Overview of projects working on next-generation secure email.
There are an increasing number of projects working on next generation secure email or email-like communication. This is an initial draft report highlighting the projects and comparing the approaches. Please help us fill in the missing details and correct any inaccuracies. To contribute to this document, fork repository found at https://github.com/OpenTechFund/secure-email and issue a pull request.
All of the technologies listed here face a common set of problems when trying to make email (or email-like communication) secure and easy to use. These problems are hard, and have defied easy solutions, because there are no quick technological fixes: at issue is the complex interaction between user experience, real world infrastructure, and security. Although no consensus has yet emerged on how best to tackle any of these problems, the diversity of projects listed in this report reflect an surge of interest in this area and an encouraging spirit of experimentation.
All the projects in this report use public-key encryption to allow a user to send a confidential message to the intendant recipient, and for the recipient to verify the authorship of the message. Unfortunately, public-key encryption is notoriously difficult to use properly, even for advanced users. The very concepts are confusing for most users: public key versus private key, key signing, key revocation, signing keys versus encryption keys, bit length, and so on.
Traditionally, public key cryptography for email has relied on either the X.509 Certificate Authority (CA) system or a decentralized "Web of Trust" (WoT) for key validation (authenticating that a particular person owns a particular key). Recently, both schemes have come under intense criticism. Repeated security lapses at many of the Certificate Authorities have revealed serious flaws in the CA system. On the other hand, in an age where we better understand the power of social network analysis and the sensitivity of the social graph, the exposure of metadata by a "Web of Trust" is no longer acceptable from a security standpoint.
This is where we are now: we have public key technology that is excessively difficult for the common user, and our only methods of key validation have fallen into disrepute. The projects listed here have plunged into this void, attempting to simplify the usage of public-key cryptography. These efforts have four elements:
The projects that use a public key as a unique identifier do not have the key validation problem, because they do no need to try to bind a human memorable identifier to a long non-memorable public key: they simply enforce the use of the public key as the user's address. For example, rather than
8b3b2213ff00e5fb684b003d005ed2fb. In place of the key validation problem, this approach raises the key exchange problem: how do two parties initially exchange long public keys with one another? This approach is taken by all the P2P projects listed here (although there do exist some P2P application that don't use public key identifiers).
Some of the major experimental approaches to solving the problem of public key discovery and validation include:
Traditional schemes for secure email have left metadata exposed. We now know that metadata is often more sensitive than message content: metadata is structured data, easily stored forever, and subject to powerful techniques of social network analysis that can can be incredibly revealing.
Metadata protection, however, is hard. In order to protect metadata, the message routing protocol must hide the sender and recipient from all the intermediaries responsible for relaying the message. This is not possible with the traditional protocol for email transport, although it will probably be possible to piggyback additional (non-backward compatible) protocols on top of traditional email transport in order to achieve metadata protection.
Alternately, some projects reject traditional email transport entirely. These decentralized peer-to-peer approaches to metadata protection generally fall into four camps: (1) directly relay the message from sender's device to recipients device; (2) relay messages through a network of friends; (3) broadcast messages to everyone; (4) relay messages through an anonymization network. The first two approaches protect metadata, but at the expense of increasing vulnerability to traffic analysis that could reveal the same metadata. The third solution faces serious problems of scalability. Pond uses the fourth method, discussed below.
The most advanced scheme in this area is the high-latency anonymizing mix-network PANORAMIX (forthcoming).
All schemes for metadata protection face the prospect of increasing Spam (since one of the primary methods used to prevent Spam is analysis of metadata). This is why some schemes with strong metadata protection make it impossible to send or receive messages to anyone you are not already in contact with. This works brilliantly for reducing Spam, but is unlikely to be a viable long term strategy for entirely replacing the utility of email.
Forward secrecy is a security property that prevents an attacker from saving messages today and then later decrypting these messages once they have captured the user's private key. Without forward secrecy, an attacker is more likely to be able to capture messages today and simply wait for computers to become powerful enough to crack the encryption by brute force. Traditional email encryption offers no forward secrecy.
