# SimpleX Chat: The World's Most Secure Messaging
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.

# The World's Most Secure Messaging

Ultimate security: SimpleX network uses the most secure end-to-end encryption, with continuous post-quantum key exchange to protect all messages and metadata.

Unique privacy: SimpleX network has no user profile IDs, not even random numbers or keys. It provides better privacy of your contacts, protecting who you talk with from network servers.

No spam: nobody can contact you unless you share 1-time link or long-term address.

Data ownership: only your device stores your profiles, contacts and messages. You can securely move your data to another device. Servers store encrypted messages only while your device is offline.

Secure decentralization: you control which servers to connect to. For security 4 different servers are used in each chat — they can't observe which IP addresses talk to each other.

#### How to connect to others

- Tap new chat button in the corner, then create 1-time link.
- Share the link with your contact via any other messenger or email - it is secure.
- Ask your contact to use the link in the app - click it after the app is installed or paste into the search field in the app.

## Por qué SimpleX es único

# #1

### La privacidad es completa

SimpleX protege la privacidad de tu perfil, contactos y metadatos, ocultándolos de los servidores SimpleX y de cualquier observador.

A diferencia de cualquier otra red de mensajería existente, SimpleX no tiene identificadores asignados a los usuarios, ni siquiera números aleatorios.

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# #2

### Estás protegido
contra el spam y el abuso

Al no tener identificadores o dirección permanente en la red SimpleX, nadie puede ponerse en contacto contigo salvo que compartas una dirección de usuario de un solo uso o una dirección temporal en forma de enlace o código QR.

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# #3

### Tu gestionas tus datos

SimpleX almacena todos los datos de usuario únicamente en los dispositivos cliente usando un formato cifrado y portable de la base de datos, la cual puede ser transferida a otro dispositivo.

Los mensajes cifrados de extremo a extremo (E2E) se mantienen temporalmente en los servidores SimpleX hasta que se entregan al destinatario, y después se borran definitivamente.

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# #4

### La red SimpleX te pertenece

La red SimpleX está completamente descentralizada y es independiente de cualquier criptodivisa o de cualquier red salvo Internet.

Puedes usar SimpleX con tus propios servidores o con los servidores proporcionados por nosotros para contactar con cualquier usuario.

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# Privacidad total de tu identidad, perfil, contactos y metadatos

A diferencia de otras redes de mensajería, SimpleX no tiene identificadores asignados a los usuarios. No depende de números de teléfono, direcciones basadas en dominios (como es el caso del email o XMPP), nombres de usuario, claves públicas o incluso números aleatorios para identificar a sus usuarios. Los operadores de servidores SimpleX desconocen cuántas personas usan sus servidores.

Para la entrega de mensajes, SimpleX usa direcciones anónimas por pares de colas de mensajes unidireccionales separadas para mensajes salientes y entrantes a través de servidores diferentes.

Este diseño protege la privacidad de la persona que se comunica contigo ocultándole a los servidores de la red SimpleX y de cualquier observador. Para ocultar tu dirección IP a los servidores puedes conectarte a los servidores SimpleX mediante Tor.

# La mejor protección contra el spam y el abuso

Al no tener identificadores en la red SimpleX, nadie puede ponerse en contacto contigo salvo que compartas una dirección de usuario de un solo uso o una dirección temporal en forma de enlace o código QR.

Incluso con la dirección de usuario opcional, aunque pueda usarse para enviar solicitudes de contacto spam, puedes cambiarla o eliminarla por completo sin perder ninguna de tus conexiones.

# Titularidad, control y seguridad de tus datos

SimpleX Chat almacena todos los datos de usuario únicamente en los dispositivos cliente usando un formato cifrado y portable de la base de datos, la cual puede ser exportada y transferida a cualquier dispositivo compatible.

Los mensajes cifrados de extremo a extremo (E2E) se mantienen temporalmente en los servidores SimpleX hasta que se entregan al destinatario, y después se borran definitivamente.

A diferencia de los servidores de redes federadas (correo electrónico, XMPP o Matrix), los servidores SimpleX no almacenan cuentas de usuario, sólo retransmiten mensajes, protegiendo así la privacidad de ambas partes.

No hay identificadores ni texto cifrado en común entre el tráfico de servidor enviado y el recibido. Si alguien te está monitorizando, difícilmente podría determinar quién se comunica con quién, incluso si el protocolo TLS se ve comprometido.

# Totalmente descentralizado, los usuarios son dueños de la red SimpleX

Puedes usar SimpleX con tus propios servidores y aún así comunicarte con personas que usen los servidores preconfigurados de la aplicación.

La red SimpleX usa un protocolo abierto y proporciona el SDK para crear chatbots, permitiendo implementar servicios con los que los usuarios interactúen con las aplicaciones SimpleX Chat — esperamos con interés ver los servicios SimpleX que creará usted.

Si estás considerando desarrollar para la red SimpleX, por ejemplo un chatbot para los usuarios de SimpleX o la integración de las librerías SimpleX Chat en tus aplicaciones móviles, por favor ponte en contacto para consejos y soporte.

