9 ms·

An analogy that was useful for explaining part of this to my (non-technical) father. Maybe others will find it helpful as well. Imagine that you want to know w

by hyperion2010 9y ago

An analogy that was useful for explaining part of this to my (non-technical) father. Maybe others will find it helpful as well. Imagine that you want to know whether someone has checked out a particular library book. The library refuses to give you access to their records and does not keep a slip inside the front cover. You can only see the record of which books you have checked out. What you do is follow the person of interest into the library whenever they return a book. You then ask the librarian for a copy of the books you want to know whether the person has checked out. If the librarian looks down and says "You are in luck, I have a copy right here!" then you know the person had checked out that book. If the librarian has to go look in the stacks and comes back 5 minutes later with the book, you know that the person didn't check out that book (this time). The way to make the library secure against this kind of attack is to require that all books be reshelved before they can be lent out again, unless the current borrower is requesting an extension. There are many other ways to use the behavior of the librarian and the time it takes to retrieve a book to figure out which books a person is reading. edit: A closer variant. Call the library pretending to be the person and ask for a book to be put on hold. Then watch how long it takes them in the library. If they got that book they will be in and out in a minute (and perhaps a bit confused), if they didn't take that book it will take 5 minutes.

technics256 9y ago
Thank you for this. Would you say this applies to both Spectre and Meltdown, or one and not the other?
- fooker 9y ago
  This is a general explanation of side channel attacks, as I understand.
  - ilitirit 9y ago
    A timing attack is one type of side channel attack. These types of timing attacks can also be used against poor/unsuitable crypto functions, or even some processes involving general computation e.g. If it takes longer to reject input A than input B, you can reason that input A is closer to the answer (similar to someone reading a paragraph until they reach the first error). Other side-channel attacks can come in the form of analysing network data, power-consumption (CPUs use more power when they are "busier")... even noise (listen for when the fans start spinning up).
    - AlphaSite 9y ago
      Is blinding the only real solution to most of these attacks against Crypto?
  - craigyk 9y ago
    yeah, I don't think it is a perfect analogy for Meltdown, I'll try one, someone correct me if I'm misunderstanding Meltdown. Let's say you want to know if your boss is away on vacation next week so you call their admin and say "you need to double-check my contact info if the boss is going to be out next week". They load up the boss' calendar to check and based on his presence next week then load up your info. Only once done, do they take the time to remember the boss didn't want you to know wether they are in or out. So you hear back, "sorry, can't tell you that, but you follow up with "OK, well can you still double check that my phone number is..." If they respond quickly with a yes, then your file is still on their screen and the boss is in fact out next week. If there is a short pause while they look it up, then the opposite.
  - hyperion2010 9y ago
    Yes, this is a general explanation of side channel attacks against some kind of caching. The more specific example tries to be closer to the type of situation that happens in Spectre, but it is not a direct analogy.
prab97 9y ago
I don't understand how this info can be used for getting what was inside the book? If my understanding of your explanation is correct, book name is analogous to memory address. When the victim (legit process) returned the book with name X (called free on the mem block X), the librarian (OS) erased all pages of the book and repurposed it for printing another book before handing it out to the evil dude(snoopy process).
- tamriel 9y ago
  The person checking out the book is a program, so they aren't the brightest. They check out the book called "how to go to facebook.com". Then they check out "how to type a password". Then they check out "Typing '1234' for Dummies". I bet you'll never figure out how to get into their facebook account.
- myrmi 9y ago
  My attempt, assuming that the books only contain one character each: The librarian has a list of books you're not allowed to take out. You request one of those books (book X), but it takes a while for search to run to see whether you're allowed to or not. While you're waiting, you say "actually, I'm not really interested in taking out book X, but if the content of that book is 'a', I'd like to take out book Y. If the content of that book is 'b', I'd like to take out book Y+1, and so on". The librarian is still waiting for the search to complete to see if you can take out book X, but doesn't have anything better to do, so looks inside it, sees that the letter is 'b', and goes and gets book Y+1 so she can hand it over to you. Now, the original check to see if you can take the first book out completes, and the librarian says "I'm sorry, I can't let you have book X, and I can't give you the book I fetched that you are allowed to take out, otherwise you'd know the content of the forbidden book." Now, you request book 'Y', which you are allowed. The librarian goes away for a few minutes, and returns with book 'Y', and hands it over to you. You request book 'Y+1', and she hands it over immediately. You request book 'Y+2', and she goes away for a few minutes again, and hands it over. You now know that Y+1 was (probably) the book she fetched when you made the forbidden request, and therefore that the letter inside the forbidden book was 'b'.
  - 9y ago
deleted 9y ago
[deleted]
314 9y ago
Your analogy is more apt for side-channel attacks in general. Here is a more specific version for Meltdown: A library has two rooms, one for general books and one for restricted books. The restricted books are not allowed out of the library, and no notes or recordings are allowed to be taken out of the restricted room. An attacker wants to sneak information out of the restricted room. To do this the pick up a pile of non-restricted books and go into the restricted room. Depending on what they read in there they rearrange the pile of non-restricted books into a particular order. A guard comes along and sees them, they are thrown out of the restricted room and their pile of non-restricted books is put on the issue desk ready to be put back into circulation. Their conspirator looks at the order of the books on the issue desk and decodes a piece of information about the book in the restricted room. They repeat this process about 500000 times a second until they have transcribed the secret book.
- lma21 9y ago
  What is the analogy behind being able to go into the restricted room?
  - foota 9y ago
    You can call into the kernel. edit: s/call into/trigger a syscall/
    - IshKebab 9y ago
      You don't even need to do that for meltdown.
  - 314 9y ago
    The restricted room is the part of the machine behind the protection. Memory reads are not checked at the tine access. They are checked when the instruction retires.
    - AlphaSite 9y ago
      On intel* this isn’t a property intrinsic to superscalar processors, other architectures check it in flight or while it’s in the issue queue, preventing this side channel.
pgt 9y ago
Fantastic explanation of cache timing attacks. This morning I was explaining spectre to non-technical people and let me tell you, "leaking L1 CPU cache memory," is a real party starter. So I'm using there librarian example going forward.
- pgt 9y ago
  I used your explanation in a longer note, "Spectre: How do side-channel attacks work?"[^1] to try and explain how side-channel attacks work (partly to myself, and partly to non-hackers). [^note]: https://www.facebook.com/notes/petrus-theron/spectre-how-do-side-channel-attacks-work/10154397550352465/?pnref=story https://www.facebook.com/notes/petrus-theron/spectre-how-do-...
  - hyperion2010 9y ago
    Thanks for the heads up!

