Copyright ©2001 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This document specifies a process for encrypting data and representing the result in XML. The data may be arbitrary data (including an XML document), an XML element, or XML element content. The result of encrypting data is an XML Encryption element which contains or references the cipher data.
This is the first draft of the "XML Encryption Syntax and Processing" specification from the XML Encryption Working Group (Activity).This version attempts to capture the consensus resulting from the 01 March 2001 face-to-face meeting and subsequent discussion on the list. However, it does contain points which are still under discussion or not well specified.
The Working Group will try to use a new namespace when changes in its syntax or processing are substantive. However, this namespace might be reused (prior to reaching Candidate Recommendation) by subsequent drafts in such a way as to cause instances using the namespace to become invalid or to change in meaning or affect the operation of existing software. Requests for a more stringent level of namespace stability should be made to the Working Group.
Publication of this document does not imply endorsement by the W3C membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite a W3C Working Draft as anything other than a "work in progress." A list of current W3C working drafts can be found at http://www.w3.org/TR/.
Please send comments to the editors (<reagle@w3.org>, <dee3@torque.pothole.com>) and cc: the list xml-encryption@w3.org(archives)
Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page in conformance with W3C policy.
This document specifies a process for encrypting data and
representing the result in XML. The data may be arbitrary data
(including an XML document), an XML element, or XML element
content. The result of encrypting data is an XML Encryption
EncryptedData
element which contains (via one of its
children's content) or identifies (via a URI reference) the cipher
data.
When encrypting an XML element or element content the
EncryptedData
element replaces the element or content
(respectively) in the encrypted version of the XML document.
When encrypting an entire XML document, the
EncryptedData
element may become the root of the new
document. And when encrypting arbitrary data, the the
EncryptedData
element may become the root of a new XML
document or become a child element in an application-chosen XML
document.
This specification uses XML Schemas [XML-schema] to describe the content model.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in RFC2119 [KEYWORDS]:
"they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"
Consequently, we use these capitalized keywords to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification [XML-NS] is described as "REQUIRED."
The design philosophy and requirements of this specification are addressed in the XML Encryption Requirements document [EncReq].
No provision is made for an explicit version number in this syntax. If a future version is needed, it will use a different namespace. The experimental XML namespace [XML-NS] URI that MUST be used by implementations of this (dated) specification is:
xmlns:enc='http://www.w3.org/2001/04/xmlenc#'
Additionally, this specification makes use of the XML Signature [XMLDSIG] namespace and schema definitions
xmlns:ds='http://www.w3.org/2000/09/xmldsig#'
This namespace is also used as the prefix for algorithm
identifiers used by this specification. While applications MUST
support XML and XML namespaces, the use of internal
entities [XML] or our
&enc;
entity or "enc
" XML
namespace prefix and defaulting/scoping conventions are
OPTIONAL; we use these facilities to provide compact and readable
examples.
The contributions of the following working group members to this specification are gratefully acknowledged: See partipants. [These names will placed within this document when it has reached a sufficient maturity.]
This section provides an overview and examples of XML Encryption syntax. The formal syntax is found in Core Encryption Syntax (section 3); the specific processing is given in Processing Rules (section 4).
Expressed in shorthand form, the EncryptedData
element has the
following structure:
<EncryptedData (Id='')? (Type='')?> <EncryptionMethod/>? <ds:KeyInfo> <EncryptedKey/>? <ds:*
/>? </ds:KeyInf
o>? <CipherData> <CipherValue>(encrypted character data)</CipherValu
e>? <CipherReference URI=''/>? </CipherDat
a> </EncryptedDat
a>
The CipherData
element envelopes or references the
raw encrypted data. If enveloping, the raw encrypted data is the
CipherValue
element's content; if referencing, the
CipherReference
element's URI
attribute
points to the location of the raw encrypted data
Consider the following fictitious payment information, which includes identification information and information approriate to a payment method (e.g., credit card, money transfer, or electronic check):
<?xml version='1.0'?> <PaymentInfo xmlns='http://example.org/paymentv2'> <Name>John Smith<Name/> <CreditCard Limit='5,000' Currency='USD'> <Number>4019 2445 0277 5567</Number> <Issuer>Bank of the Internet</Issuer> <Expiration>04/02</Expiration> </CreditCar
d> </PaymentInf
o>
This markup represents that John Smith's is using his credit card with a limit of $5,000USD.
Smith's credit card number is sensitive information! If the
application wishes to keep that information confidential, it can
encrypt the CreditCard
element:
<?xml version='1.0'?> <PaymentInfo xmlns='http://example.org/paymentv2'> <Name>John Smith<Name/> <EncryptedData Type='http://www.w3.org/2001/04/xmlenc#Element' xmlns='http://www.w3.org/2001/04/xmlenc#'> <CipherData><CipherValue>A23B45C56</CipherValue></CipherDat
a> </EncryptedDat
a> </PaymentInf
o>
By encrypting the entire CreditCard
element from
its start to end tags, the identity of the element itself is
hidden. (An evesdropper doesn't know whether he used a credit card
or money transfer.) The CipherData
element contains
the encrypted serialization of the CreditCard
element.
