Ever since former National Security Agency consultant Edward Snowden revealed mass governmental surveillance, my inbox has been barraged with announcements about new encryption tools to keep people's communications safe from snooping.

Best Secure Messaging Tools This is a ranking of encrypted messaging programs based on criteria aimed to assess whether they are well designed to make the content of the messages unreadable to anybody other than the sender and recipient. But even messages that are securely encrypted often do not obscure the identities of the sender and recipient. All rankings » Source: Electronic Frontier Foundation, ProPublica, Joseph Bonneau

But it's not easy to sort out which secret messaging tools offer true security and which ones might be snake oil. So I turned to two experts — Joseph Bonneau at Princeton and Peter Eckersley at the Electronic Frontier Foundation — for advice about what to look for in encryption tools. Working together, we chose seven technical criteria on which to rank encryption tools.

The criteria aim to assess whether the tool is designed to combat threats such as backdoors secretly built into the software, Internet eavesdroppers, or tricksters who steal the secret "keys" that users must safeguard to keep their communications secure.

Check out the results of our review.

Keep in mind, even an unbreakable encryption tool can be circumvented by hackers or spies that secretly install software on a computer or phone that hijacks communications before it is encrypted.

And even the best encryption tools still don't do enough. All the tools require both people communicating to install software. And few tools provide much anonymity – so even if your messages are unreadable by anyone but you, your contact list could still be exposed. And many of the tools are run by rag-tag teams of volunteers, which could mean that they won't last.

Still, some tools scored highly enough that users can feel confident that they take encryption seriously. "It's important to realize we're mostly grading for effort here and not execution," said Bonneau. "We're still a long way from being able to state which confidence how much security apps are actually delivering."

One program that scored well was Cryptocat, a free chat program that can be installed in any Web browser and was famously used by journalist Glenn Greenwald while he was in Hong Kong meeting with Snowden. Nadim Kobeissi created Cryptocat in 2010 as an experiment when he was a 21-year-old student at Concordia University in Montreal. "It wasn't anything serious," Kobeissi told me.

But his tool won attention after it won a prize in a New York hackathon in 2012. Since then, he has raised about $150,000 in grants to help pay developers to work on improvements to the software. He funds his Web hosting bills through donations, and he pays himself by working as a software consultant and selling Cryptocat stickers and t-shirts. "It's been an uphill battle," he says. Being recognized as a secure tool, "is a huge deal."

A lineup of three cellphone apps from San Francisco-based Open Whisper Systems also received perfect scores: Signal, for making secure phone calls on iPhone; RedPhone for secure phone calls on Android; and TextSecure, for sending secure texts on Android. All the apps are free and relatively simple to use.

The company's Signal app also tries to give users' some anonymity by using a sophisticated system called a " bloom filter," that allows users to find each other without sharing their address books. "The contacts from your device are never transmitted anywhere," says Open Whisper Systems security expert Moxie Marlinspike.

A pricier option is available from a pair of highly ranked encryption apps for Android and iPhone, Silent Text and Silent Phone. The apps are free to install but users must sign up for a $9.95 monthly subscription service.

Mike Janke, CEO of Silent Circle, says that the only way to offer real privacy is to charge users. "It takes a lot of money to have a robust, always-on and high-quality service," he said. "Most free apps don't or cannot support this," without selling ads or user data.

"Our architecture, network and technology is built to not have any user data," he says. "You pay us for a service and a product with money, not with your data or through ad dollars."

Surprisingly, some popular encryption programs didn't fare well in the rankings. Gnu Privacy Guard, an often used email encryption program, fell short of the top score because it has not been audited and past communications can be compromised if the user's secret key is stolen (by theft of a laptop, for instance). Similarly, Apple's iMessage and FaceTime encrypted texting and video calling programs lost points because its software code is not open for public review.

Also, some tools that are popular in the press didn't fare well. Wickr, a cellphone encryption app that was recently profiled on CNBC, lost points for not disclosing its underlying code or its underlying cryptographic protocols, and for not having a way for users to verify each others' identity. Wickr said it is working toward publicly releasing a white paper that will disclose its protocols and is testing a new identity verification feature that it will release soon.

Similarly, Virtru, which was recently profiled in the New York Times, received low rankings because it stores user's "secret keys" at its own computers rather than on user's computers – requiring users to trust Virtru with access to their secret messages. Virtru says it is working on a way to allow users to store their keys on their own computer if they prefer.

And some programs that sound like they might be secret—such as Snapchat and Google's off the record chats—are only encrypted in transit, but can still be read by the provider.

One problem that remains thorny for many encryption apps is giving users a way to verify that they are sending secret messages to correct person.

That was an issue when one of Edward Snowden's lawyers, Jesslyn Radack, sent an encrypted e-mail to journalist Glenn Greenwald earlier this year asking if Snowden was going to appear at the Polk Awards. By mistake, she sent the email to the public key of someone masquerading as Greenwald, who then decrypted the message and made it public.

Radack could avoided her mishap by comparing the 'fingerprint' of the fake Greenwald key with the 'fingerprint' of the key that Greenwald publishes on The Intercept's website.

Eckersley said he hopes that the next generation of encryption apps can tackle the key verification problem. "It's like we have extremely trustworthy couriers to deliver our secret packages, but we don't always have a safe way to know what address to send them to," he said.