Network Working Group E. Taft

Network Working Group E. Taft
Request for Comments: 3778 J. Pravetz
Category: Informational S. Zilles
L. Masinter
Adobe Systems
May 2004

The application/pdf Media Type

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

PDF, the 'Portable Document Format', is a general document
representation language that has been in use for document exchange on
the Internet since 1993. This document provides an overview of the
PDF format, explains the mechanisms for digital signatures and
encryption within PDF files, and updates the media type registration
of 'application/pdf'.

Table of Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
2. History . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Fragment Identifiers. . . . . . . . . . . . . . . . . . . . . 3
4. Encryption. . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Digital Signatures. . . . . . . . . . . . . . . . . . . . . . 5
6. PDF implementations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References. . . . . . . . . . . . . . . . . . . . . . . . . .
9.1. Normative References. . . . . . . . . . . . . . . . . . 10
9.2. Informative References. . . . . . . . . . . . . . . . . 10
10. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 12
11. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 14

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1. Introduction

This document is intended to provide updated information on the
registration of the MIME Media Type "application/pdf", with
particular focus on the features that help mitigate security
concerns. This document refers to features documented in the PDF
References versions 1 [1], 1.3 [2], 1.4 [3] and 1.5 [4], as updated
by errata [5].

PDF is used widely in the Internet Community. Since PDF was
introduced in 1993, it has grown to be a widely-used format for
capturing and exchanging formatted documents electronically, across
the Web, via e-mail, and, for that matter, virtually every other
document exchange mechanism.

PDF represents formatted documents. These documents may be
structured or simple. They may contain text, images, graphics, and
other multimedia content, such as video and audio. There is support
for annotations, metadata, hypertext links, and bookmarks.

PDF supports encryption and digital signatures in the document. The
encryption capability is also combined with access control
information in a way that is intended to manage the uses that a
recipient can make of a document.

PDF usage is specified in other international standards. ISO 15930-
1:2001 PDF/X [16] has been adopted as the exchange standard for
electronic documents within the Prepress community. PDF/X is a
profile of PDF that references the PDF Reference, Third edition [2],
as the source specification.

Another profile of PDF, known as PDF/A [17], is being developed for
use as an international standard as an electronic document file
format for long-term preservation. Following the work on PDF/X, the
activity is joint work between NPES (The Association for Suppliers of
Printing, Publishing and Converting Technologies) and AIIM
International (the Association for Information and Image Management,
International). AIIM is the secretariat for ISO/TC 171 SC2, Document
Imaging Applications.

PDF usage is widespread enough for 'application/pdf' to be used in
other IETF specifications. RFC 2346 [15] describes how to better
structure PDF files for international exchange of documents where
different paper sizes are used; HTTP byte range retrieval is
illustrated using application/pdf (RFC 2616 [14], Section 19.2); RFC
3297 [13] illustrates how PDF can be sent to a recipient that
identifies his ability to accept the PDF using content negotiation.

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2. History

PDF was originally envisioned as a way to communicate and view
printed information electronically across a wide variety of machine
configurations, operating systems, and communication networks in a
reliable manner.

PDF relies on the same imaging model as the PostScript page
description language to render complex text, images, and graphics in
a device and resolution-independent manner, bringing this feature to
the screen as well as the printer. To improve performance for
interactive viewing, PDF defines a more structured format than that
used by most PostScript language programs. PDF also includes
objects, such as hypertext links and annotations, that are not part
of the page itself, but are useful for building collections of
related documents and for reviewing and commenting on documents.

The application/pdf media type was first registered in 1993 by Paul
Lindner for use by the gopher protocol; the registration was
subsequently updated in 1994 by Steve Zilles.

3. Fragment Identifiers

The handling of fragment identifiers [6] is currently defined in
Adobe Technical Note 5428 [7]. This section summarizes that
material.