As an alternative scenario, it may be useful for intermediate
agents to know that John used a credit card with a particular
limit, but not the card's number, issuer, and expiration date. In
this case, the content (character data or children elements) of the
CreditCard
element is encrypted:
<?xml version='1.0'?> <PaymentInfo xmlns='http://example.org/paymentv2'> <Name>John Smith<Name/> <CreditCard Limit='5,000' Currency='USD'> <EncryptedData Type='http://www.w3.org/2001/04/xmlenc#Content' xmlns='http://www.w3.org/2001/04/xmlenc#'> <CipherData><CipherValue>A23B45C56</CipherValue></CipherDat
a> </EncryptedDat
a> </CreditCar
d> </PaymentInf
o>
Or, consider the scenario in which all the information except the actual credit card number can be in the clear, including the fact that the Number element exists:
<?xml version='1.0'?> <PaymentInfo xmlns='http://example.org/paymentv2'> <Name>John Smith<Name/> <CreditCard Limit='5,000' Currency='USD'> <Number> <EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#' Type='http://www.w3.org/2001/04/xmlenc#Content'> <CipherData><CipherValue>A23B45C56</CipherValue></CipherDat
a> </EncryptedDat
a> </Number> <Issuer>Bank of the Internet</Issuer> <Expiration>04/02</Expiration> </CreditCar
d> </PaymentInf
o>
Both CreditCard
and Number
are in the
clear, but the character data content of Number
is
encrypted.
If the application scenario requires all of the information to be encrypted, the whole document is encrypted as an octet set. This applies to arbitrary data including XML documents.
<?xml version='1.0'?> <EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#' Type='http://www.isi.edu/in-notes/iana/assignments/media-types/text/xml'> <CipherData><CipherValue>A23B45C56</CipherValue></CipherDat
a> </EncryptedDat
a>
An XML document may contain zero or more
EncryptedData
elements. However, EncryptedData
can not be the parent or child of another
EncryptedData
element. However, the actual data encrypted
can be anything, including EncryptedData
and
EncryptedKey
elements (i.e., super-encryption). During
super-encryption of an EncryptedData
or
EncryptedKey
element, one must encrypt the entire element.
Encrypting only the content of these elements, or encrypting
selected child elements is an invalid instance under the provided
schema.
For example, consider the following:
<pay:PaymentInfo
xmlns:pay='http://example.org/paymentv2'> <EncryptedData ID='ED1' Type='http://www.w3.org/2001/04/xmlenc#Element' xmlns='http://www.w3.org/2001/04/xmlenc#'> <CipherData> <CipherValue>original
EncryptedData</CipherValue> </CipherDat
a> </EncryptedDat
a> </pay:PaymentInf
o>
A valid super-encryption of
'
//EncryptedData[@ID='ED1']' would be:
<pay:PaymentInfo
xmlns:pay='http://example.org/paymentv2'> <EncryptedData ID='ED2' Type='http://www.w3.org/2001/04/xmlenc#Element' xmlns='http://www.w3.org/2001/04/xmlenc#'> <CipherData><CipherValue>new
EncryptedData</CipherValue></CipherDat
a> </EncryptedDat
a> </pay:PaymentInf
o>
where 'newEncryptedData
' is the base64 encoding of
the encrypted octet sequence resulting from encrypting the
EncryptedData
element with Id='ED1'
.
EncryptedData
and EncryptedKey
UsageEncryptedData
with Symmetric
Key (KeyName
)[s1] <EncryptedData xmlns='http://www.w3.org/2001/04/xmlenc#' Type='http://www.w3.org/2001/04/xmlenc#Element'/> [s2] <EncryptionMethod Algorithm='http://www.w3.org/2001/04/xmlenc#3des-cbc '/> [s3] <ds:KeyInfo xmlns:ds='http://www.w3.org/2000/09/xmldsig#'> [s4] <ds:KeyName
> John Smith </ds:KeyNam
e> [s5] </ds:KeyInf
o> [s6] <CipherData><CipherValue>DEADBEEF</CipherValue></CipherDat
a> [s7] </EncryptedDat
a>
[s1]
The type of data encrypted may be represented
as an attribute value as an aid in decryption and subsequent
processing. In this case, the data encrypted was an 'Element'.
Other alternatives include 'Content' of an element, or an an
external octet sequence that is identified by a media type URI.
[s2]
This (3DES CBC) is a symmetric key cipher.
[s4-s5]
The symmetric key has the name John
Smith.
[s6]
CipherData's
CipherValue
will always be a base64 encoded octet sequence
or a URI reference with any transforms necessary to obtain the
cipher data as an octet sequence.
EncryptedKey
(ReferenceList
,
ds:RetrievalMethod
, CarriedKeyName
)The following EncryptedData
structure is very
similar to the one above, except this time the key is referenced
using a ds:RetrievalMethod
:
[t01] <EncryptedData Id='ED' xmlns:enc='http://www.w3.org/2001/04/xmlenc#'> [t02] <EncryptionMethod Algorithm='http://www.w3.org/2001/04/xmlenc#aes128-cbc'/> [t03] <ds:KeyInfo
xmlns:ds='http://www.w3.org/2000/09/xmldsig#'> [t04] <ds:RetrievalMethod
URI='#EK'/ Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey"> [t05] <ds:KeyNam
e>John Doe<ds:KeyNam
e> [t06] </ds:KeyInf
o> [t07] <CipherData><CipherValue>DEADBEEF</CipherValue></CipherDat
a> [t08] </EncryptedDat
a>
[t02]
This (AES-128-CBC) is a symmetric key
cipher.
[t03]
The (AES) key is located at '#EK'.
