Internet abuse culture is a tech industry problem

After Jesse Frazelle blogged about the online abuse she receives, a common reaction in various forums[1] was "This isn't a tech industry problem - this is what being on the internet is like"[2]. And yes, they're right. Abuse of women on the internet isn't limited to people in the tech industry. But the severity of a problem is a product of two separate factors: its prevalence and what impact it has on people.

Much of the modern tech industry relies on our ability to work with people outside our company. It relies on us interacting with a broader community of contributors, people from a range of backgrounds, people who may be upstream on a project we use, people who may be employed by competitors, people who may be spending their spare time on this. It means listening to your users, hearing their concerns, responding to their feedback. And, distressingly, there's significant overlap between that wider community and the people engaging in the abuse. This abuse is often partly technical in nature. It demonstrates understanding of the subject matter. Sometimes it can be directly tied back to people actively involved in related fields. It's from people who might be at conferences you attend. It's from people who are participating in your mailing lists. It's from people who are reading your blog and using the advice you give in their daily jobs. The abuse is coming from inside the industry.

Cutting yourself off from that community impairs your ability to do work. It restricts meeting people who can help you fix problems that you might not be able to fix yourself. It results in you missing career opportunities. Much of the work being done to combat online abuse relies on protecting the victim, giving them the tools to cut themselves off from the flow of abuse. But that risks restricting their ability to engage in the way they need to to do their job. It means missing meaningful feedback. It means passing up speaking opportunities. It means losing out on the community building that goes on at in-person events, the career progression that arises as a result. People are forced to choose between putting up with abuse or compromising their career.

The abuse that women receive on the internet is unacceptable in every case, but we can't ignore the effects of it on our industry simply because it happens elsewhere. The development model we've created over the past couple of decades is just too vulnerable to this kind of disruption, and if we do nothing about it we'll allow a large number of valuable members to be driven away. We owe it to them to make things better.

[1] Including Hacker News, which then decided to flag the story off the front page because masculinity is fragile

[2] Another common reaction was "But men get abused as well", which I'm not even going to dignify with a response

Anti Evil Maid 2 Turbo Edition

The Evil Maid attack has been discussed for some time - in short, it's the idea that most security mechanisms on your laptop can be subverted if an attacker is able to gain physical access to your system (for instance, by pretending to be the maid in a hotel). Most disk encryption systems will fall prey to the attacker replacing the initial boot code of your system with something that records and then exfiltrates your decryption passphrase the next time you type it, at which point the attacker can simply steal your laptop the next day and get hold of all your data.

There are a couple of ways to protect against this, and they both involve the TPM. Trusted Platform Modules are small cryptographic devices on the system motherboard[1]. They have a bunch of Platform Configuration Registers (PCRs) that are cleared on power cycle but otherwise have slightly strange write semantics - attempting to write a new value to a PCR will append the new value to the existing value, take the SHA-1 of that and then store this SHA-1 in the register. During a normal boot, each stage of the boot process will take a SHA-1 of the next stage of the boot process and push that into the TPM, a process called "measurement". Each component is measured into a separate PCR - PCR0 contains the SHA-1 of the firmware itself, PCR1 contains the SHA-1 of the firmware configuration, PCR2 contains the SHA-1 of any option ROMs, PCR5 contains the SHA-1 of the bootloader and so on.

If any component is modified, the previous component will come up with a different measurement and the PCR value will be different, Because you can't directly modify PCR values[2], this modified code will only be able to set the PCR back to the "correct" value if it's able to generate a sequence of writes that will hash back to that value. SHA-1 isn't yet sufficiently broken for that to be practical, so we can probably ignore that. The neat bit here is that you can then use the TPM to encrypt small quantities of data[3] and ask it to only decrypt that data if the PCR values match. If you change the PCR values (by modifying the firmware, bootloader, kernel and so on), the TPM will refuse to decrypt the material.

