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hello everyone and welcome to the talk

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breaking the specification

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pdf certification my name is simon and

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this was a joint work with vladislav

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christian and york

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we are all researchers at whole

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university boho

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in the next 50 minutes we talk about pdf

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certification and how to use features

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and certification

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to change the content in a certified

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document

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we start with the basics and take a look

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on the pdf structure

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incremental updates talk about the

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differences between site and certified

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documents

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and take a look on the permission levels

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in certified documents

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so on the right side you see a simple

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pdf document and each pdf document start

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with the header

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so the header defines a pdf version

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the header is followed by the body and

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contains objects of the pdf document

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with unique object numbers so it starts

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with the object of type catalog

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which reference to the pages object

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and the pages object reference to all

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pages in the document

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as you can see there's only one page in

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the document so we

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only have one object of type

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page so the page reference to the

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contents of the document or the page

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so you see in object number four

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there contains a stream and within the

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stream

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there's the text of the pdf document and

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starts with contract

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the third part of a pdf document is a

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cross reference table

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or expert table it

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holds the entries or the byte offsets of

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the object

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defined in the body so as you can see it

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starts with one

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followed by four this means the

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first object is object number one

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followed by four entries

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so as you can see there's a 10 and 10

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means the byte of set 10

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so object number one starts at byte

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offset 10.

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the last part of a pdf document is a

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trailer the trailer reference to the

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root object

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so the root object is of type catalog

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and defines a byte offset of the cross

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reference table

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so in this example the cross reference

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table start at byte offset 191

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and these four parts build the whole pdf

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document

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if you want to change the content of a

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pdf file

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or add new content we do this usually

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within an incremental update so this

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means only the modification

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are added to the end of the pdf file

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within an update

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but the original pdf content remains

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unchanged

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so on the left side you see a simple pdf

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file and now we want to add

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an incremental update we do this by

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adding a body update a cross reference

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table update and a trailer update

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and these three parts build the

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incremental update

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so if we want to sign a pdf document

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we do this within an incremental update

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and these incremental update protects

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the document

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so there's a small part containing the

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signature value which leaves unprotected

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so if we now want to add an annotation

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to a signed pdf document

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we do this with another incremental

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update

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soon as you can see the annotation part

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is now unprotected

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so let's talk about signed pdf documents

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if you want to sign the simple pdf

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document

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we do this within an incremental update

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so as you can

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see here we see the revision one is

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signed by ceo

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but we are not limited to one signature

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we can also add another signature

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so in update 2 we add the revision 2

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which is signed by ceo2

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and we also can add annotations but if

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we want to add an annotation

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like here not for public release we see

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there is in the signature part of the

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document we see there's an annotation

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created with a warning

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so signed pdf documents there are

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multiple signatures are possible

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until assigner explicitly locks the

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document and

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adding an annotation leads to warnings

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but are not blocked

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so in certified documents the

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certification must be added before

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signing the document

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this is because the certification sets a

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permission level

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p1 p2 or p3 for the whole document

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so in certified documents up to the

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permission level

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additional signature our annotations are

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allowed

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or prohibit so let's take a look on the

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difference between signed and

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certified documents add or change

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visible content is not allowed in a

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signed or

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certified document but you can fill out

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forms

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in certified documents p2 or p3

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and inside pdf documents this usually

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leads to warnings if you do not add a

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signature at the same time

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but adding multiple signature is also

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allowed in site pdfs

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and on certified documents at level p2

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and p3 adding annotations

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leads to warnings the most pdf

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application when we do this within sign

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pdf but is allowed on a

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certified document at level p3 so on the

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right side you see a certified document

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and a signature was added and an

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annotation was added and we see on level

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p3

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there's no warnings when we add an

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annotation

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so let's start with the attacks on

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certified documents we start with the

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evil annotation attack um we use

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annotations to add to hide content and

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certify documents

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and we take a look on the user interface

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layers ui layers

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and we improve the stealthiness of the

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eaa

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so for in eaa you need a

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certified document on level p3

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so adding or changing annotations is

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allowed

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and annotations can be placed anywhere

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in the documents the pdf

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specification does not restrict the

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position or size of an

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annotation so we can add an annotation

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as big as we want

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and two annotations can overlap each

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other

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the pdf specification 2.0 defines

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26 annotation types and 18 of them

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are allowed to be used in a certified

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document

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so as you can see on the right side we

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categorize them by the danger level

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so danger level high means you can add

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text or images

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to the certified document so for

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annotation free text and redact you can

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add text

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and for notation stamp you can images

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at the level medium it means you can

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hide images or text in a certified

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document but you cannot add

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new content and on level low you only

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can

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add icons to the certified documents

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so how to use eia to change a content in

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a certified document

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we first use the annotation type stamp

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to hide content with a white image so we

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place a white image

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above the text we want to change

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after this we add a free text comment or

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a free text annotation

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to add text chosen by the attacker we

