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

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

#21

Post by tpurves »

layer8 wrote: post #12
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…

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

I think the main point the parent comment is making is that, today and historically, the great majority of contract disputes today don't involve disputing (analog or digital) signatures. People usually dispute about all the other terms rather that about the act of having signed it. [Even though analog signatures aren't particularly secure either]

However, if Microsoft signatures have now became more forgeable at scale, perhaps we could see some rash of fake contract fraud in the future where disputing fact of signature becomes more common.

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

#22

Post by ipython »

nailer wrote: post #8

I've heard the same for Windows code signing certs - that's it possible to modify the payload and have the signature still apply.

In the Authenticode spec, there is an unauthenticatedAttrbutes structure, the contents of which does not affect the resulting signature.

Dropbox’s installer used it a while ago (maybe still, haven’t checked): https://news.ycombinator.com/item?id=8204454

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

#23

Post by c00lio »

layer8 wrote: post #12
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…

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

EIDAS isn't worth the paper (pun intended) it is written on, if insecure "click a link in an email"-signature schemes such as https://help.eversign.com/hc/en-us/articles/6664408666013-Is... can claim compliance...

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

#24

Post by psychphysic »

layer8 wrote: post #12
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…

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 story short if you give someone the ability to sign on your behalf and appear to consider parties bound by that it's binding.

In a personal sense, I send out appointment letters but my secretary does it for me. I consider myself as bound to that as if I arranged it myself. I gave the secretary latitude to book appointments for me, and I usually turn up to those appointments. If that letter is signed, or if it's by my hand. Doesn't legally matter as much as you'd imagine.

[0] https://www.lexisnexis.co.uk/blog/banking-and-finance/gordon...

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

#26

Post by 1MachineElf »

c00lio wrote: post #23
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

EIDAS isn't worth the paper (pun intended) it is written on, if insecure "click a link in an email"-signature schemes such as https://help.eversign.com/hc/en-us/articles/6664408666013-Is... can claim compliance...

Disclaimer: I'm not familiar with this service or EIDAS, so my comment might be entirely off the mark.

The decision to require users to use an emailed link to view and sign a message could stem from DLP requirements. Emails are plain-text and therefore any sensitive data leaked could be irreversibly exposed. By keeping the messages inside a system that requires authentication, there is less likelihood that a someone besides the intended recipient will interact with the message. Such systems also support auditability and DLP scanning.

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

#27

Post by tialaramex »

loeg wrote: post #17
galleywest200 wrote: post #14

Earlier quoted context omitted.

I have been asked to e-sign pretty much every building/home leasing document involving myself in the past few years in the US. I am sure that this is also the case for thousands, if not millions, of other Americans.

Sure, but do you have any indication cryptography is involved in this process? In my (limited) experience as a consumer, they just take some existing PDF and add marks on top of it.

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)

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

#28

Post by tialaramex »

munchler wrote: post #25

Does this affect the security of password-protected Office documents as well? I think there's lots of valuable information locked up with simple Excel or Word passwords.

I expect not, Office last century previously had a very simple password scheme (broken essentially instantly with modern tools) and then a poor one (take a couple seconds to break typical passwords) before I think settling on a more modern design where brute force isn't practical for a decent password.

It's possible they threw that away to use this approach but seems unlikely

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

#29

Post by justincormack »

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…

"An Englishman's word is his bond" as the old saying goes, verbal contracts are enforceable. The whole financial system used to run on purely verbal contracts. There are some requirements, there has to be a consideration for example. I am not sure this is true in other legal systems though.

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

#30

Post by layer8 »

c00lio wrote: post #23
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

EIDAS isn't worth the paper (pun intended) it is written on, if insecure "click a link in an email"-signature schemes such as https://help.eversign.com/hc/en-us/articles/6664408666013-Is... can claim compliance...

I don’t know where you’re taking “click a link in an email” from, but that’s decidedly not how eIDAS works. The private keys must be hardware-protected and under the sole control of an identified signer, which usually means either a physical token or a 2FA-protected remote key in an HSM. In addition, a prior ID verification procedure like video ID or a face-to-face verification is required.
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