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Lavabit, secure email? Hardly, says infosec wizard Moxie Marlinspike

Claims of multiple security measures just 'promises,' researcher claims

Internet Security Threat Report 2014

Former Lavabit proprietor Ladar Levison claims the new Dark Mail initiative he's cooking up with the team from Silent Circle will enable email that's virtually spy-proof, but according to at least one expert, the original Lavabit service was never all that secure to begin with.

"After all," security guru Moxie Marlinspike wrote in a blog post this week, "how is it possible that a service which wasn't supposed to have access to its users' emails found itself in a position where it had no other option but to shut down in an attempt to avoid complying with a request for the contents of its users' emails?"

The main problem with Lavabit's design, according to Marlinspike, is that each Lavabit user's private encryption key was stored on the Lavabit server. The key was itself encrypted with a password, true. But every time the user wanted to read an email, that password needed to be transmitted to the server, essentially negating any security.

"Unlike the design of most secure servers, which are ciphertext in and ciphertext out, this is the inverse: plaintext in and plaintext out," Marlinspike wrote. "The server stores your password for authentication, uses that same password for an encryption key, and promises not to look at either the incoming plaintext, the password itself, or the outgoing plaintext."

Those "promises," Marlinspike says, are essentially worthless.

For one thing, because he was in possession of every user's private key, all Levison would need to do to read your Lavabit email is intercept your password as it came into his server, use it to decrypt your private key, then use the private key to decrypt your mail. This is essentially what the Lavabit server did anyway; Levison just claims he never eavesdropped.

Even if he never did, however (and no one is suggesting he did), an attacker or a rogue employee who gained access to the Lavabit systems might not have been so scrupulous.

"The cryptography was nothing more than a lot of overhead and some shorthand for a promise not to peek," Marlinspike wrote. "Even though they advertised that they 'can't' read your email, what they meant was that they would choose not to."

Finally, an attacker that found a way to eavesdrop the communications between the server and the client would effectively negate all of the security mechanisms on the Lavabit server. The encryption, the passwords, the keys – none of it would really matter to an attacker with the ability to listen in over the wire, who would be able to obtain the user's password and unlock all of the rest.

Marlinspike says his criticisms of Lavabit aren't intended as attacks on Levison, but he does worry that the current effort to release the Lavabit code as an open source project will just lead to further vulnerable services that pretend to be secure.

"I think we should celebrate and support Ladar for making the hard choice that he did to at least speak out and let his users know they'd been compromised," he wrote. "However, I think we should simultaneously be extremely critical of the technical choices and false guarantees that put Ladar in that position."

The Reg has tried to contact Levison for comment on these matters, but our emails have bounced as undeliverable. ®

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