[t04]
ds:RetrievalMethod
is used to
indicate the location of a key with type
&enc;EncryptedKey.
[t05]
ds:KeyName
provides an
alternative method of identifying the key needed to decrypt the
CipherData
. Either or both the KeyName
and KeyRetrivalMethod
could be used to identify the
key.
[t09] <EncryptedKey Id='EK'CarriedKeyName
="John Doe" [t10] xmlns='http://www.w3.org/2001/04/xmlenc#'> [t11] <EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/> [t12] <ds:KeyInfo
xmlns:ds='http://www.w3.org/2000/09/xmldsig#'> [t13] <ds:KeyNam
e>John Smith</ds:KeyNam
e> [t14] </ds:KeyInf
o> [t15] <CipherData><CipherValue>xyzabc</CipherValue></CipherDat
a> [t16] <ReferenceList> [t17] <DataReference URI='#ED'/> [t18] </ReferenceLis
t> [t19] </EncryptedKe
y>
[t09]
The EncryptedKey
element is
similar to the EncryptedData
element except that the
data encrypted is always a key value. The
CarriedKeyName
attribute is used to identify the encrypted
key value which may be referenced by the KeyName
element in ds:KeyInfo
.
[t11]
The EncryptionMethod
is the RSA
public key algorithm.
[t13]
ds:KeyName
of "John Smith" is a
property of the key necessary for decrypting (using RSA) the
CipherData
.
[t15]
The CipherData
's
CipherValue
is an octet sequence that is encoded (e.g.,
padded) by a referring encrypted object's
EncryptionMethod
. (Note, an EncryptedKey's
EncryptionMethod
is the algorithm used to encrypt these
octets and does not speak about what type of octets they are.)
[t16-18]
A ReferenceList
identifies
the encrypted objects (DataReference
and
KeyReference
) encrypted with this key. The
ReferenceList
contains a list of references to data
encrypted by the symmetric key carried within this structure.
This section provides a detailed description of the syntax and features for XML Encryption. Features described in this section are mandatory to implement unless otherwise noted. The syntax is defined via [XML-Schema] with the following XML preamble, declaration, internal entity, and import:
Schema Definition:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
"http://www.w3.org/2001/XMLSchema.dtd"
[
<!ATTLIST schema
xmlns:enc CDATA #FIXED 'http://www.w3.org/2001/04/xmlenc#'
xmlns:ds CDATA #FIXED 'http://www.w3.org/2000/09/xmldsig#'>
<!ENTITY % p ''>
<!ENTITY % s ''>
]>
<schema xmlns='http://www.w3.org/2001/XMLSchema' version='0.1'
xmlns:ds='http://www.w3.org/2000/09/xmldsig#'
xmlns:enc='http://www.w3.org/2001/04/xmlenc#'
targetNamespace='http://www.w3.org/2001/04/xmlenc#'
elementFormDefault
='qualified'>
<import namespace='http://www.w3.org/2000/09/xmldsig#'
schemaLocation='http://www.w3.org/TR/2001/CR-xmldsig-core-20010419/xmldsig-core-schema.xsd'/>
EncryptedType
EncryptedType
is the abstract type from which
EncryptedData
and EncryptedKey
are
derived. While these two latter element types are very similar with
respect to their content models, a syntactical distinction is
useful to processing.
Schema Definition: <complexType name='EncryptedType
' abstract='true'> <sequence> <element name='EncryptionMethod
' type='enc:EncryptionMethodType
' minOccurs='0'/> <element ref='ds:KeyInfo
' minOccurs='0'/> <element ref='enc:CipherData
'/> </sequence> <attribute name='Id' type='ID' use='optional'/> </complexType>
EncryptionMethod
is an optional element that
describes the encryption algorithm applied to the cipher data. If
the element is absent, the encryption algorithm is assumed to be
known by the recipient.
ds:KeyInfo
is an optional element, defined by [XMLDSIG], that carries information about
the key used to encrypt the CipherData
. The new
elements defined by this specification that may appear as children
of ds:KeyInfo
are described in subsequent
sections.
CipherData
is a mandatory element that contains the
CipherValue
or CipherReference
with the
encrypted data.
Id
is an optional attribute providing for the
standard method of assigning a string id to the element within the
document context.
CipherData
ElementThe CipherData
is a mandatory element that provides
the encrypted data. It must either contain the encrypted octet
sequence as base64 encoded text of the CipherValue
element, or provide a reference to an external location containing
the encrypted octet sequence via the CipherReference
element.
The optional set of ds:DigestMethod
and
ds:DigestValue
elements are provided for ensuring the
integrity of the encrypted data. See section 5.6 of the algorithm
specification for more information.
Schema Definition: <element name='CipherData
' type='enc:CipherDataType
'/> <complexType name='CipherDataType
'> <sequence> <choice> <element name='CipherValue
' type='ds:CryptoBinary
'/> <element ref='enc:CipherReference
'/> </choice> <sequence minOccurs='0'> <element ref='ds:DigestMethod
'/> <element ref='ds:DigestValue
'/> </sequence> </sequence> </complexType>
CipherReference
ElementIf CipherValue
is not supplied directly, the
CipherReference
identifies a source which, when processed,
yields the encrypted octet sequence.
The actual value is obtained as follows. The
CipherReference
URI
contains an identifer that
is dereferenced. Should the CipherReference
element
contain an OPTIONAL sequence of Transforms
, the data
resulting from deferenced URI is transformed as specified so as to
yield the intended cipher value. For example, if the value is
base64 encoded within an XML document; the transforms could specify
an XPath expression followed by a base64 decoding so as to extract
the octets.