A fragment identifier consists of one or more PDF-open parameters in
a single URL, separated by the ampersand (&) or pound (#) character.
Each parameter implies an action to be performed and the value to be
used for that action. Actions are processed and executed from left
to right as they appear in the character string that makes up the
fragment identifier.

The PDF-open parameters allow the specification of a particular page
or named destination to open. Named destinations are similar to the
"anchors" used in HTML or the IDs used in XML. Once the target is
specified, the view of the page in which it occurs can be specified,
either by specifying the position of a viewing rectangle and its
scale or size coordinates or by specifying a view relative to the
viewing window in which the chosen page is to be presented.

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The list of PDF-open parameters and the action they imply is:

nameddest=<name>
Open to a specified named destination (which includes a view).

page=<pagenum>
Open the specified (physical) page.

zoom=<scale>,<left>,<top>
Set the <scale> and scrolling factors. <left>, and <top> are
measured from the top left corner of the page, independent of the
size of the page. The pair <left> and <top> are optional but both
must appear if present.

view=<keyword>,<position>
Set the view to show some specified portion of the page or its
bounding box; keywords are defined by Table 8.2 of the PDF
Reference, version 1.5. The <position> value is required for some
of the keywords and not allowed for others.

viewrect=<left>,<top>,<wd>,<ht>
As with the zoom parameter, set the scale and scrolling factors,
but using an explicit width and height instead of a scale
percentage.

highlight=<lt>,<rt>,<top>,<btm>
Highlight a rectangle on the chosen page where <lt>, <rt>, <top>,
and <btm> are the coordinates of the sides of the rectangle
measured from the top left corner of the page.

All specified actions are executed in order; later actions will
override the effects of previous actions; for this reason, page
actions should appear before zoom actions. Commands are not case
sensitive (except for the value of a named destination).

4. Encryption

PDF files allow access to be controlled using encryption and
permission settings. A document's data decryption keys and
permission settings are provided by encryption handlers. An
'Encryption Dictionary' is provided in the document trailer to enable
encryption handlers to store document-specific information.
Different encryption handlers can provide for different sets of
permissions. The PDF encoding rules for password and public key
encryption handlers are specified in the PDF Reference.

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A person that is able to 'access' a document is said to be able to
open and view the document. Access is possible when a person can
provide the key with which to decrypt the document. The key is
protected and provided by the encryption handler. Encryption
handlers will normally require some sort of authentication before a
person can access the document decryption key.

Encryption of PDF files is normally applied to all string and stream
data in the document, and only to string and stream data. By
encrypting only data portions of the PDF file, random access to PDF
file contents is maintained. The data is normally encrypted using
the 40 to 128-bit RC4 [8] encryption algorithm. Use of decryption
filters allow algorithms other than RC4 to be used.

The person that has access to a document will be given certain
permissions for the document. A person that has full permissions,
including permission to save a document without encryption, is said
to be an 'owner'. A person that has restricted permissions is said
to be a 'user'. Example permissions include the ability to copy text
and other content from the PDF file, the ability to fill in form
field data, and the ability to print the PDF file. Enforcement of
permissions is the responsibility of the viewing application.

Password encryption allows the possibility of two different passwords
to be used when providing access to the document. The 'author'
password allows access to the document and full permissions,
including the permission to save the document without encryption.
The 'user' password allows access to the document, but access is
restricted by a set of permissions.

Public key encryption of PDF files uses one or more PKCS#7 [9]
objects to store information regarding recipients that are able to
open a document. Each PKCS#7 object contains a list of recipients, a
document decryption key, and permission settings that apply to all
recipients listed for that PKCS#7 object. The document decryption
key is protected with a triple-DES key that is encrypted once with
the public key of each listed recipient.

5. Digital Signatures

A digital signature can be used to authenticate the identity of a
user and the validity of a document's contents. PDF supports the
association of a digital signature with a complete record that is
needed to reproduce a visual representation of what a person saw when
they signed the PDF file. PDF digital signatures allows for multiple
signers to update and sign the same document; a subsequent user may
then view the state of the document at each point when any individual
signature was applied.

