Stuart Schechter published a good primer on the security issues surrounding two-factor authentication.
While it’s often an important security measure, it’s not a panacea. Stuart discusses the usability and security issues that you have to think about before deploying the system.
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Apple is rolling out an iOS security usability feature called Security code AutoFill. The basic idea is that the OS scans incoming SMS messages for security codes and suggests them in AutoFill, so that people can use them without having to memorize or type them.
Sounds like a really good idea, but Andreas Gutmann points out an application where this could become a vulnerability: when authenticating transactions:
Transaction authentication, as opposed to user authentication, is used to attest the correctness of the intention of an action rather than just the identity of a user. It is most widely known from online banking, where it is an essential tool to defend against sophisticated attacks. For example, an adversary can try to trick a victim into transferring money to a different account than the one intended. To achieve this the adversary might use social engineering techniques such as phishing and vishing and/or tools such as Man-in-the-Browser malware.
Transaction authentication is used to defend against these adversaries. Different methods exist but in the one of relevance here — which is among the most common methods currently used — the bank will summarise the salient information of any transaction request, augment this summary with a TAN tailored to that information, and send this data to the registered phone number via SMS. The user, or bank customer in this case, should verify the summary and, if this summary matches with his or her intentions, copy the TAN from the SMS message into the webpage.
This new iOS feature creates problems for the use of SMS in transaction authentication. Applied to 2FA, the user would no longer need to open and read the SMS from which the code has already been conveniently extracted and presented. Unless this feature can reliably distinguish between OTPs in 2FA and TANs in transaction authentication, we can expect that users will also have their TANs extracted and presented without context of the salient information, e.g. amount and destination of the transaction. Yet, precisely the verification of this salient information is essential for security. Examples of where this scenario could apply include a Man-in-the-Middle attack on the user accessing online banking from their mobile browser, or where a malicious website or app on the user’s phone accesses the bank’s legitimate online banking service.
This is an interesting interaction between two security systems. Security code AutoFill eliminates the need for the user to view the SMS or memorize the one-time code. Transaction authentication assumes the user read and approved the additional information in the SMS message before using the one-time code.
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Slashdot asks if password masking — replacing password characters with asterisks as you type them — is on the way out. I don’t know if that’s true, but I would be happy to see it go. Shoulder surfing, the threat is defends against, is largely nonexistent. And it is becoming harder to type in passwords on small screens and annoying interfaces. The IoT will only exacerbate this problem, and when passwords are harder to type in, users choose weaker ones.
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Back in March, Rolf Weber wrote about a potential vulnerability in the WhatsApp protocol that would allow Facebook to defeat perfect forward secrecy by forcibly change users’ keys, allowing it — or more likely, the government — to eavesdrop on encrypted messages.
It seems that this vulnerability is real:
WhatsApp has the ability to force the generation of new encryption keys for offline users, unbeknown to the sender and recipient of the messages, and to make the sender re-encrypt messages with new keys and send them again for any messages that have not been marked as delivered.
The recipient is not made aware of this change in encryption, while the sender is only notified if they have opted-in to encryption warnings in settings, and only after the messages have been re-sent. This re-encryption and rebroadcasting effectively allows WhatsApp to intercept and read users’ messages.
The security loophole was discovered by Tobias Boelter, a cryptography and security researcher at the University of California, Berkeley. He told the Guardian: “If WhatsApp is asked by a government agency to disclose its messaging records, it can effectively grant access due to the change in keys.”
The vulnerability is not inherent to the Signal protocol. Open Whisper Systems’ messaging app, Signal, the app used and recommended by whistleblower Edward Snowden, does not suffer from the same vulnerability. If a recipient changes the security key while offline, for instance, a sent message will fail to be delivered and the sender will be notified of the change in security keys without automatically resending the message.
WhatsApp’s implementation automatically resends an undelivered message with a new key without warning the user in advance or giving them the ability to prevent it.
Note that it’s an attack against current and future messages, and not something that would allow the government to reach into the past. In that way, it is no more troubling than the government hacking your mobile phone and reading your WhatsApp conversations that way.
An unnamed “WhatsApp spokesperson” said that they implemented the encryption this way for usability:
In WhatsApp’s implementation of the Signal protocol, we have a “Show Security Notifications” setting (option under Settings > Account > Security) that notifies you when a contact’s security code has changed. We know the most common reasons this happens are because someone has switched phones or reinstalled WhatsApp. This is because in many parts of the world, people frequently change devices and Sim cards. In these situations, we want to make sure people’s messages are delivered, not lost in transit.
He’s technically correct. This is not a backdoor. This really isn’t even a flaw. It’s a design decision that put usability ahead of security in this particular instance. Moxie Marlinspike, creator of Signal and the code base underlying WhatsApp’s encryption, said as much:
Under normal circumstances, when communicating with a contact who has recently changed devices or reinstalled WhatsApp, it might be possible to send a message before the sending client discovers that the receiving client has new keys. The recipient’s device immediately responds, and asks the sender to reencrypt the message with the recipient’s new identity key pair. The sender displays the “safety number has changed” notification, reencrypts the message, and delivers it.
The WhatsApp clients have been carefully designed so that they will not re-encrypt messages that have already been delivered. Once the sending client displays a “double check mark,” it can no longer be asked to re-send that message. This prevents anyone who compromises the server from being able to selectively target previously delivered messages for re-encryption.