All methods for forward secrecy involve a process where two parties negotiate an ephemeral key that is used for a short period of time to secure their communication. In many cases, the ephemeral key is generated anew for every single message. Traditional schemes for forward secrecy are incompatible with the asynchronous nature of email communication, since with email you still need to be able to send someone a message even if they are not online and ephemeral key generation requires a back and forth exchange between both parties.
There are several new experimental (and tricky) protocols that attempt to achieve both forward secrecy and support for asynchronous communication, but none have yet emerged as a standard. These protocols either (1) require an initial bootstrap message that is not forward secret, (2) require an initial synchronous exchange to start the process, or (3) rely on a pool of pre-generated ephemeral key pairs that can be used on first contact. When the continually changing ephemeral key for a conversation is lost by either party, then the initialization stage is performed again.
Another possible approach is to use traditional encryption with no support for forward secrecy but instead rely on a scheme for automatic key discovery and validation in order to frequently rotate keys. This way, a user could throw away their private key every few days, achieving a very crude form of forward secrecy.
Users today demand data availability: they want to be able to access their messages and send messages from any device they choose, wherever they choose, and whenever they choose. Most importantly, they don't want the loss of any particular device to result in a loss of all their data. For insecure communication, achieving data availability is dead simple: simply store everything in the cloud. For secure communication, however, we have no proven solutions to this problem. As noted above, the key management problem is also really a data availability problem.
Most of the email projects here have postponed dealing with the data availability problem. A few have used IMAP to synchronize data or developed their own secure synchronization protocol. Several of the email-like P2P approaches rely on a P2P network for data availability.
For those projects that make use of a service provider, one of the key problems is how to authenticate securely with the service provider without revealing the password (since the password is probably also used to encrypt the private key and other secure storage, so it is important that the service provider does not have cleartext access as with typical password authentication schemes). The possible schemes include:
No consensus or standard has yet emerged, although SRP has been around a while.
Most users are familiar with web-based email and the incredible convenience it offers: you can access your email from any device, and you don't need to worry about data synchronization. Developers of web-based email faces several difficult challenges when attempting to make a truly secure application. These challenges can be overcome, but not easily.
First, because the web application is loaded from the web server each time you use it, the service provider could be targeting a version of the client to you that includes a backdoor. To overcome this vulnerability, it is possible to load the code for the web application from a third party. There are two ways of doing this:
Second, even if the application is loaded from a trusted third party, web browsers are not an ideal environment for sensitive data: there are many ways for an in-browser application to leak data and web browsers are notoriously prone to security holes (it is a very difficult problem to be able to run untrusted code locally in a secure sandbox). To their credit, the browser developers are often vigilant about fixing these holes (depending on who you ask), but the browser environment is far from a secure computing environment. It continues to be, however, the most convenient environment.
Third, developers of web-based secure email face an additional challenge when dealing with offline data or data caching. Modern HTML5 apps typically store a lot of data locally on the user's device using the localStorage facility. Currently, however, no browser stores this encrypted. A secure web-based email application must either choose to not support any local storage, or develop a scheme for individually encrypting each object put in localStorage, a process which is very inefficient. Even storing keys temporarily in short lived session storage is problematic, since these can be easily read from disk later.
These challenges to do not apply to downloaded mail clients that happen to use HTML5 as their interface (Mailpile, for example).
Ghostmail is a web-based email provider using client-side encryption in the browser.
Keys: Key validation relies on trusting the provider.
STATUS: lavaboom is no longer being developed.
Lavaboom is a web-based email provider from Germany using client-side encryption in the browser.
Lavaboom's name is a tribute to the shuttered Lavabit service, although Lavaboom has no affiliation or people in common with Lavabit.