The syntax of the URI
and Transform
s
is similar to that of [XMLDSIG] reference validation. However, there is a
difference between signature and encryption processing. In [XMLDSIG] both generation and validation
processing start with the same source data and perform that
transform in the same order. In encryption, the decryptor has only
the cipher data and the specified transforms are enumerated for the
decryptor, in the order necessary to obtain the octets.
For example, if the relevant cipher value is captured within an
CipherValue
element within a different XML document,
the CipherRerence
might look as follows:
<CipherReference URI="http://www.example.com/CipherValues.xml">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<XPath xmlns:rep="&repository;">
self::text()[parent::CipherValue[@id="example1"]]
</XPath>
<Transform>
<Transform Algorithm="decode"/>
</Transforms>
</CipherReferenc
e>
Schema Definition: <element name='CipherReference
' type='enc:CipherReferenceType
'/> <complexType name='CipherReferenceType
'> <choice> <element name='Transforms' minOccurs='0'/> </choice> <attribute name='URI' type='anyURI' use='required'/> </complexType> <element name="Transforms" type="ds:TransformsType"/> <complexType name="TransformsType"> <sequence> <element ref="ds:Transform" maxOccurs="unbounded"/> </sequence> </complexType>
EncryptedData
elementThe EncryptedData
element is the core element in
the syntax. Not only does its CipherData
child contain
the encrypted data, but it's also the element that replaces the
encrypted element, or serves as the new document root.
Schema Definition: <element name='EncryptedData
' type='enc:EncryptedDataType
'/> <complexType name='EncryptedDataType
'> <complexContent> <extension base='enc:EncryptedType
'> <attribute name='Type' type='anyURI' use='optional'/> </extension> </complexContent> </complexType>
Type
is an optional attribute identifying type
information about the decrypted content. This type information
plays a key role in the behavior of compliant decryptors as
decribed in Section 4.2. If the EncryptedData
element
contains data of Type
Element
or
ElementContent
, and replaces that data in an XML Document
context, it is strongly recommended the Type
attribute
be provided. Without this information, the decryptor will be unable
to automatically restore the XML Document to its original
clear-text form.
ds:KeyInfo
ElementThere are different ways to define the key material to be used
in decrypting the CipherData
:
EncryptedData
or EncryptedKey
element specifies the associated key material:
EncryptedKey
element
ds:RetrievalMethod
element used to indicate the URI of an
EncryptedKey
or a KeyName
element used to
indicate a key known by the recipient.EncryptedKey
element specifies the
EncryptedData
or EncryptedKey
element which
needs it:
EncryptedKey
element can refer to the
EncryptedData
element via a DataReference
element.This specification defines the EncryptedKey
element
and uses the ds:RetrievalMethod
element as described
in subsequent sections.
EncryptedKey
ElementThe EncryptedKey
element is used to transport
encryption keys from the originator to a known recipient(s). It may
be used as a standalone XML document, be placed within an
application document, or appear inside an EncryptedData element as
a child of a ds:KeyInfo
element. The key value is
always encrypted to the recipient(s).
Schema Definition: <element name='EncryptedKey
' type='enc:EncryptedKeyType
'/> <complexType name='EncryptedKeyType
'> <complexContent> <extension base='enc:EncryptedType
'> <sequence> <element ref='enc:ReferenceList
' minOccurs='0'/> </sequence> <attribute name='CarriedKeyName
' type='string' use='optional'/> <attribute name='Recipient' type='string' use='optional'/> </extension> </complexContent> </complexType>
ReferenceList
is an optional element containing
pointers to data and keys encrypted using this key. The reference
list may contain multiple references to EncryptedKey
and EncryptedData
elements. This is done using
KeyReference
and DataReference
elements
repectively. These are defined below.
CarriedKeyName
is an optional attribute for
associating a user readable name with the key value. This may then
be used to reference the key using the ds:KeyName
element within ds:KeyInfo
. The same
CarriedKeyName
label, unlike an ID type, may occur multiple
times within a single document. The value of the key is to be the
same in all EncryptedKey
elements identified with the
same CarriedKeyName
label within a single XML
document
Recipient
is an optional attribute that contains a
hint as to which recipient this encrypted key value is intended
for. Its contents are application dependent.
ds:RetrievalMethod
ElementThe ds:RetrievalMethod
[XMLDSIG]
element provides a way to
express a link to an EncryptedKey element containing the key needed
to decrypt the CipherData
associated with an
EncryptedData
or EncryptedKey
element. It
always refers to an EncryptedKey
and it's
Type
is always
'http://www.w3.org/2001/04/xmlenc#EncryptedKey
'. The
ds:RetrievalMethod
with this type is always a child of
the ds:KeyInfo
element and may appear multiple times.
If there is more than one instance of a
ds:RetrievalMethod
in a ds:KeyInfo
, then the
EncryptedKey
objects referred to must contain the same
key value, possibly encrypted in different ways or for different
recipients.
Schema Definition:
<!--
<attribute name='Type' type='anyURI' use='optional'
fixed='http://www.w3.org/2001/04/xmlenc#EncryptedKey
' />
-->
ReferenceList
ElementReferenceList
is an element that contains pointers
from a key value to items encrypted by that key value
(EncryptedData
or EncryptedKey
elements).
