Internet Draft Richard D. Brown GlobeSet, Inc. Expires July 1999 January 1999 Digital Signatures for XML ------- ---------- --- --- Richard D. Brown GlobeSet, Inc. Status of This Document This draft, file name draft-brown-xml-dsig-00.txt, is intended to be become a Proposed Standard RFC. Distribution of this document is unlimited. Comments should be sent to the DSIG mailing list or to the author. This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract A syntax and procedures for the computation and verification of XML digital signatures is specified. Richard D. Brown [Page 1] INTERNET-DRAFT January 1999 Digital Signatures for XML Table of Contents Status of This Document....................................1 Abstract...................................................1 Table of Contents..........................................2 1. Introduction............................................4 2. Objective and Requirements..............................4 3. Signature Basics........................................5 3.1 Signature Element......................................5 3.2 Resource Element.......................................5 3.3 Other Attributes Element...............................6 3.4 Originator and Recipient Information Elements..........6 3.5 Key Agreement Algorithm Element........................7 3.6 Signature Algorithm Element............................8 4. Signature Principles....................................8 4.1 Enabling Signature in XML Applications.................8 4.2 Encapsulating Arbitrary Contents.......................9 4.3 Implementing Endorsement..............................10 4.4 Supporting Composite Documents........................10 4.5 Facilitating One-pass Processing......................11 5. Detailed Signature Syntax..............................12 5.1 Namespace Attributes..................................12 5.2 dsig:eval Global Attribute............................13 5.3 Uniform Resource Names................................14 5.4 Document..............................................14 5.5 DigestAlgorithm.......................................15 5.6 Algorithm.............................................16 5.7 Parameter.............................................16 5.8 Package...............................................17 5.9 ContentInfo...........................................18 5.10 Value................................................18 5.11 Signatures...........................................19 5.12 Signature............................................19 5.13 Manifest.............................................20 5.14 Resource.............................................20 5.15 Locator..............................................21 5.16 Digest...............................................21 5.17 Attributes...........................................22 5.18 Attribute............................................22 5.19 Date.................................................23 5.20 OriginatorInfo.......................................24 5.21 RecipientInfo........................................24 5.22 Identifier...........................................25 5.23 IssuerAndSerialNumber................................25 5.24 SignatureAlgorithm...................................26 5.25 Certificates.........................................26 5.26 Certificate..........................................26 5.27 Integer..............................................27 Richard D. Brown [Page 2] INTERNET-DRAFT January 1999 Digital Signatures for XML 5.28 Real.................................................28 5.29 Keyword..............................................28 5.30 Resources............................................29 6. Supported Algorithms...................................29 6.1 Digest Algorithms.....................................30 6.2 Key Agreement Algorithms..............................30 6.3 Key Exchange Aglorithms...............................30 6.4 Signature Algorithms..................................31 6.5 SHA1..................................................31 6.6 DOM-HASH..............................................31 6.7 XHASH.................................................32 6.8 PKCS12-PBE............................................32 6.9 HMAC..................................................33 6.10 DSA..................................................33 6.11 RSA..................................................33 6.12 ECDSA................................................34 7. Uniform Resource Names.................................34 8. Certificate Supplement.................................34 9. Conformance Requirements...............................35 10. Examples..............................................35 10.1 Signature DTD - Embedded Content.....................35 10.2 Signature DTD - Detached Signature...................37 10.3 Extended DTD - Domain-specific Attribute.............38 11. Signature DTD.........................................40 12. Security Considerations...............................41 References................................................42 Author's Address..........................................42 Expiration and File Name..................................42 Richard D. Brown [Page 3] INTERNET-DRAFT January 1999 Digital Signatures for XML 1. Introduction XML, the Extensible Markup Language [XML], is a syntactical standard elaborated by the World Wide Web Consortium. XML is a subset of an existing and widely used international text processing standard known as SGML (Standard Generalized Markup Language). XML is intended primarily for structuring data exchanged and served over the World Wide Web. As it is anticipated that XML will be widely used in the exchange of business and commercial data, and it has been already established that digital signatures will play an important role in enabling electronic commerce, the necessity to promote a digital signature standard for general XML documents becomes evident. Drafted with IOTP (Internet Open Trading Protocol) requirements in mind, this document has been further enhanced as comments were received and alternative proposals disclosed. It is now expected that it provides a more general solution to signing XML documents. 