Mailfence is a web-based email provider from Belgium that provides OpenPGP based end-to-end en(de)cryption and digital signing in the client-browser and uses open-source implementation (openpgpjs.org library) of RFC-4880.
Key storage and an integrated key-store: Private keys are generated in the client-browser, encrypted with the user's passphrase, and then stored on the server. The server never sees the user's passphrase (encrypted via AES256 and iterated, salted string-to-key specifier). An integrated key-store for each account (without requiring any third party plugin/add-on) is also provided - where users can manage their generated (or externally imported) key-pairs and can export, modify, revoke (with/without revocation certificate), delete, publish/import PGP public keys from public key servers. This gives users full control over their key management and freedom to use as many PGP key-pairs as they want.
Application: The service uses open standards and does not restrict users to communicate securely only with-in their platform, and allows their users to either send plain-text, plain-text and signed, signed and end-to-end encrypted emails to any external PGP compatible provider. They also provide a rich collaborative email-suite along with POP/IMAP support.
Market positioning The project came into existence after snowden revelations and holds a strong online privacy-oriented philosophy. The service donates 15% of their annual Pro-plan revenue to Electronic Frontier Foundation (EFF) and Electronic Digital Rights (EDRi) to support their fight for online privacy and digital freedom.
PrivateSky was a secure web-based email service that chose to shut down because their design was not compatible with UK law. Many in the press have said GCHQ forced the closure, which the company refutes.
The makers of the secure search engine startpage.com have announced they will be providing secure email service.
Despite the tag line as the "world's most private email," StartMail is remarkably insecure. It offers regular IMAP service and a webmail interface that supports OpenPGP, but the user still must trust StartMail entirely. For example when you authenticate, your password string is sent to StartMail, and new OpenPGP keypairs are generated on the server, not the client. The website also makes some dubious statements, such as claiming to be more secure because their TLS server certificate supports extended validation.
Verdict: oil of snake
Scramble is an OpenPGP email application that can be loaded from a website (with plans to add app store support). Additionally, you can sign up for email service from scramble.io.
Keys: Private keys are generated in the browser app, encrypted with the user's passphrase, and then stored on the server. The server never sees the user's passphrase (password is hashed using scrypt before sent to the server during account creation and authentication). The master storage secret (symmetric key) used to encrypt keys is stored in the browser's sessionStorage, which is erased when the user logs out. Keys are validated using notaries.
Infrastructure: Scramble uses a system of network perspectives to discover and validate public keys. The client will come with a list of pre-blessed notaries that can be used to query for public keys. If the notaries agree, the client will consider the key to be validated.
Limitations: Currently, attachments are not supported.
STATUS: Whiteout is currently no longer in business, but the server code might get open sourced in the future.
Whiteout is an open source IMAP/SMTP client with integrated support for OpenPGP.
It can be run in three different ways:
Keys: Key pairs are generated on the client, client-encrypted, and synchronized (via IMAP) among the various devices a user might be running Whiteout.
A browser extension modifies the behavior of the web browser (not to be confused with a browser application, which has far fewer permissions and consists of a self-contained application). Browser extensions are able to modify how the user interacts with a variety of websites. Browser extensions share many of the same advantages and disadvantages of web mail approaches.
Mailvelope is a browser extension that allows you to use OpenPGP email with traditional web-mail providers like Gmail, Yahoo, and Outlook.com.
Keys: The private key is generated for you, password protected, and stored in the browser's local storage (along with public keys). In the future, the plan is to support automatic discovery and validation of public keys using OpenPGP keyservers and message footers.
Application: When the extension detects you have opened a web page from a supported web-mail provider such as Gmail, it offers the user the opportunity to encrypt what you type in the compose window and decrypt messages you receive.
Limitations: Because of an inherent limitation in the way Mailvelope can interface with web-mail, it is not able to send OpenPGP/MIME (although it can read it fine). As mentioned elsewhere, browser storage is not a particular ideal place to be storing keys. When a web-mail provider changes their UI (or API if they happen to have one), the extension must be updated to handle the new format.
