Network Working Group H. Maruyama
Request for Comments: 2803 K. Tamura
Category: Informational N. Uramoto
IBM
April 2000
Digest Values for DOM (DOMHASH)
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 (2000). All Rights Reserved.
Abstract
This memo defines a clear and unambiguous definition of digest (hash)
values of the XML objects regardless of the surface string variation
of XML. This definition can be used for XML digital signature as well
efficient replication of XML objects.
Table of Contents
1. Introduction............................................2
2. Digest Calculation......................................3
2.1. Overview..............................................3
2.2. Namespace Considerations..............................4
2.3. Definition with Code Fragments........................5
2.3.1. Text Nodes..........................................5
2.3.2. Processing Instruction Nodes........................6
2.3.3. Attr Nodes..........................................6
2.3.4. Element Nodes.......................................7
2.3.5. Document Nodes......................................9
3. Discussion..............................................9
4. Security Considerations.................................9
References................................................10
Authors' Addresses........................................10
Full Copyright Statement..................................11
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RFC 2803 Digest Values for DOM (DOMHASH) April 2000
1. Introduction
The purpose of this document is to give a clear and unambiguous
definition of digest (hash) values of the XML objects [XML]. Two
subtrees are considered identical if their hash values are the same,
and different if their hash values are different.
There are at least two usage scenarios of DOMHASH. One is as a basis
for digital signatures for XML. Digital signature algorithms normally
require hashing a signed content before signing. DOMHASH provides a
concrete definition of the hash value calculation.
The other is to use DOMHASH when synchronizing two DOM structures
[DOM]. Suppose that a server program generates a DOM structure which
is to be rendered by clients. If the server makes frequent small
changes on a large DOM tree, it is desirable that only the modified
parts are sent over to the client. A client can initiate a request by
sending the root hash value of the structure in the cache memory. If
it matches with the root hash value of the current server structure,
nothing needs be sent. If not, then the server compares the client
hash with the older versions in the server's cache. If it finds one
that matches the client's version of the structure, then it locates
differences with the current version by recursively comparing the
hash values of each node. This way, the client can receive only an
updated portion of a large structure without requesting the whole
thing.
One way of defining digest values is to take a surface string as the
input for a digest algorithm. However, this approach has several
drawbacks. The same internal DOM structure may be represented in may
different ways as surface strings even if they strictly conform to
the XML specification. Treatment of white spaces, selection of
character encodings, entity references (i.e., use of ampersands), and
so on have impact on the generation of a surface string. If the
implementations of surface string generation are different, the hash
values would be different, resulting in unvalidatable digital
signatures and unsuccessful detection of identical DOM structures.
Therefore, it is desirable that digest of DOM is defined in the DOM
terms -- that is, as an unambiguous algorithm operating on a DOM
tree. This is the approach we take in this specification.
Introduction of namespace is another source of variation of surface
string because different namespace prefixes can be used for
representing the same namespace URI [URI]. In the following example,
the namespace prefix "edi" is bound to the URI
"http://ecommerce.org/schema" but this prefix can be arbitrary chosen
without changing the logical contents as shown in the second example.
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<?xml version="1.0"?>
<root xmlns:edi='http://ecommerce.org/schema'>
<edi:order>
:
</edi:order>
</root>
<?xml version="1.0"?>
<root xmlns:ec='http://ecommerce.org/schema'>
<ec:order>
:
</ec:order>
</root>
The DOMHASH defined in this document is designed so that the choice
of the namespace prefix does not affect the digest value. In the
above example, both the "root" elements will get the same digest
value.
2. Digest Calculation
2.1. Overview
Hash values are defined on the DOM type Node. We consider the
following five node types that are used for representing a DOM
document structure:
- Text
- ProcessingInstruction
- Attr
- Element
- Document
Comment nodes and Document Type Definitions (DTDs) do not participate
in the digest value calculation. This is because DOM does not
require a conformant processor to create data structures for these.
