Businessmen having a private conversation in a tunnel

How to secure mobile comm? Cut out the trusted third party.

The new secure communications service offered by Silent Circle intends to solve the BYOD security challenge by harnessing the computing power of smart phones for crypto key management, cutting the middle man out of the equation.

You don't have to trust us

How the service removes the trusted third party from secure communications:

Peer-to-peer encryption

Key management is done by end devices rather than a central server operated by the service provider.

Separate phone number for Silent Circle applications

Cellular provider becomes a carrier for IP traffic rather than providing phone service.


Zimmermann Real Time Transport Protocol, a VOIP crypto key agreement protocol using the Diffie-Hellman key exchange and the Secure Real Time Transport Protocol.


NSA Suite B cryptography, a public interoperable set of crypto tools that includes
• The Advanced Encryption Standard
• The Secure Hash Algorithm 2
• Elliptic curve digital signature and key agreement algorithms.


Published protocol specifications and application source code so security implementation can be verified.

“We’ve pushed the key management out to the endpoints,” said company CTO Jon Callas. “We never have the key.”

For a $20 monthly subscription users can communicate securely with each other by downloading a suite of apps for peer-to-peer encryption. Calls, texts and video are routed through the Silent Circle network, but keys are generated on the mobile devices when a call is initiated and are not held on a central server. All security information is deleted from the device when the call ends.

Much has been made of the fact that this model could make it impossible for law enforcement and intelligence agencies to listen in to calls or look at data, images and video being exchanged between secured phones. But company executives say that instead of pushback, government has been an early adopter of the service, particularly U.S. military and intelligence agencies.

“This is not 1991,” said Philip Zimmermann, the company’s president and creator of PGP (Pretty Good Privacy), the widely used e-mail encryption software.

Zimmermann is a veteran of the crypto wars of the 1990s, when the National Security Agency threatened the emergence of strong cryptography being developed commercially. “Times have changed,” he said. “Today you’re in trouble if you don’t use strong crypto.”

“The government is our largest customer,” said CEO Mike Janke. The rapid adoption of the service-based technology by the military and intelligence communities has left the company scrambling to scale up and meet demand. “We didn’t see this coming,” said Janke, a former Navy Seal.

Silent Circle focuses on the security issues raised by users bringing their personal, unmanaged mobile devices into the enterprise. Although originally envisioned primarily as a consumer tool, it has become popular with secure enterprises as a way to manage BYOD.

The Silent Suite of end-user applications for iOS and Android include Silent Phone, Silent Text, and Silent Eyes. With a subscription, each user receives a phone number for the applications, separate from the cellular number for the physical device. The application uses the customer’s cellular carrier service to establish an IP connection with Silent Circle, which routes the encrypted communications to the application phone number at the other end, encrypted end-to-end and bypassing the regular phone service.

The Silent Mail e-mail encryption app uses what the company calls an elegant solution that uses server-side key encryption rather than peer-to-peer.

Secure peer-to-peer connections use the Zimmermann Real Time Transport Protocol, a crypto key agreement protocol for voice over IP that uses the Diffie-Hellman key exchange and the Secure Real Time Transport Protocol. Encryption is done with NSA Suite B cryptography, a public interoperable set of crypto tools that include the Advanced Encryption Standard, Secure Hash Algorithm 2 and elliptic curve digital signature and key agreement algorithms.

“We built our own network, SIP servers and codecs that allow this to happen,” said Janke. But all of the crypto and security remain in the application in the users’ hands.

Zimmermann, who said he does not trust service providers, said, “I made a protocol that doesn’t trust us.” That means customers do not need to trust the Silent Circle infrastructure. The protocol specs and application source code is published so that users can confirm the security of the scheme.

Users can place a time-to-die on files that are sent, and a sender can recall or “burn” a file that is in the recipient’s application. But there are no restrictions on the recipient’s saving or copying data outside of the app.

“It’s difficult to put in restrictions and make them stick,” Callas said. “DRM (Digital Rights Management) doesn’t work.” The security of the scheme relies on the trust between the sender and recipient. “If you assume the other party isn’t trusted, it’s a very hard problem to solve. If you assume he is trusted, it is very easy.”

“The roots of what we are doing go back to STU III,” the government’s standard encrypted telephone through 2009, said Callas. But STU could not be easily deployed to consumer devices on the fly and there was the issue of central management that Silent Circle wanted to avoid. PGP is good consumer crypto, but key management is not transparent. What makes Silent Circle’s service practical is the increased computing power of smart phones that enables them to handle strong encryption and key management with an onboard app.

“We started thinking about the phone as if it were a server,” Callas said. “This would have been difficult to do five years ago.”

Although the company says in its literature it will comply with legal requests from law enforcement for customer data, it retains only minimal data about its customers, which does not include crypto keys or any way to access encrypted transmissions.

How does the company ensure that its services are not used for evil?

“We don’t,” Zimmermann said.

“We know bad people will use this,” Janke said. “It’s not our place to stop it.” But law enforcement has other tools to pursue the bad guys and the benefits of strong peer-to-peer encryption outweigh the risks, he said.


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