Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

usenix.org

Post reply on HN
Print view View original

71–80 of 158 posts

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#71

Post by mati365 »

hannob wrote: post #57

I feel there are some fundamental misconceptions about signatures that are baked into many of these older standards. There have been similar attacks on XML signatures, email signatures and PDF files (with some overlapping authors). It usually comes down to creating some complex formats where signatures are allowed to sign sub-parts of the information. This is an extremely fragile design, and should probably be consid…

Isn't the file part of the file system which can be file too? Basically everything everywhere is broken.

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#72

Post by GranPC »

mati365 wrote: post #71
hannob wrote: post #57

I feel there are some fundamental misconceptions about signatures that are baked into many of these older standards. There have been similar attacks on XML signatures, email signatures and PDF files (with some overlapping authors). It usually comes down to creating some complex formats where signatures are allowed to sign sub-parts of the information. This is an extremely fragile design, and should probably be consid…

Isn't the file part of the file system which can be file too? Basically everything everywhere is broken.

People typically do not share documents, such as signed contracts, by providing an image of their entire filesystem containing the signed file.

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#73

Post by fatfingerd »

hannob wrote: post #57

I feel there are some fundamental misconceptions about signatures that are baked into many of these older standards. There have been similar attacks on XML signatures, email signatures and PDF files (with some overlapping authors). It usually comes down to creating some complex formats where signatures are allowed to sign sub-parts of the information. This is an extremely fragile design, and should probably be consid…

Yes though there are other large classes of error, I've had editor makers insist that they want their signed document format to include code and be signable. "Is that date fixed or calculated from the current time?"

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#74

Post by kjetil »

phrz wrote: post #33
tialaramex wrote: post #27

Earlier quoted context omitted.

Usually the distinction made is between electronic signature (e.g. you scrawled your mark on a piece of paper, scanned it and now the software lets you paste that image on signature pages) and digital signature (cryptographic)

The ESIGN Act makes no distinction in the US between whether you cryptographically sign or digitally draw, etc., defining a signature as "an electronic sound, symbol, or process, attached to or logically associated with a contract or other record and executed or adopted by a person with the intent to sign the record."

In the EU (and EEA), eIDAS distinguishes between

- "electronic signatures", which can be any electronic data used to sign, like a drawn signature - "advanced electronic signature" (AdES), usually a type of digital signature (XML-DSig, PDF signature, etc.) - "qualified electronic signature (QES), which is a digital signature created by a certified signature device

QES is legally equivalent to a "wet signature", but in my experience rarely used because of cost. AdES is much more common for high-trust scenarios like loan applications. For low-trust like package delivery, a signature (or smiley) drawn on a touch device will usually do.

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#75

Post by bee_rider »

fatfingerd wrote: post #73
hannob wrote: post #57

I feel there are some fundamental misconceptions about signatures that are baked into many of these older standards. There have been similar attacks on XML signatures, email signatures and PDF files (with some overlapping authors). It usually comes down to creating some complex formats where signatures are allowed to sign sub-parts of the information. This is an extremely fragile design, and should probably be consid…

Yes though there are other large classes of error, I've had editor makers insist that they want their signed document format to include code and be signable. "Is that date fixed or calculated from the current time?"

Did they give any justification?

I guess hypothetically in an extremely niche situation— signed documents for coders who can actually read the code and decide they want to sign whatever it output—that could make sense… but I can’t imagine any market for such a tool.

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#76

Post by CobrastanJorji »

D13Fd wrote: post #11

> For documents of prime importance, such as contracts and invoices Few in the legal world actually use cryptographic signatures for signing things. It's vastly more common to use scanned hand signatures or just /s/ and an e-mail record of sign off. Why? Because it has worked that way for hundreds of years. It's pretty uncommon for there to be a dispute about the fact of signature, and even if there is, cryptographic…

Same thing with the notarization system. Is the notary system foolproof? Absolutely not! Imagine how much better notarizations could be with a bit of technology around it. A digital notary could take a photo of your document, register a hash/summary/thumbnail in realtime, and make it so that backdating or changing the document after the fact is a technical impossibility.

What do we use instead? A guy who's probably employed by the signatory promises that he stamped the document on a certain date.

But here's the thing: the current system works well enough. Sure, as crypto enthusiasts and programmers, we take offense at the current system and immediately start dreaming up notary camera devices with tamper-resistant hardware and image hashes on blockchains and whatnot, but why go and build a fancy, hard-to-fool system unless somebody actually needs it?

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#77

Post by fatfingerd »

bee_rider wrote: post #75
fatfingerd wrote: post #73

Earlier quoted context omitted.

Yes though there are other large classes of error, I've had editor makers insist that they want their signed document format to include code and be signable. "Is that date fixed or calculated from the current time?"

Did they give any justification? I guess hypothetically in an extremely niche situation— signed documents for coders who can actually read the code and decide they want to sign whatever it output—that could make sense… but I can’t imagine any market for such a tool.

They didn't want to upset people who used macros for footer page numbers and other boilerplate. Naturally the general code capabilities preceded the interest in adding signatures.

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#78

Post by est31 »

GranPC wrote: post #72
mati365 wrote: post #71

Earlier quoted context omitted.

Isn't the file part of the file system which can be file too? Basically everything everywhere is broken.

People typically do not share documents, such as signed contracts, by providing an image of their entire filesystem containing the signed file.

They also don't want to share two files, one for the file, one for the signature. Ideally, it's integrated, which means there needs to be some management. And ideally, it's also applied to the data before it's compressed, so that you can apply advanced compression and deduplication to archived documents like zopfli for example.

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#79

Post by denton-scratch »

psychphysic wrote: post #24
layer8 wrote: post #12

Earlier quoted context omitted.

At least in Europe, electronic signatures of a certain form are legally equivalent to handwritten signatures [0], and are increasingly used as such. Emails don’t provide that legal value. [0] https://en.wikipedia.org/wiki/EIDAS

In the UK at least it you email agreeing to terms and then appear to be complying you're unlikely to get out of it. Common examples are someone is sent a contract of employment unfortunately often after starting and they don't sign it. If they have been coming into work broadly in line with that contract so long as it's fair, employee and employer are bound by it. Here is an interesting edge case in the UK [0]. Long…

I don't know about other places; but in the UK, a contract doesn't even have to be written down, let alone signed.

Of course, an unwritten contract is no more valuable than the paper it's (not) written on; and either party can dispute the terms. But you can still make a valid contract with a verbal agreement and a shake of hands. But don't do this unless you trust your co-contractor!

Re: Every Signature Is Broken: Insecurity of Microsoft Office’s Ooxml Signatures

#80

Post by bee_rider »

fatfingerd wrote: post #77
bee_rider wrote: post #75

Earlier quoted context omitted.

Did they give any justification? I guess hypothetically in an extremely niche situation— signed documents for coders who can actually read the code and decide they want to sign whatever it output—that could make sense… but I can’t imagine any market for such a tool.

They didn't want to upset people who used macros for footer page numbers and other boilerplate. Naturally the general code capabilities preceded the interest in adding signatures.

Interesting!

The page numbers seem important, in the sense that the sudden removal of a bunch of pages from a contract ought to be easy to detect. But maybe your co-workers were so fundamentally honest that they couldn’t think of the duplicitous applications there.

Post reply on HN