DOMHASH is designed so that it can be computed with any XML processor
conformant to the DOM or SAX [SAX] specification.
Nodes with the node type EntityReference must be expanded prior to
digest calculation.
The digest values are defined recursively on each level of the DOM
tree so that only a relevant part needs to be recalculated when a
small portion of the tree is changed.
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Below, we give the precise definitions of digest for these types. We
describe the format of the data to be supplied to a hash algorithm
using a figure and a simple description, followed by a Java code
fragment using the DOM API and the JDK 1.1 Platform Core API only.
Therefore, the semantics should be unambiguous.
As the rule of thumb, all strings are to be in UTF-16BE [UTF16]. If
there is a sequence of text nodes without any element nodes in
between, these text nodes are merged into one by concatenating them.
A zero-length text node is always ignored.
Note that validating and non-validating XML processors may generate
different DOM trees from the same XML document, due to attribute
normalization and default attributes. If DOMHASH is to be used for
testing logical equivalence between two XML documents (as opposed to
DOM trees), it may be necessary to normalize attributes and supply
default attributes prior to DOMHASH calculation.
Some legacy character encodings (such as ISO-2022-JP) have certain
ambiguity in translating into Unicode. This is again dependent on
XML processors. Treatment of such processor dependencies is out of
scope of this document.
2.2. Namespace Considerations
To avoid the dependence on the namespace prefix, we use "expanded
names" to do digest calculation. If an element name or an attribute
name is qualified either by a explicit namespace prefix or by a
default namespace, the name's LocalPart is prepended by the URI of
the namespace (the namespace name as defined in the Namespace
specification [NAM]) and a colon before digest calculation. In the
following example, the default qualified name "order" is expanded
into "http://ecommerce.org/schema:order" while the explicit qualified
name "book:title" is expanded into "urn:loc.gov:books:title" before
digest calculation.
<?xml version="1.0"?>
<root xmlns='http://ecommerce.org/schema'
xmlns:book='urn:loc.gov:books'>
<order>
<book:title> ... </book:title>
:
</order>
</root>
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We define an expanded name (either for element or attribute) as
follows:
If a name is not qualified, the expanded name is the name itself.
If a name is qualified with the prefix "xmlns", the expanded name
is undefined.
If a name is qualified either by default or by an explicit
namespace prefix, the expanded name is URI bound to the namespace
+ ":" + LocalPart
In the following example code, we assume that the getExpandedName()
method (which returns the expanded name as defined above) is defined
in both Element and Attr interfaces of DOM.
Note that the digest values are not defined on namespace
declarations. In other words, the digest value is not defined for an
attribute when
- the attribute name is "xmlns", or
- the namespace prefix is "xmlns".
In the above example, the two attributes which are namespace
declarations do not have digest values and therefore will not
participate in the calculation of the digest value of the "root"
element.
2.3. Definition with Code Fragments
The code fragments in the definitions below assume that they are in
implementation classes of Node. Therefore, a methods call without an
explicit object reference is for the Node itself. For example,
getData() returns the text data of the current node if it is a Text
node. The parameter digestAlgorithm is to be replaced by an
identifier of the digest algorithm, such as "MD5" [MD5] and "SHA-1"
[SHA].
The computation should begin with a four byte integer that represents
the type of the node, such as TEXT_NODE or ELEMENT_NODE.
2.3.1. Text Nodes
The hash value of a Text node is computed on the four byte header
followed by the UTF-16BE encoded text string.
- TEXT_NODE (3) in 32 bit network-byte-ordered integer
- Text data in UTF-16BE stream (variable length)
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public byte[] getDigest(String digestAlgorithm) {
MessageDigest md = MessageDigest.getInstance(digestAlgorithm);
md.update((byte)0);
md.update((byte)0);
md.update((byte)0);
md.update((byte)3);
md.update(getData().getBytes("UnicodeBigUnmarked"));
return md.digest();
}
Here, MessageDigest is in the package java.security.*, one of the
built-in packages of JDK 1.1.
