# 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.

## Why SimpleX is unique

# #1

### You have complete privacy

SimpleX protects the privacy of your profile, contacts and metadata, hiding it from SimpleX network servers and any observers.

Unlike any other existing messaging network, SimpleX has no identifiers assigned to the users — not even random numbers.

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# #2

### You are protected
from spam and abuse

Because you have no identifier or fixed address on the SimpleX network, nobody can contact you unless you share a one-time or temporary user address, as a QR code or a link.

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# #3

### You control your data

SimpleX stores all user data on client devices in a portable encrypted database format — it can be transferred to another device.

The end-to-end encrypted messages are held temporarily on SimpleX relay servers until received, then they are permanently deleted.

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# #4

### You own SimpleX network

The SimpleX network is fully decentralised and independent of any crypto-currency or any other network, other than the Internet.

You can use SimpleX with your own servers or with the servers provided by us — and still connect to any user.

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# Full privacy of your identity, profile, contacts and metadata

Unlike other messaging networks, SimpleX has no identifiers assigned to the users. It does not rely on phone numbers, domain-based addresses (like email or XMPP), usernames, public keys or even random numbers to identify its users — SimpleX server operators don't know how many people use their servers.

To deliver messages SimpleX uses pairwise anonymous addresses of unidirectional message queues, separate for received and sent messages, usually via different servers.

This design protects the privacy of who you are communicating with, hiding it from SimpleX network servers and from any observers. To hide your IP address from the servers, you can connect to SimpleX servers via Tor.

# The best protection from spam and abuse

Because you have no identifier on the SimpleX network, nobody can contact you unless you share a one-time or temporary user address, as a QR code or a link.

Even with the optional user address, while it can be used to send spam contact requests, you can change or completely delete it without losing any of your connections.

# Ownership, control and security of your data

SimpleX Chat stores all user data only on client devices using a portable encrypted database format that can be exported and transferred to any supported device.

The end-to-end encrypted messages are held temporarily on SimpleX relay servers until received, then they are permanently deleted.

Unlike federated networks servers (email, XMPP or Matrix), SimpleX servers don't store user accounts, they only relay messages, protecting the privacy of both parties.

There are no identifiers or ciphertext in common between sent and received server traffic — if anybody is observing it, they cannot easily determine who communicates with whom, even if TLS is compromised.

# Fully decentralised — users own the SimpleX network

You can use SimpleX with your own servers and still communicate with people who use the servers preconfigured in the apps.

SimpleX network uses an open protocol and provides SDK to create chat bots, allowing implementation of services that users can interact with via SimpleX Chat apps — we're really looking forward to see what SimpleX services you will build.

If you are considering developing for the SimpleX network, for example, the chat bot for SimpleX app users, or the integration of the SimpleX Chat library into your mobile apps, please get in touch for any advice and support.

## Features

E2E-encrypted messages with markdown and editing

E2E-encrypted
images, videos and files

E2E-encrypted decentralized groups — only users know they exist

E2E-encrypted voice messages

Disappearing messages

E2E-encrypted
audio and video calls

Portable encrypted app storage — move profile to another device

Incognito mode —
unique to SimpleX Chat

## What makes SimpleX private

### Temporary anonymous pairwise identifiers

SimpleX uses temporary anonymous pairwise addresses and credentials for each user contact or group member.

It allows to deliver messages without user profile identifiers, providing better meta-data privacy than alternatives.

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### Out-of-band
key exchange

Many communication networks are vulnerable to MITM attacks by servers or network providers.

To prevent it SimpleX apps pass one-time keys out-of-band, when you share an address as a link or a QR code.

Tap to close

### 2-layers of
end-to-end encryption

Double-ratchet protocol —
OTR messaging with perfect forward secrecy and break-in recovery.

NaCL cryptobox in each queue to prevent traffic correlation between message queues if TLS is compromised.

Tap to close

### Message integrity
verification

To guarantee integrity the messages are sequentially numbered and include the hash of the previous message.

If any message is added, removed or changed the recipient will be alerted.

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### Additional layer of
server encryption

Additional layer of server encryption for delivery to the recipient, to prevent the correlation between received and sent server traffic if TLS is compromised.

Tap to close

### Message mixing
to reduce correlation

SimpleX servers act as low latency mix nodes — the incoming and outgoing messages have different order.

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### Secure authenticated
TLS transport

Only TLS 1.2/1.3 with strong algorithms is used for client-server connections.

Server fingerprint and channel binding prevent MITM and replay attacks.

Connection resumption is disabled to prevent session attacks.

Tap to close

### Optional
access via Tor

To protect your IP address you can access the servers via Tor or some other transport overlay network.

To use SimpleX via Tor please install Orbot app and enable SOCKS5 proxy (or VPN on iOS).

Tap to close

### Unidirectional
message queues

Each message queue passes messages in one direction, with the different send and receive addresses.

It reduces the attack vectors, compared with traditional message brokers, and available meta-data.

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### Multiple layers of
content padding

SimpleX uses content padding for each encryption layer to frustrate message size attacks.

It makes messages of different sizes look the same to the servers and network observers.

Tap to close

## SimpleX Network

SimpleX Chat provides the best privacy by combining the advantages of P2P and federated networks.