End-to-End is a Chrome extension from Google that is currently in an alpha stage. Google is solliciting feedback, and security bugs are eligible for Google’s Vulnerability Rewards Program. More background about the browser extension is available in Google's blog post announcing End-to-End.
An email client, or MUA (Mail User Agent), provides a user interface to access email from any service provider. Traditional examples of email clients include Thunderbird or Microsoft Outlook (although both these application include a lot of other functionality as well). Nearly all email clients communicate with the email service provider using IMAP or POP and SMTP, although some also support local mailboxes in mbox or Maildir format.
There are two primary advantages to the mail client approach:
The mail client approach, however, also has several disadvantages:
Ultimately, the level of email security that is possible with the custom mail client approach will always be limited. However, custom email clients may be an excellent strategy for gradually weening users away from email and to a different and more secure protocol. Most of the projects in this section see email support as a gateway to ease the transition to something that can replace email.
Bitmail is a desktop application that provides a user interface for traditional IMAP-based mail, but also supports a custom peer-to-peer protocol for relaying email through a network of friends. Bitmail will support both OpenPGP and S/MIME.
Keys: Keys are validated using a shared secret or fingerprint validation. Public keys are discovered over the P2P network. Keys are stored locally in an encrypted database.
Routing: Bitmail uses an opportunistic message distribution model where every message is sent to every neighbor. It is called "Echo" and it is very similar to the protocols used by Retroshare and Briar.
Application: Bitmail uses the Qt library for cross platform UI.
There are also plans to include a Bitmail MUA extension.
Note: I am unclear which of the previous features are planned and which are currently working.
M is a suite of native email clients with built-in end-to-end encryption.
Infrastructure: Keys are generated and stored locally. There is no central point of trust and no collection of user data.
Key discovery and validation: Public key transmitted automatically in X-Header. Trust on first use with optional fingerprint validation.
Project status: Apps for Mac & iOS ready. Outlook plugin in alpha, Android app planned.
Encryption format: Implements custom Open Source encryption format. Provable CCA security. Removes SHA1 and other broken algorithms. Encrypts subject lines. Signing of other header info prevents replay attacks and surreptitious forwarding.
Limitations: Requires download. No webmail access. PGP and S/MIME support in development. Other MUAs need to parse the X-Myn-PK header.
Mailpile is an email client designed to quickly handle large amounts of email and also support user-friendly encryption. The initial focus is on email, with plans to eventually support post-email protocols like bitmessage, flowingmail, or darkmail. Also, the developers hope to add support for XMPP-based chat in the future. Since the Mozilla foundation has not committed the resources necessary to keep Thunderbird contemporary, the Mailpile initiative holds a lot of promise as a cross-platform mail client that seeks to redesign how we interact with email.
Keys: Mailpile email encryption is based on OpenPGP (it uses your GPG keyring). Key discovery will be handled using OpenPGP keyservers and including public keys as attachments to outgoing email. Public keys are trusted on first use, with plans for validation via DANE and manual fingerprint verification (future support for a P2P protocol might include additional methods, such as Certificate Transparency or Short Authentication Strings). Currently, keys are not backed up.
Limitations: Mailpile does not currently have a scheme for recovery if your device is destroyed or a method for securely synchronizing keys among devices. Although the search index is stored encrypted on disk (if the user already has GPG installed and a key pair generated), it is encrypted in a way that requires the index to be loaded entirely into memory. Mailpile is under very active development, so these and other issues may change in the near future.
NOTE: discontinued, not under active development.
Keys: Although Parley can be used with any service provider, the Parley servers are used to publish public keys and back up client-encrypted private keys. For key discovery, Parley uses a central repository and the OpenPGP keyservers. For key validation, Parley relies on trust on first use and Parley key endorsement.
