# SimpleX blog: the latest news
SimpleX Chat - a private and encrypted messenger without any user IDs (not even random ones)! Make a private connection via link / QR code to send messages and make calls.

# Latest news

## SimpleX Chat v6.4.1: welcome your contacts, review members to protect groups, and more.

29 Jul 2025

v6.4.1 is released:

- welcome your contacts: set your profile bio and welcome message.
- protect your communities from spam and abuse: review new members ("knocking").
- set default time to delete messages for new contacts.
- improved app integrity: Linux app builds are now reproducible.

Also, we added 3 new interface languages to Android and desktop apps thanks to our users: Indonesian, Romanian and Vietnamese.

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## SimpleX network: new experience of connecting with people — available in SimpleX Chat v6.4-beta.4

03 Jul 2025

Now you can start talking to your contacts much faster, as soon as you scan the link. This technical post covers the technology that enabled this new user experience — short links and associated data of messaging queues.

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## SimpleX Chat v6.3: new user experience and safety in public groups

08 Mar 2025

v6.3 is released:

- Preventing spam and abuse in public groups.
- Group improvements: mention other members and improved performance.
- Better chat navigation: organize chats into lists and jump to found and forwarded messages.
- Privacy and security improvements: chat retention period and private media file names.

Also, we added Catalan interface language, thanks to our users and Weblate.

The last but not the least - server builds are now reproducible!

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## SimpleX network: large groups and privacy-preserving content moderation

14 Jan 2025

This post explains how server operators can moderate end-to-end encrypted conversations without compromising user privacy or end-to-end encryption.

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## Oppose digital IDs – they break the law and lead to mass scale surveillance

18 Dec 2024

Starting next year, the UK government plans to introduce digital ID cards for the young people to prove their age when visiting pubs.

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## SimpleX network: preset servers operated by Flux, business chats and more with v6.2 of the apps

10 Dec 2024

v6.2 is released:

- SimpleX Chat and Flux made an agreement to include Flux-operated servers into the app to improve metadata privacy in SimpleX network.
- Business chats for better privacy and support of your customers.
- Better user experience: open on the first unread, jump to quoted messages, see who reacted.
- Improving notifications in iOS app.

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## Servers operated by Flux - true privacy and decentralization for all users

25 Nov 2024

- Welcome, Flux — the new servers in v6.2-beta.1!
- What's the problem?
- Using two operators improves connection privacy.
- SimpleX decentralization compared with Matrix, Session and Tor.
- What's next for SimpleX decentralization?

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## Wired’s Attack on Privacy

16 Oct 2024

The Wired article by David Gilbert focusing on neo-Nazis moving to SimpleX Chat following the Telegram's changes in privacy policy is biased and misleading. By cherry-picking information from the report by the Institute for Strategic Dialogue (ISD), Wired fails to mention that SimpleX network design prioritizes privacy in order to protect human rights defenders, journalists, and everyday users who value their privacy.

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## SimpleX network: cryptographic design review by Trail of Bits, v6.1 released with better calls and user experience.

14 Oct 2024

New security audit!

Trail of Bits reviewed the cryptographic design of protocols used in SimpleX network and apps.

v6.1 is released:

- Better calls: switch audio and video during the call.
- Better iOS notifications: improved delivery, reduced traffic usage.
- Better user experience: switch chat profiles, customizable message shapes, forward up to 20 messages.

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## SimpleX network: the investment from Jack Dorsey and Asymmetric, v6.0 released with the new user experience and private message routing.

14 Aug 2024

SimpleX Chat vision and funding 2.0: past, present, future.

Announcing the investment from Jack Dorsey and Asymmetric.

v6.0 is released:

- Private message routing - enabled by default

- New chat experience:

  - connect to your friends faster.
  - new reachable interface.
  - and much more!
- Improved networking and battery usage

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## The Future of Privacy: Enforcing Privacy Standards

04 Jul 2024

It's time we shift the focus: privacy should be a non-negotiable duty of technology providers, not just a right users must fight to protect, and not something that users can be asked to consent away as a condition of access to a service.

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## SimpleX network: private message routing, v5.8 released with IP address protection and chat themes

04 Jun 2024

v5.8 is released:

- private message routing.
- server transparency.
- protect IP address when downloading files & media.
- chat themes* for better conversation privacy.
- group improvements - reduced traffic and additional preferences.
- improved networking, message and file delivery.