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The full specification for PDF digital signatures is contained in the
PDF Reference [4] section 8.7 and Appendix I; an overview is provided
here.

PDF signature information is stored in a 'signature dictionary' data
structure. A signature is created by computing a digest of the data
stored in the document. To verify the signature, the digest is
recomputed and compared with the one stored in the document.
Differences in the digest values indicate that modifications have
been made since the document was signed.

All bytes of the PDF file are covered by the signature digest,
including the signature dictionary, but excluding the signature value
itself. The range of bytes is defined and stored as the value of the
ByteRange key in the signature dictionary. The ByteRange value is an
array of integer pairs, where each pair includes a starting byte
offset and length in bytes. There are two pairs, one describing the
range of bytes preceding the signature value, and the other
describing the range of bytes that occur after the signature value.

PDF public key digital signature syntax is specified for PKCS#1 [11]
and PKCS#7 [9] signatures. In both cases, all bytes of the PDF file
are signed, with the exclusion of the PKCS#1 or PKCS#7, signature
value, objects.

The signature dictionary contains additional attributes. The
'SubFilter' attribute describes the encoding of the signature value,
and the 'Contents' attribute contains the signature value which is
normally hex (base16) encoded. There are currently three recommended
SubFilter types:

adbe.x509.rsa_sha1
In this case, the Contents key contains a DER-encoded PKCS#1 [11]
binary data object representing the signature obtained as the RSA
encryption of the byte range SHA-1 digest with the signer's
private key. When using PKCS#1, the certificate chain of the
signer is included with other signature information in the signed
document.

adbe.pkcs7.sha1
In this case, the value of Contents is a DER-encoded PKCS#7 binary
data object containing the signature. The SHA1 digest of the byte
range is encapsulated in the PKCS#7 signed-data field with
ContentInfo of type "data".

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adbe.pkcs7.detached
In this case, the value of Contents is a DER-encoded PKCS#7 binary
data object containing the signature. No data is encapsulated in
the PKCS#7 signed-data field.

If the type of signature is 'adbe.x509.rsa_sha1', the signature
dictionary includes a key named 'Cert', which contains at least the
signer's X.509 public-key certificate represented as a binary string.
The value could also be an array of strings where the first entry is
the signer's certificate and the following entries are one or more
issuer certifications from the signer's trust chain.

If the type of signature is 'adbe.pkcs7.sha1' or
'adbe.pkcs7.detached', the 'Cert' key is not used and the certificate
must be put in the PKCS#7 object stored in the 'Contents' key. The
minimum required certificate to include in the PKCS#7 object is the
signer's X.509 signing certificate. It may also optionally contain
one or more issuer certifications from the signer's trust chain.

Multiple signatures are supported using the incremental save
capabilities of PDF. When changes to a file are made and a new
signature is applied to the document, the changes are appended after
the last byte of the previously existing document and then the new
signature digest is of all bytes of the new file. In this manner,
changes can be made to a document and new signatures added to a
document without invalidating earlier signatures that have been
applied to the PDF file. Any change to a document is detected
because all bytes of the PDF file are digested.

The state of a signed document, when an earlier signature of a
multiple signature document was applied, can be viewed by extracting
the earlier set of bytes of the file and opening them in a PDF
viewing application. This process is called 'rollback' and allows
viewing of the exact state of the document when it was signed.

PDF syntax allows for 'author' and 'user' signatures. Under normal
circumstances the first signature of a document is considered an
author signature and all other signatures are considered user
signatures. Authors can specify what changes are to be allowed to
the PDF file before the author's signature is presented as invalid.
Example changes include the ability to fill in form field data, the
ability to add comments to a document, the ability to make no
changes, and the ability to make any changes. Changes are detected
by opening the existing document and the author's version of the
document and performing a complete object compare of the two
documents. Change detection is not a substitute for the legal value
of document rollback.

