XML Profile for Signature Validation Tokens
Version 1.0 - 2020-12-16 - Draft version
Registration number: 2020-62
Copyright © The Swedish Agency for Digital Government (DIGG), 2015-2020. All Rights Reserved.
Table of Contents
-
1.2. Definitions
1.2. Notation
-
2.1.1. SignatureValidationToken Signature Property
2.1.2. Multiple SVT in a signature
1. Introduction
The "Signature Validation Token" specification [SVT] defines the basic token to support signature validation in a way that can significantly extend the lifetime of a signature.
This specification defines a profile for implementing SVT with a signed XML document, and defines the following aspects of SVT usage:
- How to include reference data related to XML signatures and XML documents in an SVT.
- How to add an SVT token to a XML signature.
XML documents can have any number of signature elements, signing an arbitrary number of fragments of XML documents. The actual signature element may be included in the signed XML document (enveloped), include the signed data (enveloping) or may be separate from the signed content (detached).
To provide a generic solution for any type of XML signature an SVT is added to each XML signature element within the XML signature <ds:Object>
element.
1.1. Requirements Notation
The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
These keywords are capitalized when used to unambiguously specify requirements over protocol features and behavior that affect the interoperability and security of implementations. When these words are not capitalized, they are meant in their natural-language sense.
1.2. Definitions
The definitions in [SVT] apply also to this document.
1.2. Notation
1.2.1 References to XML Elements from XML Schemas
When referring to elements from the W3C XML Signature namespace
(http://www.w3.org/2000/09/xmldsig\#
) the following syntax is used:
<ds:Signature>
When referring to elements from the ETSI XAdES XML Signature namespace
(http://uri.etsi.org/01903/v1.3.2#
) the following syntax is used:
<xades:CertDigest>
When referring to elements defined in this specification
(http://id.swedenconnect.se/svt/1.0/sig-prop/ns
) the following syntax is used:
<svt:Element>
2. SVT in XML Documents
When SVT is provided for XML signatures then one SVT SHALL be provided for each XML signature.
An SVT embedded within the XML signature element SHALL be placed in a <svt:SignatureValidationToken>
element as defined in section 2.1.1.
2.1.1. SignatureValidationToken Signature Property
The <svt:SignatureValidationToken>
element SHALL be placed in a <ds:SignatureProperty>
element in accordance with [XMLDsig]. The <ds:SignatureProperty>
element SHALL be placed inside a <ds:SignatureProperties>
element inside a <ds:Object>
element inside a <ds:Signature>
element.
Note: [XMLDsig] requires the Target
attribute to be present in <ds:SignatureProperty>
, referencing the signature targeted by this signature property. If an SVT is added to a signature that do not have an Id
attribute, implementations SHOULD add an Id
attribute to the <ds:Signature>
element and reference that Id
in the Target
attribute. This Id
attribute and Target
attribute value matching is required by the [XMLDsig] standard, but it is redundant in the context of SVT validation as the SVT already contains information that uniquely identifies the target signature. Validation applications SHOULD not reject an SVT token because of Id
and Target
attribute mismatch, and MUST rely on matching against signature using signed information in the SVT itself.
The <svt:SignatureValidationToken>
element is defined by the following XML Schema:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified"
targetNamespace="http://id.swedenconnect.se/svt/1.0/sig-prop/ns"
xmlns:svt="http://id.swedenconnect.se/svt/1.0/sig-prop/ns">
<xs:element name="SignatureValidationToken" type="svt:SignatureValidationTokenType" />
<xs:complexType name="SignatureValidationTokenType">
<xs:simpleContent>
<xs:extension base="xs:string">
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>
The SVT token SHALL be included as a string representation of the SVT JWT. Note that this is the string representation of the JWT without further encoding. The SVT MUST NOT be represented by the Base64 encoded bytes of the JWT string.
Example:
<ds:Signature Id="MySignatureId">
...
<ds:Object>
<ds:SignatureProperties>
<ds:SignatureProperty Target="#MySignatureId">
<svt:SignatureValidationToken>
eyJ0eXAiOiJKV1QiLCJhb...2aNZ
</svt:SignatureValidationToken>
</ds:SignatureProperty>
</ds:SignatureProperties>
</ds:Object>
</ds:Signature>
2.1.2 Multiple SVT in a signature
If a new SVT is stored in a signature which already contains a previously issued SVT, implementations can choose to either replace the existing SVT or to store the new SVT in addition to the existing SVT.
If the new SVT is stored in addition to the old SVT, it SHOULD be stored in a new <ds:SignatureProperty>
element inside the existing <ds:SignatureProperties>
element where the old SVT is located.
For interoperability robustness, signature validation applications MUST be able to handle signatures where the new SVT is located in a new <ds:Object>
element.
3. SVT Claims
3.1. Signature Reference Data
The SVT SHALL contain a SigReference claims object that SHALL contain the following data:
Claim | Value |
---|---|
id |
The Id-attribute of the XML signature, if present. |
sig_hash |
The hash over the signature value bytes. |
sb_hash |
The hash over the canonicalized <ds:SignedInfo> element (the bytes the XML signature algorithm has signed to generated the signature value). |
3.2. Signed Data Reference Data
An SVT according to this profile SHALL contain one instance of the SignedData claims object for each <ds:Reference>
element in the <ds:SignedInfo>
element. The SignedData claims object shall contain the following data:
Claim | Value |
---|---|
ref |
The value of the URI attribute of the corresponding <ds:Reference> element. |
hash |
The hash of all bytes identified corresponding <ds:Reference> element after applying all identified canonicalization and transformation algorithms. These are the same bytes that is hashed by the hash value in the <ds:DigestValue> element inside the <ds:Reference> element. |
3.3. Signer Certificate References
The SVT SHALL contain a CertReference claims object. The type
claim of the CertReference claims object SHALL be either chain
or chain_hash
.
- The
chain
type SHALL be used when signature validation was performed using one or more certificates where some or all of the certificates in the chain are not present in the target signature. - The
chain_hash
type SHALL be used when signature validation was performed using one or more certificates where all of the certificates are present in the target signature.
4. JOSE Header
4.1. SVT Signing Key Reference
The SVT JOSE header MUST contain one of the following header parameters in accordance with [RFC7515], for storing a reference to the public key used to verify the signature on the SVT:
Header Parameter | Value |
---|---|
x5c |
Holds an X.509 certificate [RFC5280] or a chain of certificates. The certificate holding the public key that verifies the signature on the SVT MUST be the first certificate in the chain. |
kid |
A key identifier holding the Base64 encoded hash value of the certificate that can verify the signature on the SVT. The hash algorithm MUST be the same hash algorithm used when signing the SVT as specified by the alg header parameter. |
5. Normative References
Bradner, S., Key words for use in RFCs to Indicate Requirement Levels, March 1997.
Leiba, B., Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words, May 2017.
XML Signature Syntax and Processing Version 1.1 - W3C Recommendation 11 April 2013.