

End-to-end encryption is on by default for all direct chats. Groups are end-to-end encrypted too, but SimpleX calls them secret groups: the app sends every message and file separately to each member, which is why SimpleX says they suit only smaller groups.

Channels, added in 2026, work differently. A channel’s content is stored on chat relays, and SimpleX states that relay operators can see it. The identities of channel owners and subscribers stay hidden from the operators, from each other and from the network. SimpleX also says that any channel anyone can join with a public link should be treated as completely public, whether or not it is encrypted.

Voice and video calls are end-to-end encrypted when both devices support it. Calls run through the system WebView or browser, and on older systems a call connects without end-to-end encryption. The app shows whether a call is encrypted.

Metadata Privacy

The sign-up identifier does not exist: SimpleX Chat creates no user account on the servers. Each contact uses separate one-way message queues with random addresses, and the servers keep no profiles or contacts. With no identifier, there is no contact discovery either: nobody can look you up.

Since version 6.0 the app sends messages through two servers by default, a form of onion routing that SimpleX calls private message routing. The destination server receives the message but cannot see the sender’s IP address. The first server on the route does see it. When both the SimpleX Chat servers and the servers run by Flux, a decentralised cloud of user-operated nodes, are enabled, the app picks servers of two different operators for each connection.

The servers cannot read message content or see message size, because every message is padded to 16 KB. They delete messages once delivered or after 21 days, and files on file servers after 48 hours. By default they keep no access logs.

This metadata protection has limits, and SimpleX’s threat model lists them. A server that records all the traffic passing through it can learn part of a user’s communications graph. The protocol does not protect against an attacker targeting particular people whose identities are known: anyone who can watch two known users’ local traffic can prove that they talk to each other. On iPhone, the notification server can see how many message queues have notifications turned on and roughly how many messages arrive in each.

The privacy policy says the apps include no tracking or analytics code, which means no telemetry.

SimpleX Chat Ltd falls under UK law. Its transparency report for 2025 counts 12 requests from law enforcement in different countries, and says no responsive information was identified or provided. The report has no warrant canary.

Transparency & Verifiability

The apps and the server software are open source under the AGPL-3.0 license. The server code, written in Haskell, includes the message server and the file server, so the network is self-hostable: anyone can run servers and use them in the app. SimpleX describes the network as a relay network that no single entity controls.

Reproducible builds are documented for the servers (since version 6.3), the Linux desktop app, the terminal client and the Android app, so you can rebuild these from the public code and compare the result. SimpleX’s instructions do not cover the iOS, macOS or Windows apps.

Trail of Bits has reviewed SimpleX twice. In October 2022 two engineers spent one week on the library that handles all cryptography and networking; the chat protocol, the mobile interfaces and the push notification server were outside the scope. They found 2 medium and 2 low severity issues, and 3 were fixed in version 4.2. In July 2024 Trail of Bits reviewed the design of the protocols: messaging, the agent protocol, push notifications, file transfer, remote control of the desktop app and the chat protocol. This was a design review, not a code audit of the apps. It found 3 medium and 1 low severity issues, all hard to exploit, plus 3 informational findings; 3 were improved in version 6.1 and the rest accepted.

The company says Trail of Bits audited SimpleX again in 2026 and that the report is still to be published. Its security policy, as of October 2026, still lists that assessment as scheduled for June 2026. The policy asks for vulnerabilities to be reported by email and promises to announce a fix 7 days after release and the details after 14 days. It mentions no bug bounty.

The 2026 vulnerability in the terminal client

In February 2026 a security researcher found a zero-click remote code execution flaw in the SimpleX Chat terminal client, the command-line app. With notifications turned on, a crafted text message could run commands on the recipient’s computer without any action from the user. The researcher’s disclosure says the flaw was not present in the mobile or desktop apps.

The researcher found the flaw on 25 February and reported it on 26 February. According to the researcher, the founder confirmed the report was valid, but after about a month without a patch the researcher escalated it to the Zero Day Initiative. Version 6.5 fixed the flaw on 30 April 2026, and the researcher published the disclosure on 10 May, advising users to upgrade or turn off terminal notifications.

NVD published the flaw as CVE-2026-52103 on 26 August 2026, affecting versions before 6.5. CISA gave it a CVSS score of 9.8, critical. SimpleX has published no advisory of its own.

This was a vulnerability, not a leak of user data, so it does not count toward SimpleX’s data breach history.

Usability & Reach

On Android, SimpleX Chat is available on Google Play, F-Droid and as an APK; on iPhone, from the App Store and TestFlight. Desktop apps cover Linux, macOS and Windows, and there is a terminal client for all three. There is no web client.

