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ProtonMail, a Switzerland-based encrypted email service, recently suffered an “extremely powerful” distributed denial-of-service (DDoS) attack that has temporarily knocked it offline.

protonmail ddos attackOn Tuesday, ProtonMail tweeted out that it was experiencing a DDoS attack and that it anticipated some of its services would become temporarily unavailable. The email provider has since been working with its data center to respond to this attack, an assault which according to ProtonMail’s Twitter feed has been characterized by at least four distinct DDoS waves over the past two days.

As of this writing, ProtonMail’s website was still down.

Notwithstanding persistent issues of availability over the past few days, the email provider has published a blog post explaining what has transpired so far and what it it is doing to restore its services:

“On Tuesday November 3, 2015 ProtonMail was taken offline by an extremely powerful DDOS attack,” the post begins. After explaining what a DDoS attack is, it continues: “The attackers began by flooding our IP addresses. That quickly expanded to the datacenter in Switzerland where we have our servers. In the process of attacking us, several other tech companies and even some banks were knocked offline temporarily.”

As noted by Computing, ProtonMail is one of many “zero-knowledge” email providers that have sprung up in Germany and Switzerland following the Snowden revelations. ProtonMail protects its users’ correspondence via the use of end-to-end encryption, the same technology that developers have recently been incorporating into secure text and call mobile apps, such as WhatsApp and Signal.

Fortunate for those with a ProtonMail account, this DDoS attack has not compromised their email security:

“Even though access is limited, an important thing to note is that our core end-to-end encryption holds strong and is 100% untouched. All user data is fine and safe.”

The encrypted email service is currently looking for another data center in Switzerland to host it, though “many are afraid due to the magnitude of the attack” against it.

ProtonMail recommends that users follow along on Twitter to receive updates regarding availability.