Also, we added Persian interface language*, thanks to our users and Weblate.

* Android and desktop apps only.

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## Protecting Children's Safety Requires End-to-End Encryption

01 Jun 2024

As lawmakers grapple with the serious issue of child exploitation online, some proposed solutions would fuel the very problem they aim to solve.

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## SimpleX: Redefining Privacy by Making Hard Choices

16 May 2024

When it comes to open source privacy tools, the status quo often dictates the limitations of existing protocols and structures. However, these norms need to be challenged to radically shift how we approach genuinely private communication. This requires doing some uncomfortable things, like making hard choices as it relates to funding, alternative decentralization models, doubling down on privacy over convenience, and more.

In this post we explain a bit more about why SimpleX operates and makes decisions the way it does:

- No user accounts.
- Privacy over convenience.
- Network decentralization.
- Funding and profitability.
- Company jurisdiction.

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## SimpleX network: legally binding transparency, v5.7 released with better calls and messages

26 Apr 2024

Legally binding transparency

- We published Transparency Reports, Security Policy, and Frequently Asked Questions, and updated Privacy Policy.
- We are committed to have full consistency between marketing promises and legally binding documents.

v5.7 is released:

- quantum resistant end-to-end encryption with all contacts.
- forward and save messages without revealing the source.
- in-call sounds and switching sound sources.
- better network connection management.
- customizable profile images.

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## The dangers of metadata in messengers

16 Apr 2024

By Esra'a al Shafei

It's important not to be complacent with the current standards of messaging, where metadata aggregation is still normalized in apps falsely and dangerously marketed as "private". This is a post exploring the fundamental differences between privacy and security.

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## Why I joined SimpleX Chat - by Esra'a al Shafei

04 Apr 2024

By Esra'a al Shafei

Transitioning from a lifelong career dedicated to nonprofits, including Board roles at organizations like the Wikimedia Foundation, Access Now and Tor, my decision to join SimpleX Chat may come as a surprise to some. But, as I step into this new chapter, I want to share the insights and convictions that have guided me here, shedding light on what I think sets SimpleX Chat apart and why this move feels like an essential learning opportunity.

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## SimpleX network: real privacy and stable profits, non-profits for protocols, v5.6 released with quantum resistant e2e encryption and simple profile migration

23 Mar 2024

SimpleX network: deliver real privacy via a profitable business and non-profit protocol governance:

- community and business interests are aligned.
- the journey to the decentralized non-profit protocol governance.
- welcome, Esra'a!

v5.6 is released:

- quantum resistant end-to-end encryption (BETA) - enable it for the new contacts.
- use the app during the audio and video calls.
- migrate all app data to another device via QR code.
- blocking members and many other improvements.

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## SimpleX Chat v5.6 (beta): adding quantum resistance to Signal double ratchet algorithm

14 Mar 2024

This is a major upgrade for SimpleX Chat messaging protocols!

This post also covers various aspects of end-to-end encryption:

- Why do we need end-to-end encryption?
- Why encryption is even allowed?
- End-to-end encryption security: attacks and defence.
- How secure is encryption in different messengers?
- When can you start using quantum resistant chats?
- Next for post-quantum crypto - all direct chats, small groups and security audit

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## SimpleX Chat: free infrastructure from Linode, v5.5 released with private notes, group history and a simpler UX to connect.

24 Jan 2024

SimpleX Chat infrastructure on Linode:

- free infrastructure.
- SimpleX servers in Linode Marketplace.
- high capacity messaging servers.

v5.5 is released:

- private notes.
- group history
- simpler UX to connect to other people
- message delivery, battery usage and other improvements

Also, the app interface is now available in Hungarian and Turkish - thanks to our users.

SimpleX Chat Android and desktop apps are now available in 20 languages!

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## SimpleX Chat v5.4 - link mobile and desktop apps via quantum resistant protocol, and much better groups.

25 Nov 2023

v5.4 is released:

- Link mobile and desktop apps via secure quantum-resistant protocol. 🔗

- Better groups:

  - faster to join and more reliable.
  - create groups with incognito profile.
  - block group members to reduce noise.
  - prohibit files and media in a group.
- Better calls: faster to connect, with screen sharing on desktop.
- Many other improvements.

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## SimpleX Chat v5.3 released: desktop app, local file encryption and improved groups with directory service

25 Sep 2023

v5.3 is released:

- new desktop app! 💻
- encrypt locally stored files & media
- directory service and other group improvements
- simplified incognito mode
- better app responsiveness, stability and 40% reduced memory usage.
- new privacy settings: show last messages & save draft.