### Unlike P2P networks

All messages are sent via the servers, both providing better metadata privacy and reliable asynchronous message delivery, while avoiding many problems of P2P networks.

# Comparison with P2P messaging protocols

P2P messaging protocols and apps have various problems that make them less reliable than SimpleX, more complex to analyse, and vulnerable to several types of attack.

1. P2P networks rely on some variant of DHT to route messages. DHT designs have to balance delivery guarantee and latency. SimpleX has both better delivery guarantee and lower latency than P2P, because the message can be redundantly passed via several servers in parallel, using the servers chosen by the recipient. In P2P networks the message is passed through O(log N) nodes sequentially, using nodes chosen by the algorithm.
2. SimpleX design, unlike most P2P networks, has no global user identifiers of any kind, even temporary, and only uses temporary pairwise identifiers, providing better anonymity and metadata protection.
3. P2P does not solve MITM attack problem, and most existing implementations do not use out-of-band messages for the initial key exchange. SimpleX uses out-of-band messages or, in some cases, pre-existing secure and trusted connections for the initial key exchange.
4. P2P implementations can be blocked by some Internet providers (like BitTorrent). SimpleX is transport agnostic — it can work over standard web protocols, e.g. WebSockets.
5. All known P2P networks may be vulnerable to Sybil attack, because each node is discoverable, and the network operates as a whole. Known measures to mitigate it require either a centralized component or expensive proof of work. SimpleX network has no server discoverability, it is fragmented and operates as multiple isolated sub-networks, making network-wide attacks impossible.
6. P2P networks may be vulnerable to DRDoS attack, when the clients can rebroadcast and amplify traffic, resulting in network-wide denial of service. SimpleX clients only relay traffic from known connection and cannot be used by an attacker to amplify the traffic in the whole network.

### Unlike federated networks

SimpleX relay servers do NOT store user profiles, contacts and delivered messages, do NOT connect to each other, and there is NO servers directory.

### SimpleX network

servers provide unidirectional queues to connect the users, but they have no visibility of the network connection graph — only the users do.

## SimpleX explained

1. What users experience
2. How does it work
3. What servers see
1. What users experience

You can create contacts and groups, and have two-way conversations, as in any other messenger.

How can it work with unidirectional queues and without user profile identifiers?

2. How does it work

For each connection you use two separate messaging queues to send and receive messages via different servers.

Servers only pass messages one way, without having the full picture of user's conversations or connections.

3. What servers see

The servers have separate anonymous credentials for each queue, and do not know which users they belong to.

Users can further improve metadata privacy by using Tor to access servers, preventing corellation by IP address.

## Comparison with other protocols

|  | Signal, big platforms | XMPP, Matrix | P2P protocols
--- | --- | --- | --- | ---
Requires global identity | No - private | Yes 1 | Yes 2 | Yes 3
Possibility of MITM | No - secure 4 | Yes 5 | Yes | Yes
Dependence on DNS | No - resilient | Yes | Yes | No
Single or centralized network | No - decentralized | Yes | No - federated 6 | Yes 7
Central component or other network-wide attack | No - resilient | Yes | Yes 2 | Yes 8

1. Usually based on a phone number, in some cases on usernames
2. DNS-based addresses
3. Public key or some other globally unique ID
4. SimpleX relays cannot compromise e2e encryption. Verify security code to mitigate attack on out-of-band channel
5. If operator's servers are compromised. Verify security code in Signal and some other apps to mitigate it
6. Does not protect users' metadata privacy
7. While P2P are distributed, they are not federated — they operate as a single network
8. P2P networks either have a central authority or the whole network can be compromised — see here

## Comparison of end-to-end encryption security in different messengers

| Session | Briar | Element | Cwtch | Signal | SimpleX
--- | --- | --- | --- | --- | --- | ---
Message padding | ✗ | ✔︎1 | ✗ | ✔︎ | ✔︎1 | ✔︎
Repudiation (deniability) | ✗ | ✗ | ✗ | ✔︎2 | ✔︎3 | ✔︎
Forward secrecy | ✗ | ✔︎ | ✔︎ | ✔︎ | ✔︎ | ✔︎
Post-compromise security | ✗ | ✗ | ✗ | ✔︎ | ✗4 | ✔︎
2-factor key exchange | ✔︎ | ✔︎5 | ✔︎5 | ✔︎ | ✔︎5 | ✔︎
Post-quantum hybrid crypto | ✗ | ✗ | ✗ | ✗ | ✔︎6 | ✔︎

1. Briar pads messages to the size rounded up to 1024 bytes, Signal - to 160 bytes
2. Repudiation does not include client-server connection.
3. It appears that the usage of cryptographic signatures compromises repudiation (deniability), but it needs to be clarified.
4. Multi-device implementation compromises post-compromise security of Double Ratchet — see here.
5. 2-factor key exchange is optional via security code verification.
6. Post-quantum key agreement is "sparse" — it protects only some of the ratchet steps.

# 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.

# 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.

# 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.

# 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.

# 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.

#### 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.

#### 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.

# 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.

#### 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.

# 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.

# 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-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.

# 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.

# 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.

#### 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.

# 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.