Bitlocker uses this to encrypt the disk encryption key with the TPM. If the boot process has been tampered with, the TPM will refuse to hand over the key and your disk remains encrypted. This is an effective technical mechanism for protecting against people taking images of your hard drive, but it does have one fairly significant issue - in the default mode, your disk is decrypted automatically. You can add a password, but the obvious attack is then to modify the boot process such that a fake password prompt is presented and the malware exfiltrates the data. The TPM won't hand over the secret, so the malware flashes up a message saying that the system must be rebooted in order to finish installing updates, removes itself and leaves anyone except the most paranoid of users with the impression that nothing bad just happened. It's an improvement over the state of the art, but it's not a perfect one.

Joanna Rutkowska came up with the idea of Anti Evil Maid. This can take two slightly different forms. In both, a secret phrase is generated and encrypted with the TPM. In the first form, this is then stored on a USB stick. If the user suspects that their system has been tampered with, they boot from the USB stick. If the PCR values are good, the secret will be successfully decrypted and printed on the screen. The user verifies that the secret phrase is correct and reboots, satisfied that their system hasn't been tampered with. The downside to this approach is that most boots will not perform this verification, and so you rely on the user being able to make a reasonable judgement about whether it's necessary on a specific boot.

The second approach is to do this on every boot. The obvious problem here is that in this case an attacker simply boots your system, copies down the secret, modifies your system and simply prints the correct secret. To avoid this, the TPM can have a password set. If the user fails to enter the correct password, the TPM will refuse to decrypt the data. This can be attacked in a similar way to Bitlocker, but can be avoided with sufficient training: if the system reboots without the user seeing the secret, the user must assume that their system has been compromised and that an attacker now has a copy of their TPM password.

This isn't entirely great from a usability perspective. I think I've come up with something slightly nicer, and certainly more Web 2.0[4]. Anti Evil Maid relies on having a static secret because expecting a user to remember a dynamic one is pretty unreasonable. But most security conscious people rely on dynamic secret generation daily - it's the basis of most two factor authentication systems. TOTP is an algorithm that takes a seed, the time of day and some reasonably clever calculations and comes up with (usually) a six digit number. The secret is known by the device that you're authenticating against, and also by some other device that you possess (typically a phone). You type in the value that your phone gives you, the remote site confirms that it's the value it expected and you've just proven that you possess the secret. Because the secret depends on the time of day, someone copying that value won't be able to use it later.

But instead of using your phone to identify yourself to a remote computer, we can use the same technique to ensure that your computer possesses the same secret as your phone. If the PCR states are valid, the computer will be able to decrypt the TOTP secret and calculate the current value. This can then be printed on the screen and the user can compare it against their phone. If the values match, the PCR values are valid. If not, the system has been compromised. Because the value changes over time, merely booting your computer gives your attacker nothing - printing an old value won't fool the user[5]. This allows verification to be a normal part of every boot, without forcing the user to type in an additional password.

I've written a prototype implementation of this and uploaded it here. Do pay attention to the list of limitations - without a bootloader that measures your kernel and initrd, you're still open to compromise. Adding TPM support to grub is on my list of things to do. There are also various potential issues like an attacker being able to use external DMA-capable devices to obtain the secret, especially since most Linux distributions still ship kernels that don't enable the IOMMU by default. And, of course, if your firmware is inherently untrustworthy there's multiple ways it can subvert this all. So treat this very much like a research project rather than something you can depend on right now. There's a fair amount of work to do to turn this into a meaningful improvement in security.

[1] I wrote about them in more detail here, including a discussion of whether they can be used for general purpose DRM (answer: not really)

[2] In theory, anyway. In practice, TPMs are embedded devices running their own firmware, so who knows what bugs they're hiding.

[3] On the order of 128 bytes or so. If you want to encrypt larger things with a TPM, the usual way to do it is to generate an AES key, encrypt your material with that and then encrypt the AES key with the TPM.

[4] Is that even a thing these days? What do we say instead?

[5] Assuming that the user is sufficiently diligent in checking the value, anyway