Also, our users added 6 new interface languages - Arabic*, Bulgarian, Finnish, Hebrew*, Thai and Ukrainian.

* Android app only.

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## SimpleX Chat v5.2 released: message delivery receipts

22 Jul 2023

v5.2 is released:

- message delivery receipts! ✅
- filter favorite and unread chats
- better disappearing messages
- prohibit message reactions

- more usable groups:

  - view full original replied message in info.
  - share your address via your chat profile
  - search new and existing members

- stability improvements:

  - keep your connections working after restoring from backup.
  - restart app (Android) or reconnect servers (iOS)
  - more stable connections and message delivery

Platform evolution:

- problems of public groups and their future
- what about read receipts?

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## SimpleX Chat v5.1 released: message reactions and self-destruct passcode

23 May 2023

v5.1 is released:

- message reactions 🚀

- improved messages:

  - voice messages up to 5 minutes
  - custom time to disappear
  - message editing history
- new design and customizable themes you can share*!
- self-destruct passcode

Also, the app interface is now available in Japanese and Portuguese (Brazil)* - thanks to our users!

* Android app only.

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## SimpleX Chat: vision and funding, v5.0 released with videos and files up to 1gb

22 Apr 2023

SimpleX Chat vision and funding:

- why is it a commercial company?
- how is it funded and what is the business model?
- what is next?

v5.0 is released:

- send videos and files up to 1gb
- app passcode independent from system authentication
- networking improvements

Also, the app interface is now available in Polish - thanks to our users.

SimpleX Chat apps are now available in 10 languages!

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## SimpleX Chat v4.6 – with hidden profiles, community moderation, improved audio/video calls and reduced battery usage.

28 Mar 2023

v4.6 is released:

- ARMv7a and Android 8+ - 2x more supported devices!
- hidden chat profiles
- community moderation
- improved audio/video calls
- reduced battery usage
- SMP server monitoring: status bot and page

Also, the app interface is now available in Chinese and Spanish - thanks to our users!

Read More

## SimpleX File Transfer Protocol - a new protocol for sending large files efficiently, privately and securely.

01 Mar 2023

CLI and relays implementing the new XFTP protocol are released - you can use them now!

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## SimpleX Chat v4.5 released – with multiple chat profiles, message draft, transport isolation and Italian language!

04 Feb 2023

v4.5 is released:

- multiple chat profiles!
- message draft
- transport isolation
- reduced battery usage
- private filenames

Also, the app interface is now available in Italian - thanks to Weblate and our users!

Read More

## SimpleX Chat v4.4 released – with disappearing messages, live messages, connection security verification and French language!

03 Jan 2023

v4.4 is released:

- disappearing messages!
- live messages
- connection security verification
- support for GIFs and stickers

Also, the app interface is now available in French - thanks to Weblate and our users!

Read More

## SimpleX Chat reviews and v4.3 released – with instant voice messages, irreversible deletion of sent messages and improved server configuration.

06 Dec 2022

SimpleX Chat November reviews:

Privacy Guides recommendations.

Review by Mike Kuketz.

The messenger matrix.

Supernova review and messenger ratings.

v4.3 is released:

- instant voice messages!
- irreversible deletion of sent messages for all recipients
- improved SMP server configuration and support for server passwords
- privacy and security improvements: protect app screen, SimpleX links security, etc.

Read More

## Security assessment by Trail of Bits, the new website and v4.2 released

08 Nov 2022

"Have you been audited or should we just ignore you?"

SimpleX Chat security has been assessed by Trail of Bits, 4 issues were identified, and 3 of them are fixed in v4.2.

The new website is live: https://simplex.chat

v4.2 is released:

- group links — admins can create the links for new members to join
- auto-accept contact requests + configure whether to accept incognito and welcome message
- small things: change group member role, mark chat as unread, send stickers and GIFs from Android keyboards.
- manually switch contact or member to another address / server (BETA)
- receive files faster (BETA)

Read More

## SimpleX Chat v4.0 with encrypted database is released

28 Sep 2022

Added:

- encrypted local chat database — if you already use the app, you can encrypt the database in the app settings
- support for self-hosted WebRTC ICE servers
- improved stability of creating new connections: more reliable groups, files and contacts
- deleting files and media — for security and to save storage
- For developers — TypeScript SDK for integrating with SimpleX Chat
- support animated images in Android app
- German language in mobile apps UI

Read More

## SimpleX Chat v3.2 is released — meet Incognito mode, unique to Simplex Chat

01 Sep 2022

Added:

- Incognito mode — use a new random profile name for each contact
- use .onion server addresses with Tor
- endless scrolling and search
- choose accent color and dark mode
- reduced APK size for direct download and in F-Droid repo from 200 to 46Mb!

