atompub F. Chen
Internet-Draft M. Hondo
Expires: February 12, 2010 R. Salz
IBM
August 11, 2009
Advanced Security Processing for Atom and APP Documents
draft-rsalz-atom-adv-sec-00.txt
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Abstract
The Atom and APP specifications specify simple uses of cryptography
to sign and encrypt their documents. Each document is processed
completely, and in isolation. This document specifies additional
uses that enable selective protection or encryption of content, and
allow a "trust path" to be created across "atom:link" elements.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions and Terminology . . . . . . . . . . 3
2. Using WS-Security . . . . . . . . . . . . . . . . . . . . . . 4
3. The "atomsec:credentials" element . . . . . . . . . . . . . . 6
4. Selectively Signing parts of an Atom document . . . . . . . . 8
5. Selectively Encrypting parts of an Atom document . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Normative References . . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Crypto and Security Primer . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
The Atom and APP specifications define how to use XML Digital
Signature [W3C.REC-xmlenc-core-20021210], and XML Encryption
[W3C.REC-xmlenc-core-20021210] to prevent the contents of a document
from being modified or inadvertently disclosed. This specification
profiles how to use WS-Security [oasis-wssec-v1.1] to provide
selective protection and encryption of a document and allow a single
document to be encrypted for multiple recipients.
Linking is a key point of Atom and APP, and this document defines a
new element that can be used to establish a chain of trust, from the
document with the "atom:link" to the content being referred-to.
1.1. Notational Conventions and Terminology
The conventions and terminology of both The Atom Syndication Format
[RFC4287] and The Atom Publishing Protocol [RFC5023], known as Atom
and APP respectively, are incorporated here by reference. Within
this document, the phrase "Atom document" is used to refer to any
document defined by either RFC.
The following namespace prefixes are used in this document:
+-------+-----------------------------------------------------------+
| Prefi | Namespace URI |
| x | |
+-------+-----------------------------------------------------------+
| atom | http://www.w3.org/2005/Atom |
| | |
| app | http://www.w3.org/2007/app |
| | |
| atoms | http://www.ibm.com/xmlns/stdwip/atomext/security |
| e | |
| c | |
| | |
| dsig | http://www.w3.org/2000/09/xmldsig# |
| | |
| xenc | http://www.w3.org/2001/04/xmlenc# |
| | |
| wsse | http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-w |
| | ssecurity-secext-1.0.xsd |
| | |
| wsu | "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss- |
| | wssecurity-utility-1.0.xsd |
+-------+-----------------------------------------------------------+
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2. Using WS-Security
The "wsse:Security" element is part of a specification informally
known as WS-Security. The element is defined as a container for
security information, including security tokens, XML encryption keys,
and/or XML digital signatures for SOAP message security. This
specification uses the semantics of WS-Security processing to achieve
the goals .
An Atom or APP dcoument MUST NOT have more than one "wsse:Security"
child element. While compatible with [oasis-wssec-v1.1] processing,
the schema for the "wsse:Security" element is taken from
[oasis-wssec-v1.0].
One feature of the WS-Security header is that it should be possible
to process in a forward-streaming manner. That is, security tokens
appear before they are used (such as in signatures), and encipherment
keys appear before the encrypted data. Therefore, if present, the
"wsse:Security" element SHOULD be the first child of the Atom
document.
The "wsse:SecurityTokenReference" element can be used to help enable
streaming processing, and to reduce multiple occurrences of the same
credentials. To do this, each credential is made the child of a
"wsse:BinarySecurityToken" element, which in turn is a child of the
"wsse:Security" element. For example:
<wsse:Security>
<wsse:BinarySecurityToken
wsu:Id="ssl-cert-1"
ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3">
MII...
</wsse:BinarySecurityToken>.
...
</wsse:Security>
Note that [W3C.REC-xmlenc-core-20021210] specifies many ways to
identify key material, including Distinguished Name, Issuer/
SerialNumber, etc. Any method other than embedding the entire
certificate SHOULD NOT be used.