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6. PDF Implementations

There are a number of widely available, independently implemented,
interoperable implementations of PDF for a wide variety of platforms
and systems. Because PDF is a publicly available specification,
hundreds of companies and organizations make PDF creation, viewing,
and manipulation tools. For examples, see descriptions or tools
lists from Adobe [20], Apple [21], Ghostscript [22], Planet PDF [18],
and PDFzone.com [19].

7. Security Considerations

An "application/pdf" resource contains information to be parsed and
processed by the recipient's PDF system. Because PDF is both a
representation of formatted documents and a container system for the
resources need to reproduce or view said documents, it is possible
that a PDF file has embedded resources not described in the PDF
Reference.

Although it is not a defined feature of PDF, a PDF processor could
extract these resources and store them on the recipients system.
Furthermore, a PDF processor may accept and execute "plug-in" modules
accessible to the recipient. These may also access material in the
PDF file or on the recipients system. Therefore, care in
establishing the source, security, and reliability of such plug-ins
is recommended. Message-sending software should not make use of
arbitrary plug-ins without prior agreement on their presence at the
intended recipients. Message-receiving and -displaying software
should make sure that any non-standard plug-ins are secure and do not
present a security threat.

PDF may contain "scripts" to customize the displaying and processing
of PDF files. These scripts are expressed in a version of JavaScript
[10] based on JavaScript version 1.5 of ISO-16262 (formerly known as
ECMAScript). These scripts have access to an API that is similar to
the "plug-in" API. They are intended for execution by the PDF
processor. User agents executing such scripts or programs must be
extremely careful to insure that untrusted software is executed in a
protected environment.

In addition, JavaScript code might modify the appearance of a PDF
document. For this reason, validation of digital signatures should
take this into account.

In general, any information stored outside of the direct control of
the user -- including referenced application software or plug-ins and
embedded files, scripts or other material not covered in the PDF
reference -- can be a source of insecurity, by either obvious or

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subtle means. For example, a script can modify the content of a
document prior to its being displayed. Thus, the security of any PDF
document may be dependent on the resources referenced by that
document.

As noted above, PDF provides mechanism for helping insure the
integrity of a PDF file, Encryption (Section 4), and to be able to
digitally sign (Section 5) a PDF file. The latter capability allows
a recipient to decide if he is willing to trust the file.

Where there is concern that tampering with the PDF file might be a
problem, it is recommended that the encryption and digital signature
features be used to protect and authenticate the PDF.

In addition, PDF processors may have mechanisms that track the source
of scripts or plug-ins and will execute only those scripts or plug-
ins that meet the processors requirements for trustworthiness of the
sources.

8. IANA Considerations

This document updates the registration of 'application/pdf', a media
type registration as defined in Multipurpose Internet Mail Extensions
(MIME) Part Four: Registration Procedures [12]:

MIME media type name: application

MIME subtype name: pdf

Required parameters: none

Optional parameter: none

Encoding considerations:
PDF files frequently contain binary data, and thus must be encoded
in non-binary contexts.

Security considerations:
See Section 7 of this document.

Interoperability considerations:
See Section 6 of this document.

Published specification:
Adobe Systems Incorporated, "PDF Reference, Fourth Edition",
Version 1.5, August 2003, <http://partners.adobe.com/asn/tech/pdf/
specifications.jsp>, as amended by errata <http://
partners.adobe.com/asn/acrobat/sdk/public/docs/errata.txt>.

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Applications which use this media type:
See Section 6 of this document.

Additional information:

Magic number(s):
All PDF files start with the characters '%PDF-' using the PDF
version number, e.g., '%PDF-1.4'. These characters are in US-
ASCII encoding.

