Ars recently attempted to delve into the inner workings of the security built into Apple's iCloud service. Though we came away reasonably certain that iCloud uses industry best practices that Apple claims it uses to protect data and privacy, we warned that your information isn't entirely protected from prying eyes. At the heart of the issue is the fact that Apple can, at any time, review the data synced with iCloud, and under certain circumstances might share that information with legal authorities.

We consulted several sources to understand the implications of iCloud's security and encryption model, and to understand what types of best practices could maximize the security and privacy of user data stored in increasingly popular cloud services like iCloud. In short, Apple is taking measures to prevent access to user data from unauthorized third parties or hackers. However, iCloud isn't recommended for the more stringent security requirements of enterprise users, or those paranoid about their data being accessed by authorities.

Apple holds the (encryption) key

As we noted in our original investigation, Apple can potentially decrypt and access all data stored on iCloud servers. This includes contacts, notes, unencrypted e-mails, application preferences, Safari bookmarks, calendars, and reminders.

This was recently confirmed by a source speaking to Ars, and security researcher and forensic data analysis expert Jonathan Zdziarski agreed. "I can tell you that the iCloud terms and conditions are pretty telling about what the capabilities are at Apple with respect to iCloud, and suggests they can view any and all content," Zdziarski told Ars.

In particular, Zdziarski cited particular clauses of iCloud Terms and Conditions that state that Apple can "pre-screen, move, refuse, modify and/or remove Content at any time" if the content is deemed "objectionable" or otherwise in violation of the terms of service. Furthermore, Apple can "access, use, preserve and/or disclose your Account information and Content to law enforcement authorities" whenever required or permitted by law. Apple further says that it will review content reportedly in violation of copyright under DMCA statutes.

"If iCloud data was fully encrypted, they wouldn't be able to review content, provide content to law enforcement, or attempt to identify DMCA violations," Zdziarski told Ars.

Securosis CEO Rich Mogull agreed that iCloud's encryption model gives Apple this access.

"iCloud data is encrypted only for transport, and not on a per-user basis for the data itself," Mogull told Ars. "Apple may still encrypt data on the drives, but they have to have the key."

In other words, to provide the variety of services Apple offers, Apple must hold the encryption key to your encrypted data. "If you can access something with a webpage, that means the webserver has the key," Mogull explained. "Thus we know that Apple could access at least anything iCloud related that shows in the browser. This is true of Dropbox, box.net, and nearly everyone else—if you can see it in a browser, they can see it on the server. iCloud data isn't encrypted with a user-defined key—it's protected with keys that Apple defines and controls."

Still, vice president of products for cloud security firm Echoworx, Robby Gulri, noted that Apple is following best practices used throughout the industry. "Apple has taken the right steps to protect users' data and privacy as far as a widely public service like iCloud goes," he told Ars. "For example, data is transmitted using SSL, data is encrypted on disk using 128-bit keys, and Apple has stopped letting developers use individual UDIDs."

"Just because the data is encrypted, however, doesn't mean that it's secure," Gulri said. "In a symmetric encryption system, there's always a back door. There's always the chance, however remote, that some rogue employee could use the master key to decrypt and access your data."

Making "best practices" better

Apple insists that it takes user privacy and data security very seriously, and we noted previously that the company has policies in place to prevent the kind of rogue access Gulri refers to. However, Gulri still believes that there are some best practices that should be adopted by both cloud storage providers and mobile device vendors to maximize user privacy and security.

First, cloud services should use asymmetric public key encryption. "With asymmetric encryption, the privacy and identity of each individual user" is better protected, Gulri said, because it uses one "public" key to encrypt data before being sent to the server, and uses another, "private" key to decrypt data pulled from the server. Assuming no one but the end user has access to that private key, then no one but the user—not Apple, not Google, not the government, and not hackers—could decrypt and see the data.

Additionally, Gulri recommends that the encryption chains for cloud services be regularly audited and verified by a trusted third party. Encryption is only as strong as the weakest link in the chain, and a third-party audit could identify any weak links so they can be corrected. Furthermore, the audit can verify that an encryption process is valid and effective.

Finally, mobile device vendors need to improve the handling of public key credentials. For instance, Mail on an iPhone can use S/MIME to send and receive encrypted conversations. E-mails sent using this method can also be signed and verified as coming from a trusted sender. However, installing and managing the public and private keys necessary to use S/MIME is neither simple nor straightforward.

"It's a big problem with all mobile devices," Gulri told Ars. "Historically there hasn't been a way to push the credentials from a certificate authority over-the-air in a way that's secure and transparent to the end user." Once installed, there's also no simple way to leverage the credentials across various services.

In other words, if asymmetric encryption were easy and usable for anyone, everyone would be using it. "The reality is that the Apple way values usability over all else, including security," Gulri said. "And as it stands, PKI just isn't accessible to the average user."

The problem affects more than the average user, though. Given the fact that Apple can decrypt and access any information stored on iCloud servers, enterprise users face a potentially higher risk that sensitive corporate data could be accessed or turned over to a third party. Because of various legal requirements or corporate policies, enterprise users simply should not use iCloud. Every security expert we talked to agreed that this was advisable, and at least one source suggested that Apple itself made the same recommendations to enterprise users.

While corporate users could replace iCloud with Exchange, for example, that only covers e-mail, contacts, and calendars. Many enterprise users use iPhones (or other smartphones) for personal as well as work purposes, and iOS simply lacks any sort of method for syncing other data like bookmarks and such using a private server provided by you or your employer.

As a sort of compromise, Gulri recommends configuring iCloud to only handle certain data you are comfortable with Apple potentially accessing. "I'm a big iPhone and iPad person, and my data is on iCloud, too," he said. "If you have some concerns about what's stored on iCloud, you can change the default settings. Turn off what you don't want in the cloud—I personally only sync photos and iTunes content like songs and apps."

Ultimately, iCloud security is a matter of trust between individual users and Apple. As confirmed by industry experts, Apple takes a number of precautions to prevent unauthorized access to user data by third parties, but those precautions don't secure your data from Apple itself. If you require—or simply want—greater assurance than that, turn off whatever iCloud features you don't need. One day, mobile device vendors and cloud service providers will be able to build a user-accessible infrastructure to support asymmetric encryption, but that day is not today.

Stay tuned, as tomorrow we review a cloud storage provider that provides a more robust set of encryption options. It's not an iCloud replacement, but some users may still find it superior.

Update: We now have a new piece about whether Apple has a backdoor around your iPhone's PIN/passcode in order to help law enforcement. Check it out!