2.3.2. ProcessingInstruction Nodes
A ProcessingInstruction (PI) node has two components: the target and
the data. Accordingly, the hash is computed on the concatenation of
both, separated by 'x0000'. PI data is from the first non white
space character after the target to the character immediately
preceding the "?>".
- PROCESSING_INSTRUCTION_NODE (7) in 32 bit network-byte-ordered
integer
- PI target in UTF-16BE stream (variable length)
- 0x00 0x00
- PI data in UTF-16BE stream (variable length)
public byte[] getDigest(String digestAlgorithm) {
MessageDigest md = MessageDigest.getInstance(digestAlgorithm);
md.update((byte)0);
md.update((byte)0);
md.update((byte)0);
md.update((byte)7);
md.update(getName().getBytes("UnicodeBigUnmarked"));
md.update((byte)0);
md.update((byte)0);
md.update(getData().getBytes("UnicodeBigUnmarked"));
return md.digest();
}
2.3.3. Attr Nodes
The digest value of Attr nodes are defined similarly to PI nodes,
except that we need a separator between the expanded attribute name
and the attribute value. The '0x0000' value in UTF-16BE is allowed
nowhere in an XML document, so it can serve as an unambiguous
separator. The expanded name must be used as the attribute name
because it may be qualified. Note that if the attribute is a
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namespace declaration (either the attribute name is "xmlns" or its
prefix is "xmlns"), the digest value is undefined and the getDigest()
method should return null.
- ATTRIBUTE_NODE (2) in 32 bit network-byte-ordered integer
- Expanded attribute name in UTF-16BE stream (variable length)
- 0x00 0x00
- Attribute value in UTF-16BE stream (variable length)
public byte[] getDigest(String digestAlgorithm) {
if (getNodeName().equals("xmlns")
|| getNodeName().startsWith("xmlns:"))
return null;
MessageDigest md = MessageDigest.getInstance(digestAlgorithm);
md.update((byte)0);
md.update((byte)0);
md.update((byte)0);
md.update((byte)2);
md.update(getExpandedName().getBytes("UnicodeBigUnmarked"));
md.update((byte)0);
md.update((byte)0);
md.update(getValue().getBytes("UnicodeBigUnmarked"));
return md.digest();
}
2.3.4. Element Nodes
Element nodes are the most complex because they consist of other
nodes recursively. Hash values of these component nodes are used to
calculate the node's digest so that we can save computation when the
structure is partially changed.
First, all the attributes except for namespace declarations must be
collected. This list is sorted lexicographically by the expanded
attribute names (based on Unicode character code points). When no
surrogate characters are involved, this is the same as sorting in
ascending order in terms of the UTF-16BE encoded expanded attribute
names, using the string comparison operator String.compareTo() in
Java.
- ELEMENT_NODE (1) in 32 bit network-byte-ordered integer
- Expanded element name in UTF-16BE stream (variable length)
- 0x00 0x00
- A number of non-namespace-declaration attributes in 32 bit
network-byte-ordered unsigned integer
- Sequence of digest values of non-namespace-declaration attributes,
sorted lexicographically by expanded attribute names
- A number of child nodes (except for Comment nodes) in 32bit
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network-byte-ordered unsigned integer
- Sequence of digest values of each child node except for Comment
nodes (variable length) (A sequence of child texts is merged to one
text. A zero-length text and Comment nodes are not counted as
child)
public byte[] getDigest(String digestAlgorithm) {
MessageDigest md = MessageDigest.getInstance(digestAlgorithm);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(baos);
dos.writeInt(ELEMENT_NODE);//This is stored in network byte order
dos.write(getExpandedName().getBytes("UnicodeBigUnmarked"));
dos.write((byte)0);
dos.write((byte)0);
// Collect all attributes except for namespace declarations
NamedNodeMap nnm = this.getAttributes();
int len = nnm.getLength()
// Find "xmlns" or "xmlns:foo" in nnm and omit it.