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place this

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over the stamp and now we got

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our content placed in the document

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as you can see we can add content to a

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certified document without

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warnings in ui layer 1 which is always

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displayed when the document is opened

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and no warnings in user layer 2 which

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must be opened by the user and provide

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more detailed information about the

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signature certification

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but we have to look at another ui layer

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the ui

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layer 3 which holds a list of

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annotations added to the document

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and this layer 3 must be also opened by

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the user

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but it makes the modifications

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detectable for the user

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so let's improve the stealthiness of the

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array

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we first have to find objects of type

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anode

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you see we have two objects of type

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anode here

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which hold a value a subtype value of

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stamp or free text

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so the next step we have to change the

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subtype to null or one that is not

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specified by the specification

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so we set this to null as you can see

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and

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now the ui layer 3 holds an empty list

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so this is how we improve systems of eaa

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let's take a look on sneaky signature

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attacks

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our goal is to use the signature to add

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and hide content in a certified document

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and we take a look on the structure of

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signatures

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and on top we show how to improve the

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statusness of ssa

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the sneaky signature attack is possible

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on level p2

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and p3 because this level allows to add

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a signature to a certified document

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and the visible part of the signature

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can be placed anywhere in the document

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and the pdf specification does not

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restrict the position or size of the

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signature

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and also signature can overlap each

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other

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on this slide you see the visible part

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of a signature

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as you can see there is a graphical part

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and also a text part

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and both parts can chosen by the

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attacker

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so let's use ssa to change content in a

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certified document

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we first have to add an empty signature

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file over the content to be changed

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we can do this manually or using tools

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like apache pdf box

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which is a java library to modify or

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sign pdfs

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so in the next step we have to create a

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visible signature

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with a white background image and text

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chosen by the attacker

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in the last step we have to insert the

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signature to the empty signature files

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so now we have placed attacker choosing

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content with certified document using

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signatures

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but this leads to a problem so as you

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can see on ui layer 1

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and ui layer 2 depending on the pdf

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application

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the attack is detectable

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so let's improve the statusness of ssa

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in the first step we have to find the

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object of file type sick

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so as you can see we find file type 6 in

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the object number 7

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and object number 7 contains a reference

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to the page object

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p to the object 3 and

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the validation reference to number 8.

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the object 8 contains the signature

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value

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so in the second step we have to change

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the reference

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of the page object to our own signature

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object

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this means we change the reference 3

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to 7. so in the next step

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we have to interrupt the signature

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validation and we do this by changing

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the referenced v to the next higher

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object number so in this case we change

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the

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value 8 to the value 9.

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but this leads to another problem as you

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can see

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the signature field is now interpreted

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as an

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empty field so to prevent the signature

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field from being interpreted as an

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empty field we have to mark it as read

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only

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and we do this by adding double f1

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to the signature object seven

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so now the signature field will not be

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interpreted as

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an empty one so we finished our

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improving the stealthiness attack on ssa

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so as you can see no signature is here

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in ui layer 1

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and no signature is on your eye layer 2.

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so let's have a look on the evaluation

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and the vulnerable pdf

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applications we categorized them by the

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ui layers

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we analyzed 26 pf applications on three

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different operating systems

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so on the middle part you see the

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specification flaws

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this means the attacks without improving

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the stealthiness

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on the right part you see the

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application flaws this means the attacks

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with improving the statins we categorize

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them by the ui layers one two and three

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and a full cycle means the attack is

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undetectable on the ui layer

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a semicycle means the attack is

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undetectable on the ui layer but the

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general

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notification is shown an empty cycle

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means the attack is clearly detectable

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on the ui layer

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as you can see the application flaws

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help to increase the number of

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vulnerable applications

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for example ssa on

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ui layer 2 the number of vulnerable

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applications increased from 0 to 9

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if we improve the stair sinus and

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we see the same for eia on ui layer 3

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where the vulnerability vulnerable

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applications increase

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from 0 to 50.

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so what is the conclusion and the

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lessons learned unprotected updates are

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still a big problem in certified

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documents and also in signed pdfs

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and supposed harmless features like

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signatures or annotations

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can be used for evil things like adding

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content to a certified document

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so the specifications should be updated

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adding signatures should only be allowed

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in a predefined signature

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fields defined by the certifier before

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the document is certified

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and annotation should be limited so

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there's no reason

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why we can add an image as big as a

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whole page or add

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content like text to the to the

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certified document

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using a white list would also help to

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reduce implementation flaws

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if you want to learn more about pdf

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00:14:40,639 --> 00:14:42,880
security visit us at pdf

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00:14:42,880 --> 00:14:48,160
in security.org and on pdf demos.de

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00:14:48,160 --> 00:14:51,120
we provide for you a detector for ea and

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00:14:51,120 --> 00:14:51,920
ssa

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if you have any questions write us an

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00:14:53,680 --> 00:14:58,880
email thank you for your attention