Schema Definition: <element name='ReferenceList
'> <complexType> <sequence> <element name='DataReference
' type='enc:ReferenceType
' minOccurs='0' maxOccurs='unbounded'/> <element name='KeyReference
' type='enc:ReferenceType
' minOccurs='0' maxOccurs='unbounded'/> </sequence> </complexType> </element> <complexType name='ReferenceType
'> <sequence> <any namespace='##other' minOccurs='0' maxOccurs='unbounded'/> </sequence> <attribute name='URI' type='anyURI' use='optional'/> </complexType>
DataReference
elements are used to refer to
EncryptedData
elements that were encrypted using the key
defined in the enclosing EncryptedKey
element.
Multiple DataReference
elements can occur if multiple
EncryptedData
elements exist that are encrypted by the
same key.
KeyReference
elements are used to refer to
EncryptedKey
objects that were encrypted using the key
defined in the enclosing EncryptedKey
element.
Multiple KeyReference
elements can occur if multiple
EncryptedKey
elements exist that are encrypted by the
same key.
For both types of references one may optionally specify child
elements to aid the recipient in retrieving the
EncryptedKey
and/or EncryptedData
elements.
These could include information such as XPath transforms,
decompression transforms, or information on how to retrieve the
objects from a document storage facility.
This section describes the operations to be performed as part of encryption and decryption processing.
For each data item or key to be encrypted:
Encryption applications are recommended to use the optional
Type
attribute of EncryptedData
with the
approriate value so as to allow automated document restoration
processing as described in Section 4.2.
EncryptedData
structure including or
referencing the encrypted data and use it as the top-level element
in a new XML Document or insert it into another XML document (this
is processing is application dependent).For each item to be decrypted (either an
EncryptedData
or EncryptedKey
element):
CipherData
element. When the data is XML, the resulting
octets are interpretated as an UTF-8 encoded string of XML
characters representing an element or element content.
CipherData
contains a CipherValue
element, obtain the octet stream by de-base64ing its content.CipherData
contains a
CipherReference
element, dereference the value of the
URI
attribute and apply the specified transforms (if any) to
obtain the octet stream.EncryptedData
structure and the
Type
is "Element" or "Content"
, then place the
resulting characters in place of the EncryptedData element with the
encoding of the parent XML document if necessary. Otherwise, the
octet sequence is the final result.The specification above presumes that the data to be encrypted
is processed as an octet sequence. The application is responsible
for serializing the XML into an octet sequence that will be useful
subsequent to decryption. For instance, if the applications wishes
to canonicalize (using [XML-C14N] or
some other serialization) or encode/compress the data in an XML
packaging format, the application needs to marshal the XML
accordingly and identify the resulting type with optional the
EncryptedData
Type
attribute. The
likelihood of interoperable decryption and subsequent use will be
dependent on the decryptors support for a given type. Also, if the
data is intended to be processed both before and after decryption
(e.g., XML Signature [XMLDSIG]
validation or XSLT transform) the encryptor must be careful to
preserve information necessary for that process's success.
For interoperability purposes, the following types MUST be implemented.
EmptyElemTag
| STag
content ETag"
[XML]CharData
? ((element
|
Reference |
CDSect | PI |
Comment)
CharData
?)*" [XML]This section discusses algorithms used with the XML Encryption
specification. Entries contain the identifier to be used as the
value of the Algorithm
attribute of the
EncryptionMethod
element or other elements representing the
role of the algorithm, a reference to the formal specification and
definitions, where applicable, for the representation of keys and
the results of cryptographic operations.
All algorithms listed below have implicit parameters depending on their role. For example, the data to be encrypted or decrypted, keying material, and direction of operation (encrypting or decrypting) for encryption algorithms. Any explicit additional parameters to an algorithm appear as content elements within the element. Such parameter child elements have descriptive element names, which are frequently algorithm specific, and SHOULD be in the same namespace as this XML Encryption specification, the XML Signature specification, or in an algorithm specific namespace. An example of such an explicit parameter could be a nonce (unique quantity) provided to a key agreement algorithm.
This specification defines a set of algorithms, their URIs, and requirements for implementation. Requirements are specified over implementation, not over requirements for encryption use. Furthermore, the mechanism is extensible, alternative algorithms may be used by encryption applications.
The table below lists the categories of algorithms. Within each category, a brief name, the level of implementation requirement, and a URI are given.
The schema for EncryptionMethod is as follows:
Schema Definition: <complexType name="EncryptionMethodType" mixed="true"> <sequence> <element name="KeySize" minOccurs="0" type="KeySizeType"/> <element name="DigestMethod" minOccurs="0" type="ds:DigestMethodType"/> <element name="OAEPparams" minOccurs="0" type="OAEPparamsType"/> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) external namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
NOTE: Which child elements to the EncryptionMethod
algorithm role are allowed or required depends on the specific
value of the Algorithm
attribute URI. (Schema does not
provide a facility for expressing conditionality of child element
occurrance based on attribute value.) The presence of any child
element under EncryptionMethod
which is not permitted
by the algorithm MUST be treated as an error.
Block encryption algorithms are designed for encrypting and
decrypting data. Their identifiers appear as the value of the
Algorithm
attributes of EncryptionMethod
elements that are children of EncryptedData
.