Adding a contact works through an invitation link or QR code, since there are no phone numbers or usernames to search.

Multi-device use is limited. One profile cannot be used on two phones. The desktop app can link to the phone and use its profile, but only when both devices are on the same network.

There is no cloud backup. The app can export the chat database as a file and import it on another device. The database passphrase is random by default, and you have to set your own before you can export it.

Disappearing messages work only if both people turn them on; in groups the owners decide. Deleting a sent message for everyone has the same rule: both sides must allow it, and it works within 24 hours.

The app has several privacy features. Incognito mode gives each new contact a random profile name. Hidden profiles are protected by a password, and you can keep several profiles in one app. The app connects through Tor via Orbot or a SOCKS proxy, and a beta setting uses a separate connection and Tor circuit for each contact.

By the company’s own figures, the app has been downloaded more than 3 million times, about 480,000 people use it every month, and the network has more than 1,000 community-run servers.

Funding & Sustainability

SimpleX Chat’s funding model is mixed, and the app is free. In 2024 the company raised a $1.3 million pre-seed round led by Jack Dorsey, with the Asymmetric Capital Partners fund, after earlier money from Village Global; SimpleX says neither investor got control or a board seat. Users, including Vitalik Buterin, have donated about $650,000.

Since September 2026 SimpleX sells optional supporter badges, paid by card, Bitcoin or Monero. A badge unlocks larger files that stay available longer, creates no account and does not renew by itself. SimpleX also says large channels and communities will pay for their servers in the future.

On 20 August 2026 the company launched an equity crowdfunding round on Wefunder, where anyone can buy a stake from $100.

SimpleX Chat Ltd, a profit-seeking company, builds the apps and the network. In April 2026 it announced a SimpleX Network Foundation and a consortium agreement between the foundation and the company to govern the protocols and their licensing. The announcement said it would launch within a few months; the foundation’s board had not been named at the time.

Pros and Cons

No identifier of any kind: no phone number, no username, no random ID
Direct chats and small secret groups are end-to-end encrypted by default, with forward secrecy and post-compromise security
Post-quantum key agreement on every ratchet step of the Double Ratchet
Private message routing keeps the sender's IP address from the destination server
Servers delete messages once delivered or after 21 days, and files after 48 hours
Both the apps and the servers are open source under the AGPL-3.0 license, and anyone can run their own server
Trail of Bits reviewed the cryptography library in 2022 and the protocol design in 2024
Incognito mode, hidden profiles, several profiles in one app and Tor support

Contacts can only be added by invitation link or QR code; nobody can find you by number or name
One profile cannot be used on two phones, and the desktop app links to the phone only on the same network
No cloud backup, only a database file you export yourself
Channel content is visible to the relay operators
On older systems calls connect without end-to-end encryption
Reproducible builds are not documented for iOS, macOS or Windows, and the 2026 assessment is unpublished
A critical flaw in the terminal client took about two months to fix, according to the researcher who reported it
A profit-seeking company holds the apps, and the planned foundation had not been formed

Our Rating

Cryptography & Protocol9.0/10
Direct chats and small secret groups are end-to-end encrypted by default with the Double Ratchet, which gives forward secrecy and post-compromise security, and SimpleX adds post-quantum key agreement on every ratchet step. Channel content is visible to the relay operators, and calls connect without end-to-end encryption on older systems.
Metadata Privacy9.0/10
Accounts have no identifiers of any kind, and private message routing keeps the sender's IP address from the destination server. The first server on the route still sees that IP address, and SimpleX's own documentation says a server can learn parts of who talks to whom.
Transparency & Verifiability8.5/10
Both the apps and the servers are open source under the AGPL-3.0 license, and Trail of Bits reviewed SimpleX in 2022 and 2024. Reproducible builds are not documented for iOS, macOS or Windows, and the 2026 assessment the company mentions has not been published.
Usability & Reach5.5/10
SimpleX Chat runs on Android, iPhone, Windows, Mac and Linux, and supports calls, disappearing messages and several profiles in one app. One profile cannot be used on two phones, the desktop app links to the phone only on the same network, and the only backup is a database file you export yourself.
Funding & Sustainability6.0/10
Venture money, user donations and optional supporter badges fund the company, and the app is free with no tracking or analytics. A profit-seeking company holds the apps, and the foundation meant to govern the protocols had not been formed in the sources.

Overall8.1/10

Cryptography & Protocol 30% · Metadata Privacy 25% · Transparency & Verifiability 20% · Usability & Reach 15% · Funding & Sustainability 10%

See our rating methodology →