The fact that WhatsApp handles key changes is not a “backdoor,” it is how cryptography works. Any attempt to intercept messages in transmit by the server is detectable by the sender, just like with Signal, PGP, or any other end-to-end encrypted communication system.
The only question it might be reasonable to ask is whether these safety number change notifications should be “blocking” or “non-blocking.” In other words, when a contact’s key changes, should WhatsApp require the user to manually verify the new key before continuing, or should WhatsApp display an advisory notification and continue without blocking the user.
Given the size and scope of WhatsApp’s user base, we feel that their choice to display a non-blocking notification is appropriate. It provides transparent and cryptographically guaranteed confidence in the privacy of a user’s communication, along with a simple user experience. The choice to make these notifications “blocking” would in some ways make things worse. That would leak information to the server about who has enabled safety number change notifications and who hasn’t, effectively telling the server who it could MITM transparently and who it couldn’t; something that WhatsApp considered very carefully.
How serious this is depends on your threat model. If you are worried about the US government — or any other government that can pressure Facebook — snooping on your messages, then this is a small vulnerability. If not, then it’s nothing to worry about.
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I am on Valsorda’s side. I don’t like PGP, and I use it as little as possible. If I want to communicate securely with someone, I use Signal.
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Filippo Valsorda wrote an exellent essay on why he’s giving up on PGP. I have long believed PGP to be more trouble than it is worth. It’s hard to use correctly, and easy to get wrong. More generally, e-mail is inherently difficult to secure because of all the different things we ask of it and use it for.
Valsorda has a different complaint, that its long-term secrets are an unnecessary source of risk:
But the real issues, I realized, are more subtle. I never felt confident in the security of my long-term keys. The more time passed, the more I would feel uneasy about any specific key. Yubikeys would get exposed to hotel rooms. Offline keys would sit in a far away drawer or safe. Vulnerabilities would be announced. USB devices would get plugged in.
A long-term key is as secure as the minimum common denominator of your security practices over its lifetime. It’s the weak link.
Worse, long-term key patterns, like collecting signatures and printing fingerprints on business cards, discourage practices that would otherwise be obvious hygiene: rotating keys often, having different keys for different devices, compartmentalization. Such practices actually encourage expanding the attack surface by making backups of the key.
Both he and I favor encrypted messaging, either Signal or OTR.
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Every few years, a researcher replicates a security study by littering USB sticks around an organization’s grounds and waiting to see how many people pick them up and plug them in, causing the autorun function to install innocuous malware on their computers. These studies are great for making security professionals feel superior. The researchers get to demonstrate their security expertise and use the results as “teachable moments” for others. “If only everyone was more security aware and had more security training,” they say, “the Internet would be a much safer place.”
Enough of that. The problem isn’t the users: it’s that we’ve designed our computer systems’ security so badly that we demand the user do all of these counterintuitive things. Why can’t users choose easy-to-remember passwords? Why can’t they click on links in emails with wild abandon? Why can’t they plug a USB stick into a computer without facing a myriad of viruses? Why are we trying to fix the user instead of solving the underlying security problem?
Traditionally, we’ve thought about security and usability as a trade-off: a more secure system is less functional and more annoying, and a more capable, flexible, and powerful system is less secure. This “either/or” thinking results in systems that are neither usable nor secure.
Our industry is littered with examples. First: security warnings. Despite researchers’ good intentions, these warnings just inure people to them. I’ve read dozens of studies about how to get people to pay attention to security warnings. We can tweak their wording, highlight them in red, and jiggle them on the screen, but nothing works because users know the warnings are invariably meaningless. They don’t see “the certificate has expired; are you sure you want to go to this webpage?” They see, “I’m an annoying message preventing you from reading a webpage. Click here to get rid of me.”
Next: passwords. It makes no sense to force users to generate passwords for websites they only log in to once or twice a year. Users realize this: they store those passwords in their browsers, or they never even bother trying to remember them, using the “I forgot my password” link as a way to bypass the system completely — effectively falling back on the security of their e-mail account.
And finally: phishing links. Users are free to click around the Web until they encounter a link to a phishing website. Then everyone wants to know how to train the user not to click on suspicious links. But you can’t train users not to click on links when you’ve spent the past two decades teaching them that links are there to be clicked.
We must stop trying to fix the user to achieve security. We’ll never get there, and research toward those goals just obscures the real problems. Usable security does not mean “getting people to do what we want.” It means creating security that works, given (or despite) what people do. It means security solutions that deliver on users’ security goals without — as the 19th-century Dutch cryptographer Auguste Kerckhoffs aptly put it — “stress of mind, or knowledge of a long series of rules.”
I’ve been saying this for years. Security usability guru (and one of the guest editors of this issue) M. Angela Sasse has been saying it even longer. People — and developers — are finally starting to listen. Many security updates happen automatically so users don’t have to remember to manually update their systems. Opening a Word or Excel document inside Google Docs isolates it from the user’s system so they don’t have to worry about embedded malware. And programs can run in sandboxes that don’t compromise the entire computer. We’ve come a long way, but we have a lot further to go.
“Blame the victim” thinking is older than the Internet, of course. But that doesn’t make it right. We owe it to our users to make the Information Age a safe place for everyone — not just those with “security awareness.”
This essay previously appeared in the Sep/Oct issue of IEEE Security & Privacy.
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