...
dos.writeInt(len); // This is sorted in the network byte order
// Sort attributes lexicographically by expanded attribute
// names.
...
// Assume that `Attr[] aattr' has sorted Attribute instances.
for (int i = 0; i < len; i ++)
dos.write(aattr[i].getDigest(digestAlgorithm));
Node n = this.getFirstChild();
// Assume that adjoining Texts are merged,
// there is no 0-length Text, and
// comment nodes are removed.
len = this.getChildNodes().getLength();
dos.writeInt(len); // This is stored in the network byte order
while (n != null) {
dos.write(n.getDigest(digestAlgorithm));
n = n.getNextSibling();
}
dos.close();
md.update(baos.toByteArray());
return md.digest();
}
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2.3.5. Document Nodes
A Document node may have PI nodes before and after the root Element
node. The digest value of a Document node is computed based on the
sequence of the digest values of the pre-root PI nodes, the root
Element node, and the post-root PI nodes in this order. Comment
nodes and DocumentType nodes, if any, are ignored.
- DOCUMENT_NODE (9) in 32 bit network-byte-ordered integer
- A number of child nodes (except for Comment and DocumentType nodes)
in 32bit network-byte-ordered unsigned integer
- Sequence of digest values of each child node except for Comment and
DocumentType nodes (variable length)
public byte[] getDigest(String digestAlgorithm) {
MessageDigest md = MessageDigest.getInstance(digestAlgorithm);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(baos);
dos.writeInt(DOCUMENT_NODE);//This is stored in network byte order
// Assume that Comment and DocumentType nodes are removed and this
// node has only an Element node and PI nodes.
len = this.getChildNodes().getLength();
dos.writeInt(len); // This is stored in the network byte order
Node n = this.getFirstChild();
while (n != null) {
dos.write(n.getDigest(digestAlgorithm));
n = n.getNextSibling();
}
dos.close();
md.update(baos.toByteArray());
return md.digest();
}
3. Discussion
The definition described above can be efficiently implemented with
any XML processor that is conformant to either DOM and SAX
specification. Reference implementations are available on request.
4. Security Considerations
DOMHASH is expected to be used as the basis for digital signatures
and other security and integrity uses. It's appropriateness for
such uses depends on the security of the hash algorithm used and
inclusion of the fundamental characteristics it is desired to check
in parts of the DOM model incorporated in the digest by DOMHASH.
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References
[DOM] "Document Object Model (DOM), Level 1 Specification", October
1998, http://www.w3.org/TR/REC-DOM-Level-1/
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[NAM] Tim Bray, Dave Hollander, Andrew Layman, "Namespaces in XML",
http://www.w3.org/TR/1999/REC-xml-names-19990114.
[SAX] David Megginson, "SAX 1.0: The Simple API for XML",
http://www.megginson.com/SAX/, May 1998.
[SHA] (US) National Institute of Standards and Technology, "Federal
Information Processing Standards Publication 180-1: Secure Hash
Standard", 17 April 1995.
[URI] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[UTF16] Hoffman, P., Yergeau, F., "UTF-16, an encoding of ISO 10646",
RFC 2781, February 2000.
[XML] Tim Bray, Jean Paoli, C. M. Sperber-McQueen, "Extensible
Markup Language (XML) 1.0", http://www.w3.org/TR/1998/REC-xml-
19980210
Authors' Addresses
Hiroshi Maruyama,
IBM Research, Tokyo Research Laboratory
EMail: maruyama@jp.ibm.com
Kent Tamura,
IBM Research, Tokyo Research Laboratory
EMail: kent@trl.ibm.co.jp
Naohiko Uramoto,
IBM Research, Tokyo Research Laboratory
EMail: uramoto@jp.ibm.com
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Full Copyright Statement
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