Block encryption algorithms take, as implicit arguments, the data to encrypted or decrypted, the keying material, and their direction of operation. For all of these algorithms specified below, an initialization vector (IV) is required that is encoded with the cipher text. For user specified block encryption algorithms, the IV, if any, could be specified as being with the cipher data, as an algorithm element content, or elsewhere.
The IV is encoded with the data for the algorithms below for ease of availability to the decryption code and to emphasize its association with the cipher text. Good cryptographic practice requires that a different IV be used for every encryption.
The triple DES algorithm is described in FIPS 46-3 [DES] and ANSI X9.52 [3DES]. It is composed of three sequential DES operations. The XML Encryption 3DES consists of a DES encrypt, a DES decrypt, and a DES encrypt used in the Cipher Block Chaining (CBC) mode with 192 bits of key and a 64 bit Initialization Vector (IV). Of the key bits, the first 64 are used in the first DES operation, the second 64 bits in the middle DES operation, and the third 64 bits in the last DES operation. (Each of these 64 bits of key contain 56 effective bits and 8 parity bits.) The resulting cipher text is prefixed by the IV before being encoded in base64 for inclusion in XML output. Encryption applications MUST implement 3DES for data encryption. An example 3DES EncryptionMethod is as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#des3-cbc"/>
The AES algorithm is described in [AES]. XML Encryption implementations MUST support AES with 128 bit and 256 bit keys and MAY support AES with 192 bit keys. AES is used in the Cipher Block Chaining (CBC) mode with a 128 bit Initialization Vector (IV). The resulting cipher text is prefixed by the IV before being encoded in base64 for inclusion in XML output. An example AES EncryptionMethod is as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#aes128-cbc"/>
Simple stream encryption algorithms generate, based on the key, a stream of bytes which are XORed with the plain text bytes to produce the cipher text on encryption and with the cipher text bytes to produce plain text on decryption.
NOTE: It is critical that each simple stream encryption key (or key and initialization vector (IV) if an IV is also used) be used once only. If the same key (or key and IV) is ever used on two messages then, by XORing the two cipher texts, you can obtain the XOR of the two plain texts. This is usually very compromising.
ARCFOUR is an fast simple stream encryption algorithm that is
compatible with RSA Security's RC4(tm) algorithm. It takes an
optional KeySize
explicit parameter. In cases where
the key size is not apparent, as in the use of key agreement
methods, this parameter sets the key size. If the size of the key
to be used is apparent and disagrees with the KeySize
parameter, an error should be returned. An example of an ARCFOUR
EncryptionMethod
is as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#arcfour">
<KeySize>40</KeySize>
</EncryptionMethod>
Implementation of ARCFOUR is optional. The schema for the KeySize parameter is as follows:
Schema Definition: <simpleType name='KeySizeType'> <restriction base="integer"/> </simpleType>
Key Transport algorithms are public key encryption algorithms
especially specified for encrypting and decrypting keys. Their
identifiers appear as Algorithm
attributes to
EncryptionMethod elements that are children of
EncryptedKey
. The type of key being transported is given by
the Type
attribute of the EncryptedKey
element. This attribute value must be the URI of an encryption
algorithm.
The Key Transport algorithms given below are those used in conjunction with the Cryptographic Message Syntax (CMS) of S/MIME [CMS-Algorithms, CMS-AES].
This is the RSAES-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. The RSA-PKCS1-v1_5 algorithm takes no
explicit parameters. An example of an RSA Version 1.5
EncryptionMethod
element is:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5"/>
The CipherData
for such an encrypted key is the
base64 [MIME] encoding of the octet string
computed as per RFC 2437 [PKCS1, section
7.2.1: Encryption operation]. As specified in the EME-PKCS1-v1_5
function RFC 2437 [PKCS1, section
9.1.2.1], the value input to the key transport function is as
follows:
CRYPT ( PAD ( KEY ))
where the padding is of the following special form:
02 | PS* | 00 | key
where "|" is concatenation, "02" and "00" are fixed octets of the corresponding hexadecimal value, PS is a string of strong pseudo-random octets [RANDOM] at least eight octets long, containing no zero octets, and long enough that the value of the quantity being CRYPTed is one octet shorter than the RSA modulus, and "key" is the key being transported. The key is 168 bits for 3DES and 128, 192, or 256 bits for AES. Support of this key transport algorithm for transporting 3DES keys is mandatory to implement. Support of this algorithm for transporting AES and ARCFOUR keys is optional. RAS-OAEP is recommended for the transport of AES keys.
The resulting base64 [MIME] string is
the value of the child text node of the CipherData
element, e.g.
<CipherData> IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4 t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw= </CipherData>
This is the RSAES-OAEP-ENCRYPT algorithm described in RFC 2437
[PKCS1]. The RSA-OAEP algorithm takes as
explicit parameters a message digest function and an optional octet
string OAEPparams
. The message digest function is
indicated by the Algorithm
attribute of a child
DigestMethod
element and the octet string is the UTF-8
encoding of the text child of an optional OAEPparams
element with white space (space, tab, CR, and LF) stripped. An
example of an RSA-OAEP element is:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-oaep">
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<OAEPparams> foo </OAEPparams>
<EncryptionMethod>
The CipherData
for an RSA-OAEP encrypted key is the
base64 [MIME] encoding of the octet string
computed as per RFC 2437 [PKCS1, section
7.1.1: Encryption operation]. As described in the EME-OAEP-ENCODE
function RFC 2437 [PKCS1, section
9.1.1.1], the value input to the key transport function is
calculated use the message digest function and string specified in
the DigestMethod
and OAEPparams
elements
and using the mask generator function MGF1 specified in RFC 2437.