Read More

## SimpleX Chat v3.1 is released — with secret groups and server access via Tor

08 Aug 2022

Added:

- finally, secret chat groups — nobody but members know they exist!
- access to messaging servers via Tor on all platforms
- advanced network settings to optimize traffic usage
- published chat protocol
- new app icons

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## SimpleX Chat v3.1-beta is released — improved battery/traffic usage

23 Jul 2022

Added:

- terminal app: access to messaging servers via SOCKS5 proxy (e.g., Tor).
- mobile apps: join and leave chat groups.
- optimized battery and traffic usage — up to 90x reduction!
- two docker configurations for self-hosted SMP servers.

Read More

## SimpleX announces SimpleX Chat v3 — with encrypted calls and iOS push notifications

11 Jul 2022

Added:

- chat database export and import
- end-to-end encrypted audio/video calls
- protocol privacy and performance improvements

Also read why having users' identifiers that all other platforms use is bad for the users, and how they reduce privacy.

Read More

## SimpleX Chat v2.2 - the new privacy and security features

04 Jun 2022

Added:

- protect your chats with SimpleX Lock
- save data and avoid sharing you are online — manually accepting images
- avoid visiting websites of the links you send — disable link previews
- identify any lost messages in the chat

Read More

## SimpleX Chat v2.1 - better conversation privacy

24 May 2022

Clear conversations without deleting contacts

Read More

## SimpleX Chat v2.0 - sending images and files in mobile apps

11 May 2022

Read how SimpleX delivers messages without having user profile identifiers of any kind.

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## Instant notifications for SimpleX Chat mobile apps

04 Apr 2022

Design of private instant notifications on Android and for push notifications for iOS.

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## SimpleX announces SimpleX Chat mobile apps for iOS and Android

08 Mar 2022

Brand new mobile apps with battle-tested Haskell core.

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## SimpleX announces SimpleX Chat public beta for iOS

14 Feb 2022

Our first prototype of mobile UI for iOS is available!

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## SimpleX announces SimpleX Chat v1

12 Jan 2022

Major protocol changes address all design mistakes identified during concept review by an independent expert.

Read More

## SimpleX announces SimpleX Chat v0.5

08 Dec 2021

Support for long-term user addresses in terminal app.

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## SimpleX announces SimpleX Chat v0.4

14 Sep 2021

Terminal app now supports groups and file transfers.

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## Announcing SimpleX Chat Prototype!

12 May 2021

Prototype chat app for the terminal (console).

Read More

## Simplex Chat

22 Oct 2020

The prototype of SimpleX Messaging Server implementing SMP protocol.

Read More

# Address portability

Similarly to phone number portability (the ability of the customer to transfer the service to another provider without changing the number), the address portability means the ability of a communication service customer to change the service provider without changing the service address. Many federated networks support SRV records to provide address portability, but allowing service users to set up their own domains for the addresses is not as commonly supported by the available server and client software as for email.

# Federated network

Federated network is provided by several entities that agree upon the standards and operate the network collectively. This allows the users to choose their provider, that will hold their account, their messaging history and contacts, and communicate with other providers' servers on behalf of the user. The examples are email, XMPP, Matrix and Mastodon.

The advantage of that design is that there is no single organization that all users depend on, and the standards are more difficult to change, unless it benefits all users. There are several disadvantages: 1) the innovation is slower, 2) each user account still depends on a single organization, and in most cases can't move to another provider without changing their network address – there is no address portability, 3) the security and privacy are inevitably worse than with the centralized networks.

Federation on Wikipedia

# Anonymous credentials

The credential that allows proving something, e.g. the right to access some resource, without identifying the user. This credential can either be generated by a trusted party or by the user themselves and provided together with the request to create the resource. The first approach creates some centralized dependency in most cases. The second approach does not require any trust - this is used in SimpleX network to authorize access to the messaging queues.

