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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

#61

Post by nightpool »

tptacek wrote: post #54

Signed OOXML Word files are ZIP files defined by what appear to be 3 manifests: a content-types.xml file, a document.rels.xml file that maps symbolic IDs to filenames, and the "Package Info" section of the signature block, which supplies hashes for all signed files. The rendering of an OOXML Word file starts from "document.xml". Here are the attacks, as I understand them: 1. OOXML doesn't sign content-types.xml. It a…

There's also Attack #6, which doesn't get front-line billing in the paper but I think does a great job exemplifying how much of a security theater this whole product space of "signed documents" is:

"We discovered that on macOS, it is sufficient to include a `sig1.xml` [file] without any content to force the application to show a security banner stating that the document is protected by a signature"

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

#62

Post by nightpool »

"Signed documents" are a joke and everybody knows it. As the XKCD goes:

    How to use PGP to verify that an email is authentic:
    Look for this text at the top
    -----BEGIN PGP SIGNED MESSAGE-----
    If it's there, the email is probably fine
The abstract only mentions it offhandedly, but I think the least emphasized bug in their paper is actually the most important, because it really drives this point home:

"We discovered that on macOS, it is sufficient to include a sig1.xml without any content to force the application to show a security banner stating that the document is protected by a signature"

BEGIN PGP SIGNED MESSAGE indeed.

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

#63

Post by [deleted] »

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…

[deleted]

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

#64

Post by magicalhippo »

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.

Here in Norway, I've had to use my banks 2-factor BankID[1] to sign my loan, house purchase contract etc. According to their documentation[2] it absolutely relies on cryptography, including SEID-SDO[3]

[1]: https://www.bankid.no/bedrift/bankid-signering/

[2]: https://confluence.bankidnorge.no/confluence/pdoidclc/techni...

[3]: https://www.nets.eu/developer/E-Signing/overview/Pages/Signe...

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

#65

Post by D13Fd »

ClarityJones wrote: post #37
tpurves wrote: post #21

Earlier quoted context omitted.

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 scal…

The question of signing generally arises with dueling forms, where there's no dispute that "a" 20-page contract was signed on June 4, 2017, and that this is page 20, with all of the signatures, but... What did that contract say in Paragraph G on Page 12? Perhaps the bank only retained page 20, and is saying that they used the same master form for every mortgage and the master form says "this" on page 12. Or one party…

Signing a PDF via "/s/ Name E. Name" and sending it via e-mail solves the "lost pages" issue, and doesn't require the parties to use cryptographic techniques.

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

#66

Post by bogeholm »

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’ve found the per-country overview of general legality of digital signatures on DocuSign [0] quite informative.

As an example, in DK an email constitutes a valid contract for most purposes.

[0] https://www.docusign.com/products/electronic-signature/legal...

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

#67

Post by D13Fd »

c00lio wrote: post #31
logifail wrote: post #20

Earlier quoted context omitted.

> In general [..] ...but apparently not always. Not something I'd want to bet my business or reputation on. "AUSTRIA’s Federal Administrative Court [..] declared a framework contract from Austrian Federal Railways (ÖBB) to Stadler for the delivery of up to 186 double-deck trains to be null and void due to an alleged formal error in the qualified electronic signature of the offer." (September 2021) https://www.railjou…

That is about a public tender, where there are "form requirements", meaning that certain documents and signatures have to conform to the required form. Usually that means "in writing, on paper, with signatures made by hand" or the new legally accepted replacement of secure electronic signatures. Sometimes it might even mean signing the paper in front of a notary, e.g. for contracts regarding the sale of real estate.…

> But in general, outside clearly defined exceptions, contracts and declarations do not have a form requirement, so even oral agreement or "conclusive behaviour" (sign language, point-and-grunt, silently carrying an item to the cash register) are absolutely valid.

Correct in the US, although it's worth mentioning the Statute of Frauds and the Uniform Commercial Code:

https://en.wikipedia.org/wiki/Statute_of_frauds

https://en.wikipedia.org/wiki/Uniform_Commercial_Code

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

#68

Post by riceart »

D13Fd wrote: post #65
ClarityJones wrote: post #37

Earlier quoted context omitted.

The question of signing generally arises with dueling forms, where there's no dispute that "a" 20-page contract was signed on June 4, 2017, and that this is page 20, with all of the signatures, but... What did that contract say in Paragraph G on Page 12? Perhaps the bank only retained page 20, and is saying that they used the same master form for every mortgage and the master form says "this" on page 12. Or one party…

Signing a PDF via "/s/ Name E. Name" and sending it via e-mail solves the "lost pages" issue, and doesn't require the parties to use cryptographic techniques.

Without cryptographic techniques that is all easily forged.

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

#69

Post by D13Fd »

eduction wrote: post #55
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…

And for thousands of years humans got on with no written contracts at all. That doesn’t change the fact that as society evolves there are good reasons for agreements to evolve with it. I’m sure the first hundred years or so of written contracts there were people rolling their eyes at the idea and others “doing it wrong”. It’s possible (likely?) we’ll reach a point where strong properly implemented digital signatures…

> This stuff is hard but probably worth it ultimately.

I think you're understating "hard."

What about the large proportion of contracts that are still signed in paper? (All of those waivers you fill out at the doctor's office? Paper. Buy a car? Paper. Mortgage? Paper. Hire a contractor for your house? Paper.) Is your scheme going to render those invalid? Are we outlawing paper agreements?

Consider also:

* Many people do not have a computer or smartphone.

* Any scheme (particularly one that does not involve pretty hardcore identify verification) is going to suffer from most of the same limitations as a regular e-signature, in that either side can claim that it wasn't actually them who clicked the button.

* Technology changes rapidly and the entities that are most involved in contracts change very slowly.

* Corporations have a huge incentive to push back on anything that adds friction to customers signing contracts.

Do you think we'll really push through all of that, just to fight the extremely uncommon circumstance where a party denies that they signed a contract? Especially where cryptographic signatures wouldn't even fully solve the problem? I think it's unlikely.

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

#70

Post by toast0 »

riceart wrote: post #68
D13Fd wrote: post #65

Earlier quoted context omitted.

Signing a PDF via "/s/ Name E. Name" and sending it via e-mail solves the "lost pages" issue, and doesn't require the parties to use cryptographic techniques.

Without cryptographic techniques that is all easily forged.

The point is forgery of signatures and contracts is not typically an issue.

In contract disputes, there's usually no dispute of if a contract was signed. Sometimes there's a dispute over which contract was signed, but then each party may have a signature on a contract or not. Much more often there's no disagreement on the contract or that it was signed, but on the terms.

It's nice that electronic signing can solve the issue of validity of signatures, but it's not that big of a deal, because it wasn't that much of an issue; and that's why e-signing has devolved into 'click a button to enter a signature' without any sort of cryptography.

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