## Características

Mensajes cifrados E2E
con sintáxis markdown y edición

Cifrado E2E
de imágenes, vídeos y archivos

Grupos descentralizados cifrados E2E, sólo los usuarios saben de su existencia

Mensajes de voz cifrados E2E

Mensajes temporales

Llamadas y videollamadas
con cifrado E2E

Almacenamiento portable y cifrado, podrás transferir tu perfil a otro dispositivo

Modo Incógnito
exclusivo de SimpleX Chat

## Qué hace que SimpleX sea privado

### Identificadores por pares temporales y anónimos

SimpleX usa direcciones y credenciales temporales y anónimas por pares para cada contacto o miembro del grupo.

Esto permite entregar mensajes sin identificadores del perfil de usuario y proporcionar mayor privacidad de metadatos que las alternativas.

Pulsa para cerrar

### Intercambio de claves
fuera de banda

Muchas redes de comunicaciónes son vulnerables a ataques MITM por parte de los servidores o los proveedores de red.

Para evitarlo las aplicaciones SimpleX pasan claves de un solo uso fuera de banda cuando compartes un enlace de dirección o un código QR.

Pulsa para cerrar

### Doble capa de cifrado
de extremo a extremo

Protocolo de 'Double Ratchet' —
Mensajería OTR con secreto perfecto hacia adelante y recuperación de intrusión.

Y si el protocolo TLS se ve comprometido, NaCL cryptobox en cada cola de mensajes para prevenir la correlación de tráfico entre las colas.

Pulsa para cerrar

### Verificación de la
integridad del mensaje

Para garantizar la integridad los mensajes se numeran secuencialmente e incluyen el hash del mensaje anterior.

Si se añade, elimina o modifica algún mensaje, el destinatario es avisado.

Pulsa para cerrar

### Capa de cifrado
adicional en el servidor

Capa de cifrado adicional desde el servidor al destinatario, para prevenir la correlación entre el tráficos de entrada y salida del servidor si el protocolo TLS se ve comprometido.

Pulsa para cerrar

### Mezcla de mensajes
para prevenir la correlación

Los servidores SimpleX actúan como nodos de mezcla de baja latencia, los mensajes entrantes y salientes siguen un orden diferente.

Pulsa para cerrar

### Transporte TLS
seguro y autenticado

Para las conexiones cliente servidor se usan exclusivamente el protocolo TLS 1.2/1.3 con algoritmos robustos.

La huella digital del servidor y la vinculación de canales evitan los ataques de respuesta y MITM.

La reanudación de la conexión está deshabilitada para evitar ataques de sesión.

Pulsa para cerrar

### Acceso opcional
a través de Tor

Para proteger tu dirección IP, puedes acceder a los servidores a través Tor u otra red superpuesta de transporte.

Para usar SimpleX a través de Tor, instala la aplicación Orbot y activa el proxy SOCKS5 (o VPN en iOS).

Pulsa para cerrar

### Colas de mensajes
unidireccionales

Cada cola de mensajes transmite los mensajes en un sol sentido. entre las direcciónes de envío y recepción.

Esto reduce los vectores de ataque y los metadatos disponibles en comparación con los brokers de mensajes tradicionales.

Pulsa para cerrar

### Múltiples capas
de contenido acolchado

SimpleX utiliza relleno de contenido en cada capa de cifrado para frustrar los ataques al tamaño de los mensajes.

Esto hace que mensajes de distintos tamaños parezcan iguales desde el punto de vista de servidores y observadores de red.

Pulsa para cerrar

## Red SimpleX

SimpleX Chat proporciona la mejor privacidad al combinar las ventajas de P2P y las redes federadas.

### Distinta de las redes P2P

Todos los mensajes se envían a través de los servidores, proporcionando mejor privacidad para los metadatos y entrega asíncrona fiable, evitando al mismo tiempo muchos de los problemas asociados a las redes P2P.

# Comparativa con protocolos de mensajería P2P

Los protocolos y aplicaciones de mensajería P2P presentan varios problemas que los hacen menos fiables que SimpleX, más complejos de analizar y más vulnerables a ciertos tipos de ataques.

1. Para enrutar mensajes, las redes P2P se basan en alguna variante de DHT. Los diseños DHT tienen que equilibrar la garantía de entrega y latencia. SimpleX ofrece mayor garantía de entrega y menor latencia que P2P ya que el mensaje puede transmitirse en paralelo y de forma redundante a través de varios servidores elegidos por el destinatario. En las redes P2P el mensaje se transmite secuencialmente por O(log N) nodos, usando nodos elegidos por el algoritmo.
2. Por diseño SimpleX, a diferencia de la mayoría de las redes P2P, no tiene identificadores globales de usuario de ningún tipo, ni siquiera temporales, y sólo usa identificadores temporales por pares, lo que proporciona un mejor anonimato y protección de los metadatos.
3. P2P no resuelve el problema del ataque MITM , y la mayoría de implementaciones existentes no usan mensajes fuera de banda para el intercambio de claves inicial. SimpleX usa mensajes fuera de banda, o en algunos casos conexiones seguras y de confianza preexistentes para el intercambio inicial de claves.
4. Algunos proveedores de Internet pueden bloquear las aplicaciones P2P (como BitTorrent). SimpleX es independiente del transporte. Puede trabajar con protocolos web estándar, por ejemplo WebSockets.
5. Todas las redes P2P conocidas pueden ser vulnerables al ataque Sybil porque cada nodo es susceptible de ser descubierto y la red funciona como una unidad. Las medidas de mitigación conocidas requieren un componente centralizado o bien costosas pruebas de trabajo. La red SimpleX no tiene la capacidad para descubrir los servidores, está fragmentada y funciona como múltiples subredes aisladas haciendo imposibles los ataques a toda la red.
6. Las redes P2P pueden ser vulnerables al ataque DRDoS, en el que los clientes retransmiten y amplifican el tráfico provocando el bloqueo del servicio en la red. Los clientes SimpleX sólo transmiten el tráfico desde una conexión conocida y no pueden ser usados por un atacante para amplificar el tráfico en toda la red.