Digital credential on Wikipedia

# Blockchain

In a wide sense, blockchain means a sequence of blocks of data, where each block contains a cryptographic hash of the previous block, thus providing integrity to the whole chain. Blockchains are used in many communication and information storage systems to provide integrity and immutability of the data. For example, BluRay disks use blockchain. SimpleX messaging queues also use blockchain - each message includes the hash of the previous message, to ensure the integrity – if any message is modified it will be detected by the recipient when the next message is received. Blockchains are a subset of Merkle directed acyclic graphs.

In a more narrow sense, particularly in media, blockchain is used to refer specifically to distributed ledger, where each record also includes the hash of the previous record, but the blocks have to be agreed by the participating peers using some consensus protocol.

Wikipedia

# Merkle directed acyclic graph

Also known as Merkle DAG, a data structure based on a general graph structure where node contains the cryptographic hashes of the previous nodes that point to it. Merkle trees are a subset of Merkle DAGs - in this case each leaf contains a cryptographic hash of the parent.

This structure by design allows to verify the integrity of the whole structure by computing its hashes and comparing with the hashes included in the nodes, in the same way as with blockchain.

The motivation to use DAG in distributed environments instead of a simpler linear blockchain is to allow concurrent additions, when there is no requirement for a single order of added items. Merkle DAG is used, for example, in IPFS and will be used in decentralized SimpleX groups.

Wikipedia.

# Break-in recovery

The quality of the end-to-end encryption scheme allowing to recover security against a passive attacker who observes encrypted messages after compromising one (or both) of the parties. Also known as recovery from compromise or break-in recovery. Double-ratchet algorithm has this quality.

# Double ratchet algorithm

It is used by two parties to exchange end-to-end encrypted messages. The parties will use some key agreement protocol to agree on the initial shared secret key.

Double Ratchet algorithm provides perfect forward secrecy and post-compromise security. It is designed by Signal, and used in SimpleX Chat and many other secure messengers. Most experts consider it the state-of-the-art encryption protocol in message encryption.

# Centralized network

Centralized networks are provided or controlled by a single entity. The examples are Threema, Signal, WhatsApp and Telegram. The advantage of that design is that the provider can innovate faster, and has a centralized approach to security. But the disadvantage is that the provider can change or discontinue the service, and leak, sell or disclose in some other way all users' data, including who they are connected with.

# Content padding

Also known as content padding, it is the process of adding data to the beginning or the end of a message prior to encryption. Padding conceals the actual message size from any eavesdroppers. SimpleX has several encryption layers, and prior to each encryption the content is padded to a fixed size.

Wikipedia.

# Decentralized network

Decentralized network is often used to mean "the network based on decentralized blockchain". In its original meaning, decentralized network means that there is no central authority or any other point of centralization in the network, other than network protocols specification. The advantage of decentralized networks is that they are resilient to censorship and to the provider going out of business. The disadvantage is that they are often slower to innovate, and the security may be worse than with the centralized network.

The examples of decentralized networks are email, web, DNS, XMPP, Matrix, BitTorrent, etc. All these examples have a shared global application-level address space. Cryptocurrency blockchains not only have a shared address space, but also a shared state, so they are more centralized than email. Tor network also has a shared global address space, but also a central authority. SimpleX network does not have a shared application-level address space (it relies on the shared transport-level addresses - SMP relay hostnames or IP addresses), and it does not have any central authority or any shared state.

# Defense in depth

Originally, it is a military strategy that seeks to delay rather than prevent the advance of an attacker, buying time and causing additional casualties by yielding space.

In information security, defense in depth represents the use of multiple computer security techniques to help mitigate the risk of one component of the defense being compromised or circumvented. An example could be anti-virus software installed on individual workstations when there is already virus protection on the firewalls and servers within the same environment.

SimpleX network applies defense in depth approach to security by having multiple layers for the communication security and privacy:

- double ratchet algorithm for end-to-end encryption with perfect forward secrecy and post-compromise security,
- additional layer of end-to-end encryption for each messaging queue and another encryption layer of encryption from the server to the recipient inside TLS to prevent correlation by ciphertext,
- TLS with only strong ciphers allowed,
- mitigation of man-in-the-middle attack on client-server connection via server offline certificate verification,
- mitigation of replay attacks via signing over transport channel binding,
- multiple layers of message padding to reduce efficiency of traffic analysis,
- mitigation of man-in-the-middle attack on client-client out-of-band channel when sending the invitation,
- rotation of delivery queues to reduce efficiency of traffic analysis,
- etc.