In this document, anywhere a certificate is referenced, the content
may instead be a "wsse:SecurityTokenReference" to a token in the
"wsse:Security" element. For example, the following two document
fragments are semantically equivalent:
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<dsig:KeyInfo>
<dsig:X509Data>
<wsse:SecurityTokenReference>
<wsse:Reference URI="#ssl-cert-1"/>
</wsse:SecurityTokenReference>
</dsig:X509Data>
<dsig:KeyInfo>
<dsig:KeyInfo>
<dsig:X509Data>
<dsig:X509Certificate>
MII...
</dsig:X509Certificate>
</dsig:X509Data>
<dsig:KeyInfo>
A reference other than a direct reference using the "wsse:Reference"
element MUST NOT be used.
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3. The "atomsec:credentials" element
The "atomsec:credentials" element is a child element of an "atom:
link" element. It specifies one or more security tokens that may be
used to validate the provenance of the data being linked-to. The
"use" attribute specifies how the credentials are to be used. Two
values are specified, "content" and "transport". If the attribute is
not present, it is equivalent to providing it with the value
"content".
atomSecCredentials =
element atomsec:credentials {
atomCommonAttributes*,
attribute use { "content" | "transport" } ?,
(extensionElement*)
}
The "transport" value specifies that the credentials can be used to
validate the SSL certificate of the server containing the linked-to
data. The "content" value specifies that the credentials can be used
to validate any digital signatures on the linked-to data.
An "atom:link" element can have no more than two "atom:credentials"
elements, one for each of content and transport credentials.
The child elements of the "atomsec:credentials" element SHOULD be
"dsig:KeyInfo" elmenents used to locate X.509 certificate data.
These certificates can be used by the relying party to verify the
specified trust chain.
A party that has out of band knowledge MAY ignore this element.
Here is an example of an Atom document pointing to signed content:
<atom:link href="http://www.example.org/2003/12/13/foo">
<atomsec:credentials>
<!-- This is the certificate that should be signing the content
at the href. Or the signer's CA... -->
<dsig:KeyInfo>
<dsig:X509Data>
<dsig:X509Certificate>
MII...
</dsig:X509Certificate>
</dsig:X509Data>
</dsig:KeyInfo>
</atomsec:credentials>
</atom:link>
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Here is another example, one that specifies the CA that issued the
SSL/TLS certificate used by the server for the host www.example.com:
<atom:link href="https://www.example.org/2003/12/13/foo">
<atomsec:credentials use="transport">
<!-- SSL CA, presumably not a typical "browser trusted" one. -->
<dsig:KeyInfo>
<dsig:X509Data>
<dsig:X509Certificate>
MII...
</dsig:X509Certificate>
</dsig:X509Data>
</dsig:KeyInfo>
</atomsec:credentials>
</atom:link>
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4. Selectively Signing parts of an Atom document
A "dsig:Signature" element in the "wsse:Security" element may be used
to sign one or more elements of an Atom document. The signature
should be detached, with one "dsig:Reference" for each item being
signed.
The processing rules are defined by sections 8.2 and 8.4 of
[oasis-wssec-v1.1] with the following additions:
o The SOAP message envelope is not used and the "wsse:Security"
element MUST be the first element of the root document.
o Each element being signed MUST have a unique "xml:id" attribute if
not already present.
o The enveloped-signature transform as defined by
[W3C.REC-xmlenc-core-20021210] SHOULD be used to sign the root
document. If there are additional signed elements, they MUST be
signed first so that the root signature will include its
descendant's signatures.
Here is an example:
<feed xmlns="http://www.w3.org/2005/Atom" xml:id="id3">
<wsse:Security>
<wsu:Timestamp xml:id="id1">
<wsu:Created>XXXX-XX-XXT02:41:15Z</wsu:Created>
<wsu:Expires>XXXX-XX-XXT02:46:15Z</wsu:Expires>
</wsu:Timestamp>
<wsse:BinarySecurityToken xml:id="SecurityToken-df8e8299-0d67-4eaf-bf5e-21c4dc6342ad"
ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3">
MII...