Infrastructure: All users of the Parley client also sign up for the Parley service, but they use their existing email account. The Parley server acts as a proxy that uses context.io for email storage (context.io is a commercial service that provides a REST API for a traditional IMAP account). The Parley server also handles key discovery, validation, and backup. Both the client and server are released as free software.
Application: Parley is currently bundled into an executable using Appcelerator. The Parley client does not speak IMAP or SMTP directly. Rather, uses the email REST API of context.io.
Limitations: All user email is stored by context.io, albeit in OpenPGP format. Metadata is exposed to context.io, however (in addition to your service provider).
Traditionally, email is a federated protocol: when you send an email it travels from your computer, to the server of your email provider, to the server of the recipient's provider, and finally to the recipient's computer. The key idea with self-hosted email is to cut out the middleman and run your own email server.
In the United States, much of the interest in self-hosted email is driven by the Supreme Court's current (and particularly odd) interpretation of the 4th amendment called the "Third-Party Doctrine". Essentially, you have much weaker privacy protections in the US if you entrust any of your data to a third party. Additionally, the Court has so far afforded much greater protections to items physically inside your home. "Aha!" say the hackers and the lawyers, "we will just put email in the home."
Unfortunately, it is not so simple. There are some major challenges to putting email servers in everyone's home:
Self-hosted email is an intriguing "legal hack", albeit one that faces many technical challenges.
The Dark Mail Alliance has said they want to support self-hosting for the server component of the system. No details yet.
From its early conception, part of FreedomBox was "email and telecommunications that protects privacy and resists eavesdropping". Email, however, is not currently being worked on as part of FreedomBox. (as far as I can tell).
Although Mailpile is primarily a mail client, the background Python component can read the Maildir format for email. This means you could install Mailpile on your own server running a Mail Transfer Agent (MTA) like postfix or qmail. You would then access your mail remotely by connecting to your server via a web browser.
Mail-in-a-box helps people set up their own email address, on a server they own or lease. It turns any machine running the latest version of Ubuntu into a functioning email server, using modern email protocols and encryption (IMAP, SMTP, greylisting, DKIM, SPF, and DNSSEC). It includes a rudimentary webmail interface (Roundcube), and helps you set up an SSL certificate for web and email.
Advantages: For a simple, one-server setup, someone with some command line experience (or a willingness to learn), it's pretty straightforward to get an email address working quickly.
Limitations: Setting up an email server is the easy part, maintaining the service over time can be tricky. There still aren't well-developed tools to help troubleshoot and resolve problems (for example, a broken spam detector, or overloaded memory).
The "infrastructure" projects give a service provider the opportunity to offer secure email accounts to end-users. By modifying how both email clients and email servers work, these projects have the potential to deploy greater security measures than are possible with a client-only approach. For example:
The goal of both projects in this category is to build systems where the service provider is untrusted and cannot compromise the security of its users.
Despite the potential of this approach, there are several unknown factors that might limit its appeal:
The Dark Internet Mail Alliance (DIME) is a specification for a complete secure email-like communication system. It will include both a client application and server software.
Keys: Key pairs will be generated on the user's device and uploaded to the service provider. Keys, called "Signets," are validating using a system similar to DNSSec/DANE.
Routing: Message routing is obfuscated using automatic aliases for addresses.
LEAP includes both a client application and turn-key system to automate the process of running a secure service provider. Currently, this includes user registration and management, help tickets, billing, VPN service, and secure email service. The secure email service is based on OpenPGP.
Keys: Key pairs are generated on the user's device. Keys, and all user data, are stored in a client-encrypted database that is synchronized among the user's devices and backed up to the service provider. Keys are automatically validated using a combination of provider endorsement and network perspective (coming soon). Keys are discovered via the OpenPGP keyservers, the OpenPGP header, email footers, and a custom HTTP-based discovery protocol.
Infrastructure: LEAP follows a traditional federated client/server architecture. The client is designed to work with any LEAP-compatible service provider (with plans to support legacy IMAP providers in the future). For security reasons, users are encouraged to get the application from LEAP and not their service provider.