Wikipedia

# End-to-end encryption

A communication system where only the communicating parties can read the messages. It is designed to protect message content from any potential eavesdroppers – telecom and Internet providers, malicious actors, and also the provider of the communication service.

End-to-end encryption requires agreeing cryptographic keys between the sender and the recipient in a way that no eavesdroppers can access the agreed keys. See key agreement protocol. This key exchange can be compromised via man-in-the-middle attack, particularly if key exchange happens via the same communication provider and no out-of-band channel is used to verify key exchange.

Wikipedia

# Forward secrecy

Also known as perfect forward secrecy, it is a feature of a key agreement protocol that ensures that session keys will not be compromised even if long-term secrets used in the session key exchange are compromised. Forward secrecy protects past sessions against future compromises of session or long-term keys.

Wikipedia

# Post-compromise security

The quality of the end-to-end encryption scheme allowing to recover security against a passive attacker who observes encrypted messages after compromising one (or both) of the parties. Also known as recovery from compromise or break-in recovery. Double-ratchet algorithm has this quality.

# Man-in-the-middle attack

The attack when the attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communicating with each other.

This attack can be used to compromise end-to-end encryption by intercepting public keys during key exchange, substituting them with the attacker's keys, and then intercepting and re-encrypting all messages, without altering their content. With this attack, while the attacker does not change message content, but she can read the messages, while the communicating parties believe the messages are end-to-end encrypted.

Such attack is possible with any system that uses the same channel for key exchange as used to send messages - it includes almost all communication systems except SimpleX, where the initial public key is always passed out-of-band. Even with SimpleX, the attacker may intercept and substitute the key sent via another channel, gaining access to communication. This risk is substantially lower, as attacker does not know in advance which channel will be used to pass the key.

To mitigate such attack the communicating parties must verify the integrity of key exchange - SimpleX and many other messaging apps, e.g. Signal and WhatsApp, have the feature that allows it.

Wikipedia.

# Message padding

Also known as content padding, it is the process of adding data to the beginning or the end of a message prior to encryption. Padding conceals the actual message size from any eavesdroppers. SimpleX has several encryption layers, and prior to each encryption the content is padded to a fixed size.

Wikipedia.

#### Double ratchet algorithm

It is used by two parties to exchange end-to-end encrypted messages. The parties will use some key agreement protocol to agree on the initial shared secret key.

# Key agreement protocol

Also known as key exchange, it is a process of agreeing cryptographic keys between the sender and the recipient(s) of the message. It is required for end-to-end encryption to work. Unless it is possible to secure the key exchange via some second factor, e.g. security code verification, it can be vulnerable to man-in-the-middle attack.

Wikipedia

#### End-to-end encryption

A communication system where only the communicating parties can read the messages. It is designed to protect message content from any potential eavesdroppers – telecom and Internet providers, malicious actors, and also the provider of the communication service.

# Key exchange

Also known as key exchange, it is a process of agreeing cryptographic keys between the sender and the recipient(s) of the message. It is required for end-to-end encryption to work. Unless it is possible to secure the key exchange via some second factor, e.g. security code verification, it can be vulnerable to man-in-the-middle attack.

Wikipedia

# MITM attack

The attack when the attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communicating with each other.

This attack can be used to compromise end-to-end encryption by intercepting public keys during key exchange, substituting them with the attacker's keys, and then intercepting and re-encrypting all messages, without altering their content. With this attack, while the attacker does not change message content, but she can read the messages, while the communicating parties believe the messages are end-to-end encrypted.

Such attack is possible with any system that uses the same channel for key exchange as used to send messages - it includes almost all communication systems except SimpleX, where the initial public key is always passed out-of-band. Even with SimpleX, the attacker may intercept and substitute the key sent via another channel, gaining access to communication. This risk is substantially lower, as attacker does not know in advance which channel will be used to pass the key.

To mitigate such attack the communicating parties must verify the integrity of key exchange - SimpleX and many other messaging apps, e.g. Signal and WhatsApp, have the feature that allows it.

Wikipedia.

# Non-repudiation

Non-repudiation

# Onion routing

A technique for anonymous communication over a computer network that uses multiple layers of message encryption, analogous to the layers of an onion. The encrypted data is transmitted through a series of network nodes called "onion routers," each of which "peels" away a single layer, revealing the data's next destination. The sender remains anonymous because each intermediary knows only the location of the immediately preceding and following nodes.