</wsse:BinarySecurityToken>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
<Reference URI="#id1">
<Transforms>
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<DigestValue>cHt...</DigestValue>
</Reference>
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<Reference URI="#id2">
<Transforms>
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<DigestValue>Oj2...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>SWz...</SignatureValue>
<KeyInfo>
<wsse:SecurityTokenReference>
<wsse:Reference URI="#SecurityToken-df8e8299-0d67-4eaf-bf5e-21c4dc6342ad"/>
</wsse:SecurityTokenReference>
</KeyInfo>
</Signature>
<!-- This is the enveloped signature, which covers all the other
signatures and hence must be generated after all the other
signatures are in place. -->
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
<Reference URI="#id3">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
<DigestValue>cRY...</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>QAw...</SignatureValue>
<KeyInfo>
<wsse:SecurityTokenReference>
<wsse:Reference
URI="#SecurityToken-df8e8299-0d67-4eaf-bf5e-21c4dc6342ad"/>
</wsse:SecurityTokenReference>
</KeyInfo>
</Signature>
</wsse:Security>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
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<id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
<entry xml:id="id1">
<title>Atom-Powered Robots Run Amok</title>
<link href="http://example.org/2003/12/13/atom03"/>
<id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
<updated>2003-12-13T18:30:02Z</updated>
<summary>Some text.</summary>
<content type="xhtml" xml:lang="en" xml:base="http://diveintomark.org/">
<div xmlns="http://www.w3.org/1999/xhtml">
<p><i>[Update: The Atom draft is finished.]</i></p>
</div>
</content>
</entry>
</feed>
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5. Selectively Encrypting parts of an Atom document
The processing rules for encrypting parts of an Atom document are
defined by sections 9.4.1 and 9.4.2 of [oasis-wssec-v1.1] with the
following additions:
o The SOAP message envelope is not used and the "wsse:Security"
element MUST be the first element of the root document.
o Use WS-Security processing to encrypt the full document. If the
encrypted result does not have to be valid according to the Atom
schema's, then the [W3C.REC-xmlenc-core-20021210] standard SHOULD
be used.
Here is an example:
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom" xml:id="id3">
<wsse:Security>
<xenc:EncryptedKey>
<xenc:EncryptionMethod
Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/>
<dsig:KeyInfo>
<wsse:SecurityTokenReference>
<wsse:KeyIdentifier
ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3SubjectKeyIdentifier">
YPArt...
</wsse:KeyIdentifier>
</wsse:SecurityTokenReference>
</dsig:KeyInfo>
<xenc:CipherData xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
<xenc:CipherValue>
JG0...
</xenc:CipherValue>
</xenc:CipherData>
<xenc:ReferenceList>
<xenc:DataReference URI="#id1"/>
<xenc:DataReference URI="#id2"/>
</xenc:ReferenceList>
</xenc:EncryptedKey>
</wsse:Security>
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
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</author>
<id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
<!-- this encrypted an atom:entry element. -->
<xenc:EncryptedData Id="id1"
Type="http://www.w3.org/2001/04/xmlenc#Element">
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#tripledes-cbc"/>
<CipherData xmlns="http://www.w3.org/2001/04/xmlenc#">
<CipherValue>
Clm...
</CipherValue>
</CipherData>
</xenc:EncryptedData>
<!-- this encrypted another atom:entry element. -->
<xenc:EncryptedData Id="id2" Type="http://www.w3.org/2001/04/xmlenc#Element">
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#tripledes-cbc"/>
<CipherData xmlns="http://www.w3.org/2001/04/xmlenc#">
<CipherValue>
Clm...
</CipherValue>
</CipherData>
</xenc:EncryptedData>
</feed>
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6. IANA Considerations
None.
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7. Security Considerations
This entire document is about securing Atom documents.
Authentication and authorization (e.g., to delete a update or delete
a posting via APP) are not discussed here.
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8. Normative References
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, December 2005.
[RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing
Protocol", RFC 5023, October 2007.
[W3C.REC-xmldsig-core-20080610]
Hirsch, F., Roessler, T., Eastlake, D., Reagle, J., and D.