The desired output length for EME-OAEP-ENCODE is one byte shorter
than the RSA modulus.
Encryption applications MUST implement RSA-OAEP for the transport of 128 and 256 bit AES keys. They MAY implement RSA-OAEP for the transport of 192 bit AES keys, 3DES keys, and ARCFOUR keys.
A Key Agreement algorithm provides for the agreement to a shared
secret quantity based on certain types of compatible public keys
from both the sender and the recipient. Information to determine
the key associated with the originator is indicated by an optional
OriginatorKeyInfo
parameter child of an
AgreementMethod
element while that associated with the
recipient is indicated by an optional
RecipientKeyInfo
. The AgreementMethod
element
appears as the content of a ds:KeyInfo
since, like
other ds:KeyInfo
children, it yields a key. This
ds:KeyInfo
is in turn a child of an
EncryptedData
or EncryptedKey
and, in fact, the
Type
attribute in this grandparent
EncryptedData
or EncryptedKey
is an implicit
parameter to the key agreement computation. In addition, the sender
may include a Nonce
element under
AgreementMethod
to assure that different keying material is
generated even for repeated agreements using the same sender and
recipient public keys. For example:
<EncryptedData> <EncryptionMethod Algorithm="Example:Block/Algorithm" <KeySize>80</KeySize> </EncryptionMethod> <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> <AgreementMethod Algorithm="Example:Agreement/Algorithm"> <Nonce> foo </Nonce> <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"> <OriginatorKeyInfo> <KeyValue>...originator...</KeyValue> </OriginatorKeyInfo> <RecipientKeyInfo> <KeyValue>...recipient...</KeyValue> </RecipientKeyInfo> </AgreementMethod> </ds:KeyInfo> <CipherData>...</CipherData> </EncryptedData>
If the agreed key is being used to wrap a key, rather than data
as above, then AgreementMethod
would appear inside a
ds:KeyInfo
inside an EncryptedKey
element.
The Schema for AgreementMethod
is as follows:
Schema Definition: <element name="AgreementMethod" type="enc:AgreementMethodType"/> <complexType name="AgreementMethodType" mixed="true"> <sequence> <element name="Nonce" minOccurs="0" type="string"/> <element name="DigestMethod" minOccurs="0" type="ds:DigestMethodType"> <element name="OriginatorKeyInfo" minOccurs="0" type="ds:KeyInfoType"> <element name="RecipientKeyInfo" minOccurs="0" type="ds:KeyInfoType"> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) external namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
The AgreementMethod
will derive some shared secret
octet sequence ZZ. The amount of actual keying material needed will
then be calculated as follows:
Keying Material = KM(1) | KM(2) | ...
where "|" is byte stream concatenation and
KM(counter) = DigestAlg ( EncryptionAlg | ZZ | counter | Nonce | KeySize ).
DigestAlg is the message digest algorithm specified by the
DigestMethod
child of AgreementMethod
.
EncryptionAlg is the URI of the encryption algorithm, including
possible key wrap algorithms, in which the derived keying material
is to be used ("Example:Block/Algorithm" in the example above), not
the URI of the agreement algorithm. Nonce
is the UTF-8
serialization of the text child of the Nonce
child of
AgreementMethod
, if present, with white space (space,
tab, CR, and LF) stripped. If the Nonce
element is
absent, it is null. Counter is a one byte counter. KeySize is the
size in bits of the key to be derived from the shared secret as the
UTF-8 string for the corresponding decimal integer with only digits
in the string and no leading zeros. For some algorithms the key
size is inherent in the URI. For others, such as ARCFOUR, it may be
explicitly provided. For example, the initial (KM(1)) calculation
for the example above, with ZZ not replaced by the binary shared
secret octet sequence and the binary "1" counter byte represented
as %01, would be calculated as follows:
SHA-256 ( Example:Block/AlgorithmZZ%01foo40 )
Each application of DigestAlg will produces some number of bytes of keying material. From the concatenated string of one or more KM's, enough leading bytes are taken to meet the need for an actual key and the remainder discarded. For example, if DigestAlg is SHA1 which produces 20 octets of hash, then for 128 bit AES the first 16 bytes from KM(1) would be taken and the remaining 4 bytes discarded. For 256 bit AES, all of KM(1) suffixed with the first 12 bytes of KM(2) would be taken and the remaining 8 bytes of KM(2) discarded.
Diffie-Hellman keys can appear directly within
KeyValue
elements or be obtained by
ds:RetrievalMethod
fetches as well as appearing in
certificates and the like. The above identifier can be used as the
value of the Type
attribute of Reference
or ds:RetrievalMethod
elements.
