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About the contributor
Duff Johnson

A veteran of the electronic document space, Duff Johnson is an independent consultant, Executive Director of the PDF Association and ISO Project co-Leader (and US TAG chair) for ISO 32000 and ISO 14289.
More contributions
Talking about electronic documents

We’ve done PDF Day events and technical conferences across Europe, in the US, in Australia, and elsewhere. This Electronic Document Conference is the first PDF Association event that’s open to all technologies pertaining to documents. It’s about explor …

Happy new logo!

2006: The PDF/A Competence Center A new year brings new things, and 2019 is no exception! The “four red blocks” logo was first created for the PDF/A Competence Center in 2006. When that organization became the PDF Association in 2011, the design was ad …

Save-The-Date: PDF Day France, Toulouse, April 4, 2019

PDF Day France will be the first French-speaking event of the PDF Association, organised by our member ORPALIS. It will take place in Toulouse which is the home ground of Airbus and we are very happy that Airbus will present a case study around its usage of PDF in their document management environment!

Electronic Document Conference: Call for Papers

Prospective presenters at the Electronic Document Conference 2019 are invited to submit high-quality original proposals for 25-minute presentations on subjects of interest to developers and technical product managers concerned with electronic document implementations.

Have we passed ‘peak PDF’?

How do we gain insight into how users’ views of documents are shifting? Google Trends is an increasingly interesting source of high-level marketplace data. By aggregating Google’s search data over time, reporting a term’s popularity as compared with all other searches.

SHA-1 is gone… and it’s deprecated in PDF 2.0 as well

Collision attack: same hashes!As we reported yesterday, those in the crypto world are well-aware that Google and others have proven that SHA-1, the venerable cryptographic hash function standard, is dead. The consequences, however, are yet to be determined. Somewhere, it is safe to assume, between very very bad and catastrophic.

Among many other potential points of disruption, this recently announced SHA-1 collision attack, as PC World reported, can break code repositories that use the Subversion (SVN) revision control system. To prove the point, it seems, the WebKit browser engine repository became corrupted after someone committed two different PDF files with the same SHA-1 hash to it.

The team responsible for uncovering this vulnerability state on their website, SHAttered.io:

  • It is now practically possible to craft two colliding PDF files and obtain a SHA-1 digital signature on the first PDF file which can also be abused as a valid signature on the second PDF file.
  • For example, by crafting the two colliding PDF files as two rental agreements with different rent, it is possible to trick someone to create a valid signature for a high-rent contract by having him or her sign a low-rent contract.

The could have used a variety of different file-types to make this point, but PDF files, of course, contain more information and benefit from more trust.

What does this have to do with PDF?

The PDF specification up through ISO 32000-1 (PDF 1.7) allowed use of SHA-1 for a variety of hashing functions. However, in PDF 2.0, SHA-1 is formally deprecated for use in digital signatures. This means that a PDF 2.0 writer should not use SHA-1 to make a message digest, and a PDF 2.0 reader may reject signatures that still use SHA-1.

Does this mean that PDFs that were signed using the SHA-1 algorithm in the past suddenly become invalid? In principle, it is now proven that the contents of such a PDF can be changed without invalidating the signature. However, the problem only exists in situations where companies didn’t upgrade their document systems to the latest standards.

PDF 2.0 to the rescue!

“For those who are stuck with SHA-1 in their existing repositories of PDF documents, PDF 2.0’s new Document Security Store (DSS) including Validation-Related Information (VRI), as well as a document time-stamp (DTS) signature,” says iText founder Bruno Lowagie. “The document time-stamp signature (subtype ETSI.RFC3161) is an additional signature that should use a more recent hashing algorithm to create the message digest. The procedure of adding a DSS and a document time-stamp should be repeated before the certificate of the last signature that was added expires, or when there are indications that the algorithms that were used, whether the cryptographic hash function or the encryption algorithm, could be jeopardized,” Lowagie said.

What PDF developers can do about it

PDF 2.0 interop flyer coverPDF developers can test their software – and its response to SHA-1 – in the PDF 2.0 context long before it gets to customers. In January, the PDF Association announced two PDF 2.0 interop workshop events in the UK, and USA, to help PDF developers test their PDF 2.0 files or implementations against others.

The death of SHA-1 makes an excellent case for testing new encryption models and circumstances. Billions of PDF files worldwide rely on secure digital signatures, encryption and other features that use hashing to disambiguate documents. PDF 2.0 is an excellent opportunity to negate this risk for PDF users.


Tags: SHA-1, hash
Categories: Archives & Libraries, DRM, Digital Signatures, Document Management, PDF 2.0