EPUB Open Container Format (OCF) 3.0

› 1.1 Purpose and Scope

This section is informative

This specification, EPUB Open Container Format (OCF) 3.0, defines a file format and processing model for encapsulating the sets of related resources that comprise one or more EPUB® Publications into a single-file container.

This specification is one of a family of related specifications that compose EPUB 3, the third major revision of an interchange and delivery format for digital publications based on XML and Web Standards. It is meant to be read and understood in concert with the other specifications that make up EPUB 3:

OCF is the required container technology for EPUB Publications. OCF may play a role in the following workflows:

The OCF specification defines the rules for structuring the file collection in the abstract: the "abstract container". It also defines the rules for the representation of this abstract container within a ZIP archive: the "physical container". The rules for ZIP physical containers build upon the ZIP technologies used by [ODF]. OCF also defines a standard method for obfuscating embedded fonts for those EPUB Publications that require this functionality.

This specification supersedes Open Container Format (OCF) 2.0.1 [OCF2]. Refer to [EPUB3Changes] for information on differences between this specification and its predecessor.

› 1.2 Terminology

› 1.3 Conformance Statements

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

All sections of this specification are normative except where identified by the informative status label "This section is informative". The application of informative status to sections and appendices applies to all child content and subsections they may contain.

All examples in this specification are informative.

› 1.4 Content Conformance

› 1.5 Reading System Conformance

An EPUB Reading System must meet all of the following criteria:

› 2.1 Overview

This section is informative

An OCF Abstract Container defines a file system model for the contents of the container. The file system model uses a single common root directory for all of the contents of the container. All (non-remote) resources for embedded Publications are located within the directory tree headed by the container’s root directory, although no specific file system structure is mandated for this. The file system model also includes a mandatory directory named META-INF that is a direct child of the container's root directory and is used to store the following special files:

Complete conformance requirements for the various files in META-INF are found in META-INF.

› 2.2 File and Directory Structure

The virtual file system for the OCF Abstract Container must have a single common root directory for all of the contents of the container.

The OCF Abstract Container must include a directory named META-INF that is a direct child of the container's root directory. Requirements for the contents of this directory are described in META-INF.

The file name mimetype in the root directory is reserved for use by OCF ZIP Containers, as explained in OCF ZIP Container.

All other files within the OCF Abstract Container may be in any location descendant from the container's root directory except within the META-INF directory.

It is recommended that the contents of each of the individual Publications be stored within its own dedicated directory under the container's root.

› 2.3 Relative IRIs for Referencing Other Components

Files within the OCF Abstract Container must reference each other via Relative IRI References ([RFC3987] and [RFC3986]). For example, if a file named chapter1.html references an image file named image1.jpg that is located in the same directory, then chapter1.html might contain the following as part of its content:

<img src="image1.jpg" alt="…" />
                

For Relative IRI References, the Base IRI [RFC3986] is determined by the relevant language specifications for the given file formats. For example, the CSS specification defines how relative IRI references work in the context of CSS style sheets and property declarations. Note that some language specifications reference RFCs that preceded RFC3987, in which case the earlier RFC applies for content in that particular language.

Unlike most language specifications, the Base IRIs for all files within the META-INF directory use the root directory for the Abstract Container as the default Base IRI. For example, if META-INF/container.xml has the following content:

<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
    <rootfiles>
        <rootfile full-path="OEBPS/Great Expectations.opf"
            media-type="application/oebps-package+xml" />	
    </rootfiles>
</container>
            

then the path OEBPS/Great Expectations.opf is relative to the root directory for the OCF Abstract Container and not relative to the META-INF directory.

› 2.4 File Names

The term File Name represents the name of any type of file, either a directory or an ordinary file within a directory within an OCF Abstract Container.

For a given directory within the OCF Abstract Container, the Path Name is a string holding all directory File Names in the full path concatenated together with a / (U+002F) character separating the directory File Names. For a given file within the Abstract Container, the Path Name is the string holding all directory File Names concatenated together with a / character separating the directory File Names, followed by a / character and then the File Name of the file.

The File Name restrictions described below are designed to allow Path Names and File Names to be used without modification on most commonly used operating systems. This specification does not specify how an OCF Processor that is unable to represent OCF File and Path Names would compensate for this incompatibility.