File extension(s): .pdf

Macintosh File Type Code(s): "PDF "

For further information:
Adobe Developer Support <[email protected]>
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
http://www.adobe.com/support/main.html

Intended usage: COMMON

Author/Change controller:
Adobe Developer Support <[email protected]>
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
http://www.adobe.com/support/main.html

9. References

9.1. Normative References

[1] Adobe Systems Incorporated, "Portable Document Format Reference
Manual", Version 1.0, ISBN: 0-201-62628-4, Addison-Wesley, New
York NY, 1993.

[2] Adobe Systems Incorporated, "PDF Reference, Second Edition",
Version 1.3, ISBN: 0-201-61588-6, Addison-Wesley, New York NY,
2000.

[3] Adobe Systems Incorporated, "PDF Reference, Third Edition",
Version 1.4, ISBN: 0-201-75839-3, Addison-Wesley, New York NY,
November 2001.

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[4] Adobe Systems Incorporated, "PDF Reference, Fourth Edition",
Version 1.5, August 2003, <http://partners.adobe.com/asn/tech/
pdf/specifications.jsp>.

[5] Adobe Systems Incorporated, "Errata for PDF Reference, Fourth
Edition", December 2003, <http://partners.adobe.com/asn/
acrobat/sdk/public/docs/errata.txt>.

[6] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.

[7] Adobe Systems Incorporated, "PDF Open Parameters", Technical
Note 5428, May 2003, <http://partners.adobe.com/asn/acrobat/
sdk/public/docs/PDFOpenParams.pdf>.

[8] Rivest, R., "RC4 - an unpublished, trade secret encryption
algorithm", November 1993, <http://www.rsasecurity.com/rsalabs/
faq/3-6-3.html>.

[9] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
1.5", RFC 2315, March 1998.

[10] Adobe Systems Incorporated, "Acrobat JavaScript Scripting
Reference", Technical Note 5431, September 2003, <http://
partners.adobe.com/asn/acrobat/sdk/public/docs/AcroJS.pdf>.

[11] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards
(PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC
3447, February 2003.

9.2. Informative References

[12] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", BCP
13, RFC 2048, November 1996.

[13] Klyne, G., Iwazaki, R. and D. Crocker, "Content Negotiation for
Messaging Services based on Email", RFC 3297, July 2002.

[14] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.

[15] Palme, J., "Making Postscript and PDF International", RFC 2346,
May 1998.

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[16] International Standards Organization, "Graphic technology --
Prepress digital data exchange -- Use of PDF -- Part 1:
Complete exchange using CMYK data (PDF/X-1 and PDF/X-1a)", ISO
15930-1:2001, November 2002.

[17] Association for Information and Image Management, "PDF-Archive
Committee home page", December 2003,
<http://www.aiim.org/pdf_a/>.

[18] Planet PDF, "Planet PDF Tools List", December 2003, <http://
www.planetpdf.com/>.

[19] InternetBiz.net, "PDF software from the PDF zone toolbox",
December 2003, <http://www.pdfzone.com/toolbox/>.

[20] Adobe Systems Incorporated, "Adobe products page", December
2003, <http://www.adobe.com/products/>.

[21] Apple Computer, Inc., "Apple Mac OS X Features - Preview",
December 2003, <http://www.apple.com/macosx/features/preview/>.

[22] Artifex Software, Inc, "Ghostscript", December 2003, <http://
www.ghostscript.com/>.

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10. Authors' Addresses

Edward A. Taft
Adobe Systems
345 Park Ave
San Jose, CA 95110
US

EMail: [email protected]

James D. Pravetz
Adobe Systems
345 Park Ave
San Jose, CA 95110
US

EMail: [email protected]

Stephen Zilles
Adobe Systems
345 Park Ave
San Jose, CA 95110
US

Phone: +1 408 536 7692
EMail: [email protected]

Larry Masinter
Adobe Systems
345 Park Ave
San Jose, CA 95110
US

Phone: +1 408 536 3024
EMail: [email protected]
URI: http://larry.masinter.net

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11. Full Copyright Statement

Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
[email protected].

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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