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WTF is a KDF?

blog.dataparty.xyz

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Re: WTF is a KDF?

#91

Post by nullagent »

tptacek wrote: post #84

For whatever it's worth --- probably not much if you don't do any cryptography engineering --- the definition provided here is of a password KDF. There are other important kinds of KDF, and they don't necessarily have the goal of stretching an untenably low-entropy secret into a facsimile of an actual key. HKDF, for instance, is often the way you extract keys from a Diffie-Hellman exchange.

Thanks for the feedback, I was worried people may take it that way. I've clarified that KDFs are broader than just passwords.

Re: WTF is a KDF?

#92

Post by WhereIsTheTruth »

rhplus wrote: post #39
mr_mitm wrote: post #13

It really annoys me how everyone takes that statement that a 20 character password has been cracked at face value. If it was randomly generated, it is physically impossible to crack even if it was hashed with MD5. It's also unlikely that somebody memorized 20 random characters. It is much more likely that the passphrase was weak because it's the title of a Wikipedia article or contained in some public word list. Also…

He says his BitLocker drive was on a work computer. If it was managed through Active Directory then those key may have been recoverable through his employer. https://learn.microsoft.com/en-us/windows/security/informati...

Little backdoors

Re: WTF is a KDF?

#93

Post by jeroenhd »

vladvasiliu wrote: post #66
jeroenhd wrote: post #46

Earlier quoted context omitted.

TL;DR: secure boot is hard, TPM security is a challenge. FDE works fine with encrypted boot, it's just not the default for Ubuntu. I just ticked the "encrypt my install" box as far as I know, I only found out that my /boot wasn't encrypted after the fact. For evil maid attacks, encrypting /boot or not shouldn't really matter unless there's a known exploit for your bootloader. IMO it doesn't matter much because before…

> Microsoft can get around this problem because their root keys are preloaded into almost every secure boot capable device you can find. They can pre-sign a bootloader and distribute it to hundreds of millions of computers, whereas every Linux distro has to jump through hoops and use the Microsoft-signed loader as an inbetween stage or have the user set up their own keys and signature system. But then, they can also…

> But then, they can also (technically) sign a bogus bootloader if asked nice-enough, like by a state agency.

Definitely! However, the chance of them doing that is relatively small and the usability problem is a lot bigger than the risk of falling victim of an NSA red letter order.

> You can also have TPM + PIN since a few versions of systemd ago. I have set this up on my work laptop and seems to work well enough.

That's great! Do you happen to know a guide on how to set that up? The guides I can find use TPM to auto-unlock OR provide a PIN/password.

Re: WTF is a KDF?

#94

Post by vladvasiliu »

jeroenhd wrote: post #93
vladvasiliu wrote: post #66

Earlier quoted context omitted.

> Microsoft can get around this problem because their root keys are preloaded into almost every secure boot capable device you can find. They can pre-sign a bootloader and distribute it to hundreds of millions of computers, whereas every Linux distro has to jump through hoops and use the Microsoft-signed loader as an inbetween stage or have the user set up their own keys and signature system. But then, they can also…

> But then, they can also (technically) sign a bogus bootloader if asked nice-enough, like by a state agency. Definitely! However, the chance of them doing that is relatively small and the usability problem is a lot bigger than the risk of falling victim of an NSA red letter order. > You can also have TPM + PIN since a few versions of systemd ago. I have set this up on my work laptop and seems to work well enough. Th…

> Do you happen to know a guide on how to set that up?

Here's the Arch Wiki, but since this depends on systemd, it should be universal. The man page of systemd-cryptenroll is actually quite helpful.

https://wiki.archlinux.org/title/Trusted_Platform_Module#sys...

Re: WTF is a KDF?

#95

Post by Aeolun »

mr_mitm wrote: post #45
Aeolun wrote: post #38

Earlier quoted context omitted.

You know this because you think it is mathematically impossible?

Not mathematically, but physically. You need astronomical quantities of energy. Bruce Schneier pointed this out in "Applied Cryptography" (1996): https://www.schneier.com/blog/archives/2009/09/the_doghouse_... (unfortunately the exponents are not displayed correctly here) > One of the consequences of the second law of thermodynamics is that a certain amount of energy is necessary to represent information. To record a…

I’m not sure how we go from 20 character string to 256 bits? Guessing the 20 character string is a lot more likely than cycling through every state of a 256 bit key.

Re: WTF is a KDF?

#96

Post by swapfile »

upofadown wrote: post #44

An interesting bit from the linked ElcomSoft article: >Brute force attacks became not just faster, but much smarter as well. The user’s existing passwords are an excellent starting point. These passwords can be pulled from the user’s Google Account, macOS, iOS or iCloud keychain, Microsoft Account, or simply extracted from the user’s computer. The user’s existing passwords give a hint at what character groups are lik…

This is why you let a computer choose. By doing that you end up forcing the attacker to brute force, which we all know is mathematically unfeasible given enough entropy.

Re: WTF is a KDF?

#97

Post by Gasp0de »

jeroenhd wrote: post #26
Gasp0de wrote: post #22

I think the most likely situation is this: Bitlocker can save backup keys in the cloud where they can probably be subpoenad by the police. If the victim used a password manager without encryption (e.g. browser) and used or saved the same password somewhere else, they might have gotten it this way.

Bitlocker saves the recovery key to the cloud, not the unlock PIN. Both can be used to recover access to the data, but the recovery key is not directly related to the password chosen. Furthermore, the article talks about LUKS, which is decidedly not related to Bitlocker. I doubt they would've spent the ridiculous amount of money you need to crack LUKS though. Most likely, the password was a phrase that could be brute…

I'm sorry if my comment was unclear. The person whose laptops were confiscated by the police stated that his bitlocker encrypted windows pc was also cracked by the police.

My point was: They could have gotten the bitlocker recovery key, then found his password somewhere on the windows pc and used that to "crack" the LUKS encrypted partition.

Re: WTF is a KDF?

#98

Post by Gasp0de »

dist-epoch wrote: post #36
Gasp0de wrote: post #22

I think the most likely situation is this: Bitlocker can save backup keys in the cloud where they can probably be subpoenad by the police. If the victim used a password manager without encryption (e.g. browser) and used or saved the same password somewhere else, they might have gotten it this way.

That's not true, Bitlocker offers the option to save the encryption keys. It's your choice and it's clearly labeled: https://www.top-password.com/blog/wp-content/uploads/2018/10... Besides, the user also had a Windows laptop encrypted with Bitlocker, and the police failed to crack that one (see top comment).

I think you are mistaken, the original article in french states that the BitLocker computer was not cracked by the local police, but was accessed later by some cyber-department of the police: https://nantes.indymedia.org/posts/87395/une-lettre-divan-en....

You are right that you can choose to backup recovery keys to the cloud, I just guessed that this would be one possible scenario.

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