In the context of an OCF Abstract Container, File and Path Names must meet all of the following criteria:

› 2.5 META-INF

All OCF Abstract Containers must include a directory called META-INF. This directory contains the files specified below. Files other than the ones defined below may be included in the META-INF directory; OCF Processors must not fail when encountering such files.

<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
    <rootfiles>
        <rootfile full-path="OEBPS/My Crazy Life.opf"
            media-type="application/oebps-package+xml" />
    </rootfiles>
</container>
                    

<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
    <rootfiles>
        <rootfile full-path="SVG/Sandman.opf"
            media-type="application/oebps-package+xml" />
        <rootfile full-path="XHTML/Sandman.opf"
            media-type="application/oebps-package+xml" />
    </rootfiles>
</container>
                    

<encryption
    xmlns ="urn:oasis:names:tc:opendocument:xmlns:container"
    xmlns:enc="http://www.w3.org/2001/04/xmlenc#"
    xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
    <enc:EncryptedKey Id="EK">
        <enc:EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/>
        <ds:KeyInfo>
            <ds:KeyName>John Smith</ds:KeyName>
        </ds:KeyInfo>
        <enc:CipherData>
            <enc:CipherValue>xyzabc</enc:CipherValue>
        </enc:CipherData>
    </enc:EncryptedKey>
    <enc:EncryptedData Id="ED1">
        <enc:EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#kw-aes128"/>
        <ds:KeyInfo>
            <ds:RetrievalMethod URI="#EK"
                Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey"/>
        </ds:KeyInfo>
        <enc:CipherData>
            <enc:CipherReference URI="image.jpeg"/>
        </enc:CipherData>
    </enc:EncryptedData>
</encryption>
                    

<signatures xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
    <Signature Id="sig" xmlns="http://www.w3.org/2000/09/xmldsig#">
        <SignedInfo>
            <CanonicalizationMethod 
                Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
            <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
            <Reference URI="#Manifest1">
                <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
            </Reference>
        </SignedInfo>
        <SignatureValue>…</SignatureValue>
        <KeyInfo>
            <KeyValue>
                <DSAKeyValue>
                    <P>…</P><Q>…</Q><G>…</G><Y>…</Y> 
                </DSAKeyValue>
            </KeyValue>
        </KeyInfo>
        <Object>
            <Manifest Id="Manifest1">
                <Reference URI="OEBFPS/book.xml">                    
                    <Transforms>                                                
                        <Transform
                            Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>                        
                    </Transforms>
                    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                    <DigestValue></DigestValue>
                </Reference>
                <Reference URI="OEBFPS/images/cover.jpeg">
                    <Transforms>                                                
                        <Transform
                            Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>                        
                    </Transforms>
                    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                    <DigestValue></DigestValue>
                </Reference>
            </Manifest>
        </Object>
    </Signature> 
</signatures>
                    

› 3.1 Overview

This section is informative

An OCF ZIP Container is a physical single-file manifestation of an abstract container.

› 3.2 ZIP File Requirements

An OCF ZIP Container uses the ZIP format as specified by [ZIP APPNOTE], but with the following constraints and clarifications:

The following constraints apply to particular fields in the OCF ZIP Container archive:

› 3.3 OCF ZIP Container Media Type Identification

OCF ZIP Containers must include a mimetype file as the first file in the Container, and the contents of this file must be the MIME type string application/epub+zip.

The contents of the mimetype file must not contain any leading padding or whitespace, must not begin with the Unicode signature (or Byte Order Mark), and the case of the MIME type string must be exactly as presented above. The mimetype file additionally must be neither compressed nor encrypted, and there must not be an extra field in its ZIP header.

› 4.1 Introduction

This section is informative

Since an OCF Zip Container is fundamentally a ZIP file, commonly available ZIP tools can be used to extract any unencrypted content stream from the package. On some systems, the contents of the ZIP file may appear like any other native container (e.g., a folder). While the ability to do this is quite useful, it can pose a problem for an Author who wishes to include a third-party font.

Many commercial fonts allow embedding, but embedding a font implies making it an integral part of the Publication, not providing the original font file along with the content. Since integrated ZIP support is so ubiquitous in modern operating systems, simply placing the font in the ZIP archive is insufficient to signify that the font is not intended to be reused in other contexts. This uncertainty can undermine the otherwise very useful font embedding capability of EPUB Publications.