technics256 9y ago

Thank you for this. Would you say this applies to both Spectre and Meltdown, or one and not the other?

fooker 9y ago
This is a general explanation of side channel attacks, as I understand.
- ilitirit 9y ago
  A timing attack is one type of side channel attack. These types of timing attacks can also be used against poor/unsuitable crypto functions, or even some processes involving general computation e.g. If it takes longer to reject input A than input B, you can reason that input A is closer to the answer (similar to someone reading a paragraph until they reach the first error). Other side-channel attacks can come in the form of analysing network data, power-consumption (CPUs use more power when they are "busier")... even noise (listen for when the fans start spinning up).
  - AlphaSite 9y ago
    Is blinding the only real solution to most of these attacks against Crypto?
- craigyk 9y ago
  yeah, I don't think it is a perfect analogy for Meltdown, I'll try one, someone correct me if I'm misunderstanding Meltdown. Let's say you want to know if your boss is away on vacation next week so you call their admin and say "you need to double-check my contact info if the boss is going to be out next week". They load up the boss' calendar to check and based on his presence next week then load up your info. Only once done, do they take the time to remember the boss didn't want you to know wether they are in or out. So you hear back, "sorry, can't tell you that, but you follow up with "OK, well can you still double check that my phone number is..." If they respond quickly with a yes, then your file is still on their screen and the boss is in fact out next week. If there is a short pause while they look it up, then the opposite.
- hyperion2010 9y ago
  Yes, this is a general explanation of side channel attacks against some kind of caching. The more specific example tries to be closer to the type of situation that happens in Spectre, but it is not a direct analogy.