Application: The client application works with any existing MUA by exposing a local IMAP/SMTP server that the MUA can connect to. There is a Thunderbird extension to automate configuration of the account in Thunderbird. The client application communicates with the service provider using a custom protocol for synchronizing encrypted databases. The application is a very small C program that launches the Python code. The user interface is written using Qt.
Limitations: In the current implementation, security properties of forward secrecy and metadata production are not end-to-end. Instead, the client relies on the service provider to ensure these properties. This limitation is due to some inherent limitations in the existing protocols for secure email. As with many of the other projects, LEAP's plan is to invisibly upgrade to a post-email protocol when possible in order to overcome these limitations.
Pixelated is an OpenPGP group email solution based on the LEAP Platform.
It consists of:
Pixelated aims at fostering encryption and federation of email services. Its approach is to combine enhanced usability of email encryption for the email user with facilitation of email server set up and maintenance of secure email hosting for system administrators.
The Pixelated MUA can be run either as a web front end on the server or on the client machine. In a group setting, both setups can be run in parallel.
Keys: Private keys are generated and stored in the MUA (either on the server or the client machine, depending on where the MUA runs while generating keys) and encrypted with the user's passphrase. Hence, depending on where the MUA lives, the keys are stored either on the server or on the user’s machine.
Federation: Pixelated follows the idea of infrastructure as code to facilitate the setup and maintenance of email servers. It builds on LEAP's nicknym protocol to discover and validate public keys.
Limitations: In some set ups of Pixelated, the user's private keys are stored on the server in order to facilitate usability for a broad public (handling of key material). Pixelated assumes some level of trust to the server (e.g. self hosted for a particular organisation) and a high level of federation. The same limitions as described for LEAP also apply to Pixelated.
There are several projects to create alternatives to email that are more secure yet still email-like.
These projects share some common advantages:
Except for Pond, all these alternatives take a pure peer-to-peer approach. As such, they face particular challenges:
Most of these challenges have possible technological solutions that might make peer-to-peer approaches the most attractive option in the long run. For example, researchers may discover ways to make P2P networks less battery intensive. For this reason, it is important that research continue in this area. However, in the long run we are all dead and peer-to-peer approaches face serious hurdles before they can achieve the kind of user experience demanded today.
Bitmessage is a peer-to-peer email-like communication protocol. It is totally decentralized and places no trust on any organization for services or validation.
Bote mail (aka IMail) is an email-like communication protocol that uses the anonymizing network I2p for transport. Bote mail stores messages in a global distribute hash table for up to 100 days, during which time the client has an opportunity to download and store the message.
Keys: Bote mail uses public-key based addresses. You can create as many identities as you want, each identity corresponding to a ECDSA or NTRU key-pair.
Application: Users interact with the user interface through a webmail interface, although the client is running locally.
The Dark Mail Alliance plans to incorporate traditional email, a federated email alternative, and a second email alternative that is pure peer-to-peer. Details are not yet forthwith.
Enigmabox is a device that you install on your local network between your computer and the internet. It acts as secure proxy, providing VPN, and communication services analogous to email and VoIP. In order to communicate with another user, they must also have an enigmabox.
Data is routed peer-to-peer directly from one enigmabox to another using cjdns, a system of virtual mesh networking in which IP addresses are derived from public keys. End to end encryption of messages is provided entirely by the cjdns transport layer.
With this scheme, message are forward secret, but not entirely asynchronous. At some point, both the sender and recipient must have their enigmaboxes online at the same time.
ePOST is a cooperative, serverless email system. The actual version is 2.4.8 from 2007 and no longer under active development.
P2P secure, encrypted email system.
The block chain confirms that your mail is genuine and your sent mail was received at the other end.
Pond is an email-like messaging application with several unique architectural and cryptographic features that make it stand out in the field. Note that Adam Langley has updated to state that the project is in stasis, and has been so for several years. As of September 2017, the project website is down.