A DH public key consists of three quantities, a large Prime p, a
"Generator" g, and "Public" such that Public = g**x mod p. The
corresponding private key is x. Because a Prime and Generator can
be safely shared over many DH keys, they may be known from the
application environment and are optional. The schema for a
DHKeyValue
is as follows:
Schema:
<element name="DHKeyValue" type="enc:DHKeyValueType"/>
<complexType name="DHKeyValueType">
<sequence>
<element name="Prime" type="ds:CryptoBinary" minOccurs="0"/>
<element name="Generator" type="ds:CryptoBinary" minOccurs="0"/>
<element name="Public" type="ds:CryptoBinary"/>
</sequence>
</complexType>
Diffie-Hellman (DH) key agreement involves the derivation of shared secret information based on compatible DH keys from the sender and recipient. Two DH public keys are compatible if they have the same prime and generator. If, for the second one, Y = g**y mod p, then the two parties can calculate the shared secret ZZ = ( g**(x*y) mod p ) even though each knows only their own private key and the other party's pubic key. Leading zero bytes MUST be maintained in ZZ so it will be the same length, in bytes, as p. We require that p be at least 512 bits and g at least 160 bits. There are numerous other complex security considerations in the selection of g, p, and a random x as described in [ESDH].
Diffie-Hellman key agreement is optional to implement. An example of a DH AgreementMethod element is as follows:
<AgreementMethod Algorithm="http://www.w3.org/2001/04/xmlenc#dh">
<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#"><X509Data><X509Certificate>
...
</X509Certificate></X509Data></KeyInfo>
</AgreementMethod>
Symmetric Key Wrap algorithms are shared secret key encryption
algorithms especially specified for encrypting and decrypting
symmetric keys. Their identifiers appear as Algorithm
attributes to EncryptionMethod
elements that are
children of EncryptedKey
. The type of the key being
wrapped is indicated by the Type
attribute of
EncryptedKey
.
Some key wrap algorithms make use of the Key Checksum defined in CMS [CMS-Algorithms]. This is used to provide an integrity check value for the key being wrapped. The algorithm is
The type of the key being wrapped is given by the
Type
attribute of the parent EncryptedKey
element. XML Encryption applications MUST support 3DES wrapping of
3DES keys and may optionally support 3DES wrapping of AES keys. An
example of a 3DES Key Wrap EncryptionMethod
element is
a as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#kw-3des"/>
The following algorithm wraps (encrypts) a key (the wrapped key, WK) under a 3DES key-encryption-key (KEK):
The following algorithm unwraps (decyrpts) a key:
The above specification is that given in [CMS-Algorithms].
Implementation of AES key wrap as specified by NIST/NSA/CMS will be mandatory for AES 128 and AES 256 and recommended for AES 192 -- when it's completely specified.
Message digest algorithms are used in CipherData
to
insure integrity, in AgreementMethod
as part of the
key derivation, and within RSA-OAEP encryption as a hash function,
and in connection with the HMAC Message Authentication Code method
as described in [XMLDSIG].)
The SHA-1 algorithm [SHA] takes no
explicit parameters. XML encryption applications MUST implement
SHA-1. An example of an SHA-1 DigestMethod
element
is:
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
A SHA-1 digest is a 160-bit string. The content of the
DigestValue
element shall be the base64 encoding of this bit
string viewed as a 20-octet octet stream. For example, the
DigestValue
element for the message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be:
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
The SHA-256 algorithm [SHA] takes no
explicit parameters. It is RECOMMENDED that XML encryption
applications implement SHA-256. An example of an SHA-256
DigestMethod
element is:
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha256"/>
A SHA-256 digest is a 256-bit string. The content of the
DigestValue
element shall be the base64 encoding of this bit
string viewed as a 32-octet octet stream.
The SHA-512 algorithm [SHA] takes no
explicit parameters. XML encryption applications MAY implement
SHA-512. An example of an SHA-512 DigestMethod
element
is:
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha512"/>
A SHA-512 digest is a 512-bit string. The content of the
DigestValue
element shall be the base64 encoding of this bit
string viewed as a 64-octet octet stream.
The RIPEMD-160 algorithm [RIPEMD-160] takes no explicit
parameters. XML encryption applications may implement RIPEMD-160.
An example of an RIPEMD-160 DigestMethod
element
is:
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#ripemd160"/>
A RIPEMD-160 digest is a 160-bit string. The content of the
DigestValue
element shall be the base64 encoding of
this bit string viewed as a 20-octet octet stream.
XML Signature [XMLDSIG] is optional to implement for XML encryption applications. It is the recommended way to provide key based authentication.
If XML is to be encrypted it must first be serialized into an octet stream If it is to be later decrypted into a different environment and it is desired to preserve such aspects of its original environment as namespace prefix bindings, the value of attributes in the "xml" namespace, etc., then the Canonical XML With Comments version of the XML should be the serialization that is encrypted [Canon]. Although this is not, properly speaking, a part of the encryption/decryption process, it is RECOMMENDED that XML encryption applications implement Canonical XML With Comments and they MAY also implement Canonical XML (without comments).
The application of both encryption and digital signatures over portions of an XML document can make subsequent decryption and signature verification difficult. In particular, when verifying a signature one must be know whether the signature was computed over the encrypted or unencrypted representation of elements.
A separate, but important, issue is introducing cryptographic vulnerabilities when combining digital signatures and encryption over a common XML element. Hal Finney has suggested that encrypting digitally signed data, while leaving the digital signature in the clear, may allow plaintext guessing attacks.
In accordance with the requirements document [EncReq
] the interaction of
encryption and signing is an application issue and out of scope of
the specification. However, we make the following
recommendations:
Where a symmetric key is shared amongst multiple recipients, that symmetric key should only be used for the data intended for those multiple recipients because even if they are not directed to information encrypted in that symmetric key, they may be able to discover and decyrpt it.
...
LaMacchia
. Aug 09 2000.LaMacchia
, Maruyama, Schaad, Simon. December
2000.McCahill
. December 1994.