Solo, "XML Signature Syntax and Processing (Second
Edition)", World Wide Web Consortium Recommendation REC-
xmldsig-core-20080610, June 2008,
<http://www.w3.org/TR/2008/REC-xmldsig-core-20080610>.
[W3C.REC-xmlenc-core-20021210]
Eastlake, D. and J. Reagle, "XML Encryption Syntax and
Processing", World Wide Web Consortium Recommendation REC-
xmlenc-core-20021210, December 2002,
<http://www.w3.org/TR/2002/REC-xmlenc-core-20021210>.
[oasis-wssec-v1.0]
Nadalin, A., Kaler, C., Hallam-Baker, P., and R. Monzillo,
"Web Services Security: SOAP Message Security 1.0", OASIS
Standard 200401, March 2004, <http://docs.oasis-open.org/
wss/2004/01/
oasis-200401-wss-soap-message-security-1.0.pdf>.
[oasis-wssec-v1.1]
Nadalin, A., Kaler, C., Hallam-Baker, P., and R. Monzillo,
"Web Services Security: SOAP Message Security 1.1", OASIS
Standard (WS-Security 2004), February 2006, <http://
docs.oasis-open.org/wss/v1.1/
wss-v1.1-spec-os-SOAPMessageSecurity.pdf>.
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Appendix A. Crypto and Security Primer
This section provides a very brief overview of common cryptographic
techniques and how they are used. It is intended only to provide the
smallest amount of information so that the rest of this document is
understandable. It is not normative.
Public-key cryptography is done using two numbers that are related to
each other. One of these numbers is public and can be known or used
by anyone, while the other is private and should only be known by its
"owner." The numbers have two very important properties. First,
anything encrypted with the public key can only be decrypted with the
private key. This means that anyone can generate text that is only
legible to the key's owner. The second is that anything that is
encrypted with the private key can be decrypted by anyone who has the
public key. This allows anyone receiving such an encrypted item to
know that it came from the key holder. The most common public-key
algorithms are RSA and Elliptic-Curve.
A message digest, or hash takes an arbitrary sequence of bytes and
returns a fixed-size identifier. The most important property is that
it is statistically unlikely for two different streams to result in
the same hash. The most useful hash algorithms are SHA-1 and SHA-
256.
A message digest encrypted with a private key is a signature. Anyone
with the original document can generate their own digest, decrypt the
received one, and compare them. If the values do not match, then the
document has been modified. The most common signature method is RSA/
SHA-n.
In symmetric cryptography, the same key is used for encryption and
decryption. This means that both sender and receiver have to have
the same key. Common symmetric (or shared-secret) algorithms are AES
and triple-DES.
A common technique is to "wrap" a symmetric key by encyrpting it with
the recipient's public key. The bulk of the data is then encrypted
using the symmetric key. This allows multiple recipients to see the
data by only encrypting the symmetric key multiple times, rather than
the entire data.
An X.509 certificate is a datum that contains a name and other
identifying information, and a public key that can be associated with
that name. The certificate also contains information such as its
validity period, the ways in which the keys are to be used, and so
on. For details see the IETF PKI Profile, RFC XXX. A certificate is
signed by a CA, and common practice is for the entity receiving a
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certificate to be configured to trust everything signed by the CA, or
perhaps the CA's CA, or further up the chain.
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Authors' Addresses
Fred (Xiangfu) Chen
IBM
1 Rogers Street
Cambridge, MA 02142
USA
Phone: +1 617-693-6313
Email: chenfr@us.ibm.com
URI: http://www.ibm.com
Maryann Hondo
IBM
1 Rogers Street
Cambridge, MA 02142
USA
Phone: +1 617-693-1236
Email: mhondo@us.ibm.com
URI: http://www.ibm.com
Rich Salz
IBM
1 Rogers Street
Cambridge, MA 02142
USA
Phone: +1 617-693-3808
Email: rsalz@us.ibm.com
URI: https://www.ibm.com/developerworks/mydeveloperworks/blogs/soma/
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