Message Encryption: Pond uses Axolotl for asynchronous forward secret messages where the key is frequently ratcheted (akin to OTR, but more robust).
Routing: Pond uses a unique architecture where every user relies on a service provider for receiving messages, but sent messages are delivered directly to the recipient's server (over Tor). This allows for strong metadata protection, but does not suffer from the other problems that peer-to-peer systems typically do. In order to prevent excessive Spam under this scheme, Pond uses a clever system of group signatures to allow the server to check if a sender is authorized to deliver to a particular user without leaking any information to the server.
Keys: Pond uses Panda, a system for secure peer validation using short authentication strings.
Pond's advantages include:
Pond's disadvantages include:
Pond is an exciting experiment in how you could build a very secure post-email protocol. Although Pond currently uses non-human identifiers, Pond could be easily modified to use traditional email [email protected] identifiers (because it relies on service providers for message reception). The requirement in Pond that both parties pre-exchange keys could also be modified to allow users to set up addresses that could receive messages from anyone, albeit at the cost of likely Spam. Currently, Pond uses Tor to anonymize message routing, but the Tor network was designed for low-latency. Pond could potentially use a more secure anonymization network that was designed for higher-latency asynchronous messages.
Ultimately, Pond's unique design makes it a very strong candidate for incorporation into a messaging application that could automatically upgrade from email to Pond should it detect that both parties support it.
RetroShare is a free and open source, Friend-2-Friend and secure decentralised communication platform that offers chat and file sharing features.
Key validation: messages may be sent to your pre-arranged friends, and to their friends.
Written in: C++, Qt
Source code: https://github.com/RetroShare/RetroShare
Platforms: Windows, Mac, GNU/Linux
ZeroMail is a p2p email-like communication protocol using Bitcoin cryptography and the BitTorrent network. It is part of ZeroNet which includes e.g. p2p web, Talk, and Board.
There are many projects that are centralized email-like messaging platforms. They take one of three forms:
These systems are not strictly "secure email," even if they look and smell like email, because you cannot actually send a secure message to an email user on another system. You can send them a message that they can read after visiting another site, but this is not really email. Also, there is no way for someone using an external email account to send a secure email to a user of one of these systems.
This approach is able to increase usability by centralizing the problems of key authenticity. Hopefully, these systems will evolve into real secure email solutions over time.
Enlocked uses OpenPGP, but does not use public key cryptography: sent messages are simply included as an attachment that is symmetrically encrypted with a passphrase.
ProtonMail is a web-based mail provider from Switzerland using client-side encryption in the browser. It uses two separate passwords: one to authenticate and one to unlock encryption secrets. Emails sent to and from other ProtonMail users are end to end encrypted. You can encrypt emails to other users using a custom symmetric encryption scheme that requires the recipient to visit the ProtonMail website and to know a shared secret.
The developers promote their service as being secure because of its location in Switzerland and therefore user data are protected by strict Swiss privacy laws.
End to end encrypted messaging that uses your mobile phone to directly route a message to the recipient. For routing, it does a lookup in a central registry to discover the recipient's device. A mini IMAP server to allows you to check messages via a desktop MUA.
Tutanota is an open source installable webclient and includes apps for Android, iOS and web browsers. A message to another user on the system uses public-key encryption. Emails to users outside the system are symmetrically encrypted with a pre-shared secret (the message is viewable on the Tutanota website).
Keys: Private keys are generated in the browser app, encrypted with the user's password, and then stored on the server. The server never sees the user's password. Key validation requires trusting the Tutanota server.
Walnut is a proprietary secure email-like messaging application. You can also use it as an email client, but emails sent and received from/to non-walnut users are not end-to-end encrypted.
There are many technologies that don't belong in this document because they either (a) are not trying to make encrypted email-like communication easier, (b) use some kind of weird proprietary escrow system, or (c) we just don't know enough about them yet. Here is a place to store links to such projects.