### Distinta de las redes federadas

Los servidores de SimpleX NO almacenan perfiles de usuario, contactos, o mensajes entregados. NO contactan entre sí y NO existe un directorio de servidores.

### Red Simplex

los servidores proporcionan colas unidireccionales para conectar a los usuarios pero no ven la gráfica de conexiónes red, sólo los usuarios pueden verla.

## SimpleX explicado

1. Experiencia del usuario
2. Cómo funciona la red
3. Qué ven los servidores
1. Experiencia del usuario

Puedes crear contactos y grupos, y mantener conversaciones bidireccionales igual que en cualquier aplicación de mensajería.

¿Cómo puede funcionar con colas unidireccionales y sin identificadores de usuario?

2. Cómo funciona la red

Por cada conexión se usan dos colas de mensajes separadas, para que el envío y la recepción se realicen a través de servidores diferentes.

Los servidores sólo transmiten en un sentido para no disponer de la conversación completa o las conexiones del usuario.

3. Qué ven los servidores

Para cada cola los servidores disponen de credenciales separadas y anónimas, por lo que desconocen a qué usuarios pertenecen.

El usuario puede mejorar aún más la privacidad de sus metadatos, haciendo uso de la red Tor para acceder a los servidores, evitando así la correlación por dirección IP.

## Comparación con otros protocolos

|  | Signal, grandes plataformas | XMPP, Matrix | Protocolos P2P
--- | --- | --- | --- | ---
Requiere de identidad global | No - privado | Sí 1 | Sí 2 | Sí 3
Posibilidad de MITM | No - seguro 4 | Sí 5 | Sí | Sí
Dependencia del DNS | No - resiliente | Sí | Sí | No
Red única o centralizada | No - descentralizado | Sí | No - federado 6 | Sí 7
Componente central u otro ataque en toda la red | No - resiliente | Sí | Sí 2 | Sí 8

1. Generalmente basada en un número de teléfono, y en algunos casos en nombres de usuario
2. Direcciones basadas en DNS
3. Clave pública o algun otro ID único a nivel global
4. Los servidores no pueden comprometer el cifrado E2E. Para evitar posibles ataques, verifica el código de seguridad mediante un canal alternativo
5. Si los servidores del operador se ven comprometidos. Verifica el código de seguridad en Signal y otras aplicaciónes para mitigarlo
6. No protege la privacidad de los metadatos del usuario
7. A pesar de que las redes P2P son distribuidas, no son federadas. Funcionan como una única red
8. Las redes P2P, o bien disponen de una autoridad central, o bien la red completa podría verse comprometida — ver aquí

## Comparativa del cifrado de extremo a extremo en los distintos mensajeros

| Session | Briar | Element | Cwtch | Signal | SimpleX
--- | --- | --- | --- | --- | --- | ---
Relleno de mensajes | ✗ | ✔︎1 | ✗ | ✔︎ | ✔︎1 | ✔︎
Repudio (negabilidad) | ✗ | ✗ | ✗ | ✔︎2 | ✔︎3 | ✔︎
Secreto hacia adelante | ✗ | ✔︎ | ✔︎ | ✔︎ | ✔︎ | ✔︎
Seguridad tras ser comprometido | ✗ | ✗ | ✗ | ✔︎ | ✗4 | ✔︎
Intercambio de claves de dobre factor | ✔︎ | ✔︎5 | ✔︎5 | ✔︎ | ✔︎5 | ✔︎
Cifrado híbrido postcuántico | ✗ | ✗ | ✗ | ✗ | ✔︎6 | ✔︎

1. Briar rellena los mensajes hasta el tamaño redondeado a 1024 bytes, Signal a 160 bytes
2. El repudio no incluye la conexión cliente‑servidor.
3. Parece que el uso de firmas criptográficas compromete el repudio (negabilidad) pero es necesario aclararlo.
4. La implementación multi‑dispositivo compromete la seguridad posterior a ser comprometida del Double Ratchet — ver aquí.
5. El intercambio de clave de doble factor es opcional mediante la verificación del código de seguridad.
6. El acuerdo de claves postcuántico es "parcial", solo protege determinados pasos del ratchet.

# 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.

# 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.

# 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.