The most widely used onion network is Tor.

Some elements of SimpleX network use similar ideas in their design - different addresses for the same resource used by different parties, and additional encryption layers. Currently though, SimpleX messaging protocol does not protect sender network address, as the relay server is chosen by the recipient. The delivery relays chosen by sender that are planned for the future would make SimpleX design closer to onion routing.

Wikipedia

# Overlay network

Nodes in the overlay network can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. Tor, for example, is an overlay network on top of IP network, which in its turn is also an overlay network over some underlying physical network.

SimpleX Clients also form a network using SMP relays and IP or some other overlay network (e.g., Tor), to communicate with each other. SMP relays, on another hand, do not form a network.

Wikipedia

# Non-repudiation

The property of the cryptographic or communication system that allows the recipient of the message to prove to any third party that the sender identified by some cryptographic key sent the message. It is the opposite to repudiation. While in some context non-repudiation may be desirable (e.g., for contractually binding messages), in the context of private communications it may be undesirable.

Wikipedia

# Repudiation

The property of the cryptographic or communication system that allows the sender of the message to plausibly deny having sent the message, because while the recipient can verify that the message was sent by the sender, they cannot prove it to any third party - the recipient has a technical ability to forge the same encrypted message. This is an important quality of private communications, as it allows to have the conversation that can later be denied, similarly to having a private face-to-face conversation.

See also non-repudiation.

# Pairwise pseudonymous identifier

Generalizing the definition from NIST Digital Identity Guidelines, it is an opaque unguessable identifier generated by a service used to access a resource by only one party.

In the context of SimpleX network, these are the identifiers generated by SMP relays to access anonymous messaging queues, with a separate identifier (and access credential) for each accessing party: recipient, sender and and optional notifications subscriber. The same approach is used by XFTP relays to access file chunks, with separate identifiers (and access credentials) for sender and each recipient.

# Peer-to-peer

Peer-to-peer (P2P) is the network architecture when participants have equal rights and communicate directly via a general purpose transport or overlay network. Unlike client-server architecture, all peers in a P2P network both provide and consume the resources. In the context of messaging, P2P architecture usually means that the messages are sent between peers, without user accounts or messages being stored on any servers. Examples are Tox, Briar, Cwtch and many others.

The advantage is that the participants do not depend on any servers. There are multiple downsides to that architecture, such as no asynchronous message delivery, the need for network-wide peer addresses, possibility of network-wide attacks, that are usually mitigated only by using a centralized authority. These disadvantages are avoided with proxied P2P architecture.

Wikipedia.

# Proxied peer-to-peer

Network topology of the communication system when peers communicate via proxies that do not form the network themselves. Such design is used in Pond, that has a fixed home server for each user, and in SimpleX, that uses multiple relays providing temporary connections.

# Perfect forward secrecy

Also known as perfect forward secrecy, it is a feature of a key agreement protocol that ensures that session keys will not be compromised even if long-term secrets used in the session key exchange are compromised. Forward secrecy protects past sessions against future compromises of session or long-term keys.

Wikipedia

# Post-quantum cryptography

Any of the proposed cryptographic systems or algorithms that are thought to be secure against an attack by a quantum computer. It appears that as of 2025 there is no system or algorithm that is proven to be secure against such attacks, or even to be secure against attacks by massively parallel conventional computers, so a general recommendation is to use post-quantum hybrid cryptography - combining post-quantum and traditional algorigthms.

Wikipedia

# Post-quantum hybrid crypto

Any of the proposed cryptographic systems or algorithms that are thought to be secure against an attack by a quantum computer. It appears that as of 2025 there is no system or algorithm that is proven to be secure against such attacks, or even to be secure against attacks by massively parallel conventional computers, so a general recommendation is to use post-quantum hybrid cryptography - combining post-quantum and traditional algorigthms.

Wikipedia

# Recovery from compromise

The quality of the end-to-end encryption scheme allowing to recover security against a passive attacker who observes encrypted messages after compromising one (or both) of the parties. Also known as recovery from compromise or break-in recovery. Double-ratchet algorithm has this quality.

# User identity

In a communication system it refers to anything that uniquely identifies the users to the network. Depending on the communication network, it can be a phone number, email address, username, public key or a random opaque identifier. Most messaging networks rely on some form of user identity. SimpleX appears to be the only messaging network that does not rely on any kind of user identity - see this comparison.