ProtonMail

ProtonMail is an end-to-end encrypted email service founded in 2013 in Geneva, Switzerland by scientists who spent time at the CERN research facility. ProtonMail uses client-side encryption to protect email content and user data before they are sent to ProtonMail servers, unlike other common email providers such as Gmail and Outlook.com. The service can be accessed through a webmail client, the Tor network, or dedicated iOS and Android apps.
ProtonMail is run by Proton Technologies AG, a company based in the Canton of Geneva. The company also operates ProtonVPN, a VPN service. ProtonMail received initial funding through a crowdfunding campaign. Although the default account setup is free, the service is sustained by optional paid services. Initially invitation-only, ProtonMail opened up to the public in March 2016. In 2017, ProtonMail had over users, and grew to over 5 million by September 2018, 20 million by the end of 2019, and over 50 million in 2020.
History.
Development.
On 16 May 2014, ProtonMail entered into public beta. It was met with enough response that after three days they needed to temporarily suspend beta signups to expand server capacity. Two months later, ProtonMail received from 10,576 donors through a crowdfunding campaign on Indiegogo, while aiming for . During the campaign, PayPal froze ProtonMail's PayPal account, thereby preventing the withdrawal of worth of donations. PayPal stated that the account was frozen due to doubts of the legality of encryption, statements that opponents said were unfounded. The restrictions were lifted the following day.
On 18 March 2015, ProtonMail received from Charles River Ventures and the Fondation Genevoise pour l'Innovation Technologique (Fongit). On 14 August 2015, ProtonMail released major version 2.0, which included a rewritten codebase for its web interface. On 17 March 2016, ProtonMail released major version 3.0, which saw the official launch of ProtonMail out of beta. With a new interface for the web client, version 3.0 also included the public launch of ProtonMail's iOS and Android beta applications.
On 19 January 2017, ProtonMail announced support through Tor, at the hidden service address protonirockerxow.onion. On 21 November 2017, ProtonMail introduced ProtonMail Contacts, a zero-access encryption contacts manager. ProtonMail Contacts also utilizes digital signatures to verify the integrity of contacts data. On 6 December 2017, ProtonMail launched ProtonMail Bridge, an application that provides end-to-end email encryption to any desktop client that supports IMAP and SMTP, such as Microsoft Outlook, Mozilla Thunderbird, and Apple Mail, for Windows and MacOS.
On 25 July 2018, ProtonMail introduced address verification and PGP support, making ProtonMail interoperable with other PGP clients. In December 2019, ProtonMail launched "ProtonCalendar", a fully encrypted calendar.
The source code for the back-end remains closed source. However, ProtonMail released the source code for the web interface under an open-source license. ProtonMail also open sourced their mobile clients for iOS and Android, as well the ProtonMail Bridge app. All of their source code can be found on GitHub.
In September 2020, it was known that Protonmail has joined the Coalition for App Fairness which aims to gain better conditions for the inclusion of their apps in app stores.
DDoS attacks.
From 3 to 7 November 2015, ProtonMail was under several DDoS attacks that made the service largely unavailable to users. During the attacks, the company stated on Twitter that it was looking for a new data center in Switzerland, saying, "many are afraid due to the magnitude of the attack against us".
In July 2018, ProtonMail reported it was once more suffering from DDoS attacks. CEO Andy Yen claimed that the attackers had been paid by an unknown party to launch the attacks. In September 2018, one of the suspected ProtonMail attackers was arrested by British law enforcement and charged in connection with a series of other high-profile cyberattacks against schools and airlines.
Nationwide block in Belarus.
On 15 November 2019, Proton confirmed that Belarus had issued a nationwide block of ProtonMail and ProtonVPN IP addresses. The block was no longer in place 4 days later. No explanation was given to ProtonMail for the block, nor for the block being lifted.
Block in Russia.
On 29 January 2020, the Russian Federal Service for Supervision of Communications, Information Technology and Mass Media reported that it had implemented a complete block of ProtonMail services within Russia. As a reason for the block, it cited ProtonMail's refusal to give up information relating to accounts that allegedly sent out spam with terror threats. However, ProtonMail claimed that it did not receive any requests from Russian authorities regarding any such accounts. In response to the block, the ProtonMail Twitter account recommended legitimate users circumvent the block via VPNs or Tor.
In March 2020, the company announced that even though the Russia ban was not particularly successful and the service continues to be largely available in Russia without utilising a VPN, ProtonMail will be releasing new anti-censorship features in both ProtonMail and ProtonVPN desktop and mobile apps which will allow more block attempts to be automatically circumvented.
Encryption.
ProtonMail uses a combination of public-key cryptography and symmetric encryption protocols to offer end-to-end encryption. When a user creates a ProtonMail account, their browser generates a pair of public and private RSA keys:
This symmetrical encryption happens in the user's web browser using AES-256. Upon account registration, the user is asked to provide a login password for their account. 
A lost login password can be recovered by sending an e-mail to Protonmail Support. Two of the questions that are asked, in order for Support to provide renewed access to the account are:
This implies that these data are readable by support agents and hence by data analysis services. They constitute meta-data, so that networks of communicating accounts along with subject headers can be charted.
ProtonMail also offers users an option to log in with a two-password mode which requires a login password and a mailbox password.
Upon logging in, the user has to provide both passwords. This is to access the account and the encrypted mailbox and its private encryption key. The decryption takes place client-side either in a web browser or in one of the apps. The public key and the encrypted private key are both stored on ProtonMail servers. Thus ProtonMail stores decryption keys only in their encrypted form so ProtonMail developers are unable to retrieve user emails or reset user mailbox passwords. This system absolves ProtonMail from:
ProtonMail exclusively supports HTTPS and uses TLS with ephemeral key exchange to encrypt all Internet traffic between users and ProtonMail servers. Their 4096-bit RSA SSL certificate is signed by QuoVadis Trustlink Schweiz AG and supports Extended Validation, Certificate Transparency, Public Key Pinning, and Strict Transport Security. Protonmail.com holds an "A+" rating from Qualys SSL Labs.
In September 2015, ProtonMail added native support to their web interface and mobile app for Pretty Good Privacy (PGP). This allows a user to export their ProtonMail PGP-encoded public key to others outside of ProtonMail, enabling them to use the key for email encryption. The ProtonMail team plans to support PGP encryption from ProtonMail to outside users.
Email sending.
An email sent from one ProtonMail account to another is automatically encrypted with the public key of the recipient. Once encrypted, only the private key of the recipient can decrypt the email. When the recipient logs in, their mailbox password decrypts their private key and unlocks their inbox.
Emails sent from ProtonMail to non-ProtonMail email addresses may optionally be sent in plain text or with end-to-end encryption. With encryption, the email is encrypted with AES under a user-supplied password. The recipient receives a link to the ProtonMail website on which they can enter the password and read the decrypted email. ProtonMail assumes that the sender and the recipient have exchanged this password through a backchannel. Such emails can be set to self-destruct after a period of time.
Two-factor authentication.
ProtonMail currently supports two-factor authentication with TOTP tokens for its login process. As of October 2019, according to official ProtonMail blog, U2F support for YubiKey and FIDO physical security keys is currently under development and will be available soon after the release of v4.0.
Account types.
, ProtonMail offers the following plans:
In popular culture.
ProtonMail was mentioned in the 2015 Bear Grylls novel "Ghost Flight".
ProtonMail was featured in seasons 1, 3, and 4 of the American TV drama series "Mr. Robot".
ProtonMail was also featured in the 2019 films "Sound of Metal" and "Knives Out".