In order to discourage reuse of the font, some font vendors may allow use of their fonts in EPUB Publications if those fonts are bound in some way to the Publication. That is, if the font file cannot be installed directly for use on an operating system with the built-in tools of that computing device, and it cannot be directly used by other EPUB Publications.

It is beyond the scope of this document to provide a digital rights management or enforcement system for font files. It instead defines a method of obfuscation that will require additional work on the part of the final OCF recipient to gain general access to any included fonts. It is the hope of the IDPF that this will meet the requirements of most font vendors. No claim is made in this document or by the IDPF, that this constitutes encryption, nor does it guarantee that the font file will be secure from copyright infringement. The defined mechanism will simply provide a stumbling block for those who are unaware of the license details of the supplied font. It will not prevent a determined user from gaining full access to the font. Given an OCF Container, it is possible to apply the algorithms defined to extract the raw font file. Whether this satisfies the requirements of individual font licenses remains a question for the licensor and licensee.

› 4.2 Obfuscation Algorithm

The algorithm employed to obfuscate the font file consists of modifying the first 1040 bytes (~1KB) of the font file. In the unlikely event that the file is less than 1040 bytes, then the entire file will be modified. The key for the algorithm is generated using the instructions as given in the section Generating the Obfuscation Key. To obfuscate the original data, the result of performing a logical exclusive or (XOR) on the first byte of the raw file and the first byte of the key is stored as the first byte of the embedded font file. This process is repeated with the next byte of source and key, until all bytes in the key have been used. At this point, the process continues starting with the first byte of the key and 21st byte of the source. Once 1040 bytes have been encoded in this way (or the end of the source is reached), any remaining data in the source is directly copied to the destination. In pseudo-code, this is the algorithm:

set source to font file
set destination to obfuscated file
set keyData to key for font
set outer to 0
while outer < 52 and not (source at EOF)
    set inner to 0
    while inner < 20 and not (source at EOF)
        read 1 byte from source     //Assumes read advances file position
        set sourceByte to result of read
        set keyByte to byte inner of keyData
        set obfuscatedByte to (sourceByte XOR keyByte)
        write obfuscatedByte to destination
        increment inner
    end while
    increment outer
end while
if not (source at EOF) then
    read source to EOF
    write result of read to destination
end if 
            

To get the original font data back, the process is simply reversed. That is, the source file becomes the obfuscated data and the destination file will contain the raw font data.

› 4.3 Generating the Obfuscation Key

The key used in the obfuscation algorithm is derived from unique identifer(s) of the Publication(s) in the Container, as required by the EPUB Publications 3.0 specification and detailed in Unique Identifier [Publications30]. In order to create the key, the unique identifiers of all Publications contained in the container must be concatenated in the order that the Publications appear in container.xml and a space (Unicode code point U+0020) inserted between each identifier. Before generating this string, all whitespace characters as defined by the XML 1.0 specification [XML], section 2.3 are removed from the individual identifiers. Specifically the Unicode code points U+0020, U+0009, U+000D and U+000A must be stripped from each identifier before it is added to the concanenated space-delimited string. An SHA-1 digest of the UTF-8 representation of this string should be generated as specified by the Secure Hash Standard [SHA-1]. This digest is then directly used as the key for the algorithm described in Obfuscation Algorithm.

› 4.4 Specifying Obfuscated Resources

All encrypted data in an OCF Abstract Container must have an entry in the encryption.xml file accompanying the Publication (see Encryption – META-INF/encryption.xml), which includes fonts obfuscated using the method described here. For such obfuscated fonts, in the encryption.xml file, the EncryptionMethod element child of the EncryptedData must have an Algorithm attribute with the value http://www.idpf.org/2008/embedding. The presence of this attribute signals the use of the algorithm described in this specification. All resources that have been obfuscated using this approach must be listed in the CipherData element.

<encryption 
    xmlns="urn:oasis:names:tc:opendocument:xmlns:container" 
    xmlns:enc="http://www.w3.org/2001/04/xmlenc#">
    <enc:EncryptedData> 
        <enc:EncryptionMethod Algorithm="http://www.idpf.org/2008/embedding"/> 
        <enc:CipherData> 
            <enc:CipherReference URI="OEBPS/Fonts/BKANT.TTF"/>  
        </enc:CipherData> 
    </enc:EncryptedData>  
</encryption> 

                

To prevent trivial copying of the embedded font to other Publications, the explicit key must not be provided in the encryption.xml file. Reading systems must derive the key from the package's Unique Identifier.