prab97 9y ago

I don't understand how this info can be used for getting what was inside the book? If my understanding of your explanation is correct, book name is analogous to memory address. When the victim (legit process) returned the book with name X (called free on the mem block X), the librarian (OS) erased all pages of the book and repurposed it for printing another book before handing it out to the evil dude(snoopy process).

tamriel 9y ago
The person checking out the book is a program, so they aren't the brightest. They check out the book called "how to go to facebook.com". Then they check out "how to type a password". Then they check out "Typing '1234' for Dummies". I bet you'll never figure out how to get into their facebook account.
myrmi 9y ago
My attempt, assuming that the books only contain one character each: The librarian has a list of books you're not allowed to take out. You request one of those books (book X), but it takes a while for search to run to see whether you're allowed to or not. While you're waiting, you say "actually, I'm not really interested in taking out book X, but if the content of that book is 'a', I'd like to take out book Y. If the content of that book is 'b', I'd like to take out book Y+1, and so on". The librarian is still waiting for the search to complete to see if you can take out book X, but doesn't have anything better to do, so looks inside it, sees that the letter is 'b', and goes and gets book Y+1 so she can hand it over to you. Now, the original check to see if you can take the first book out completes, and the librarian says "I'm sorry, I can't let you have book X, and I can't give you the book I fetched that you are allowed to take out, otherwise you'd know the content of the forbidden book." Now, you request book 'Y', which you are allowed. The librarian goes away for a few minutes, and returns with book 'Y', and hands it over to you. You request book 'Y+1', and she hands it over immediately. You request book 'Y+2', and she goes away for a few minutes again, and hands it over. You now know that Y+1 was (probably) the book she fetched when you made the forbidden request, and therefore that the letter inside the forbidden book was 'b'.
- 9y ago

deleted 9y ago

[deleted]

314 9y ago

Your analogy is more apt for side-channel attacks in general. Here is a more specific version for Meltdown: A library has two rooms, one for general books and one for restricted books. The restricted books are not allowed out of the library, and no notes or recordings are allowed to be taken out of the restricted room. An attacker wants to sneak information out of the restricted room. To do this the pick up a pile of non-restricted books and go into the restricted room. Depending on what they read in there they rearrange the pile of non-restricted books into a particular order. A guard comes along and sees them, they are thrown out of the restricted room and their pile of non-restricted books is put on the issue desk ready to be put back into circulation. Their conspirator looks at the order of the books on the issue desk and decodes a piece of information about the book in the restricted room. They repeat this process about 500000 times a second until they have transcribed the secret book.

lma21 9y ago
What is the analogy behind being able to go into the restricted room?
- foota 9y ago
  You can call into the kernel. edit: s/call into/trigger a syscall/
  - IshKebab 9y ago
    You don't even need to do that for meltdown.
- 314 9y ago
  The restricted room is the part of the machine behind the protection. Memory reads are not checked at the tine access. They are checked when the instruction retires.
  - AlphaSite 9y ago
    On intel* this isn’t a property intrinsic to superscalar processors, other architectures check it in flight or while it’s in the issue queue, preventing this side channel.

pgt 9y ago

Fantastic explanation of cache timing attacks. This morning I was explaining spectre to non-technical people and let me tell you, "leaking L1 CPU cache memory," is a real party starter. So I'm using there librarian example going forward.

pgt 9y ago
I used your explanation in a longer note, "Spectre: How do side-channel attacks work?"[^1] to try and explain how side-channel attacks work (partly to myself, and partly to non-hackers). [^note]: https://www.facebook.com/notes/petrus-theron/spectre-how-do-side-channel-attacks-work/10154397550352465/?pnref=story https://www.facebook.com/notes/petrus-theron/spectre-how-do-...
- hyperion2010 9y ago
  Thanks for the heads up!