2. Objective and Requirements The objective of this document is to propose syntax and procedures for the computation and verification of digital signatures applicable to general XML documents. This proposal has been established in light of the requirements that have been gathered while reviewing diverse projects and alternative approaches such as IOTP [draft-ietf-trade-iotp-v1.0-protocol-*.txt], eCheck [x], BIPS [x], SDML [x], and XMLDSIG [x]. The following requirements have been identified: -- The solution shall provide a means for building authentication into XML applications, but shall also propose an XML alternative to binary signature syntax for signing arbitrary contents. -- The solution shall provide indifferently for digital signature and message authentication codes, considering symmetric and asymmetric authentication schemes as well as dynamic negotiation of keying material. -- The solution shall provide a mechanism that eases the production of composite documents that consist of the combination by addition or deletion of authenticated blocks of information, while preserving verifiability of the origin and authenticity of these blocks of information. Richard D. Brown [Page 4] INTERNET-DRAFT January 1999 Digital Signatures for XML -- The solution shall enable authentication of part or totality of an XML document. -- The solution shall enable authentication of internal and external resources. -- The solution shall provide for extended signature functionality such as co-signature, endorsement, plurality of recipients, etc. 3. Signature Basics 3.1 Signature Element This specification consists primarily of the definition of an XML element known as the Signature element. This element is comprised of two sub-elements. The first one is a set of authenticated attributes, known as the signature Manifest, which comprises such things as a unique reference to the resource being authenticated and an indication of the keying material and algorithms being used. The second sub-element consists of the digital signature value. (resource information block) (originator information block) (recipient information block) (other attributes) (signature algorithms information block) (encoded signature value) The digital signature is not computed directly from the pieces of information to be authenticated. Instead, the digital signature is computed from a set of authenticated attributes (the Manifest), which include a reference to, and a digest of, these pieces of information. The authentication is therefore "indirect". 3.2 Resource Element The Resource element consists of a unique and unambiguous reference to the resource being authenticated. It is constructed of a locator, Richard D. Brown [Page 5] INTERNET-DRAFT January 1999 Digital Signatures for XML a fingerprint, and optionally a content-type qualifier. (digest information block) The resource locator is implemented as a simple XML Link [XLink]. This not only provides a unique addressing scheme for internal and external resources, but also facilitates authentication of composite documents. 3.3 Other Attributes Element The Attributes element consists of a collection of Attribute elements that could be used for inserting specific pieces of information directly into the Signature element. An Attribute element is constructed of a type, a criticality, and a value. (ANY attribute value) The attribute value consists of ANY content that is defined in the application DTD. Nevertheless, to facilitate the adoption of such as 'signing-time.' 3.4 Originator and Recipient Information Elements The purpose of the Originator and Recipient information elements consists of providing identification and keying material for these respective parties. Richard D. Brown [Page 6] INTERNET-DRAFT January 1999 Digital Signatures for XML (identification information block) (keying material information block) (identification information block) (keying material information block) The actual content of these two elements depends on the authentication scheme being used and the existence or non-existence of a prior relationship between the parties. In some circumstances, it may be quite difficult to distinguish between identification and keying material information. A unique reference to a digital certificate provides for both. This may also stand true for an account number when a prior relationship exists between the parties. The Originator information element is mandatory. Depending on the existence or non-existence of a prior relationship with the recipient, this block either refers to a public credential such as a digital certificate or displays a unique identifier known by the recipient. The Recipient information element may be used when a document contains multiple signature information blocks, each being intended for a particular recipient. A unique reference in the Recipient information block helps the recipients identify their respective Signature information block. The Recipient information element may also be used when determination of the authentication key consists of a combination of keying material provided by both parties. This would be the case, for example, when establishing a key by means of Diffie Hellman [Schneier] Key Exchange algorithm. 3.5 Key Agreement Algorithm Element The Key Agreement Algorithm element indicates the algorithm to be used for deriving a one-time session key from a master key. Usage of one-time session key prevents some kinds of attack that require a large volume of cipher-text to be produced by a given key. (algorithm information block) Richard D. Brown [Page 7] INTERNET-DRAFT January 1999 Digital Signatures for XML 3.6 Signature Algorithm Element The Signature Algorithm element indicates the algorithm to be used for computation of the signature value. (algorithm information block) In consideration of the requirements stated previously, this document uses the terminology of "signature" for qualifying indifferently signature and authentication schemes. Therefore, the signature algorithm mentioned above might refer to a signature algorithm such as DSS or to a message authentication code (MAC) such as HMAC. 4. Signature Principles 4.1 Enabling Signature in XML Applications As mentioned previously, this specification provides a means for building authentication into general XML applications. The mechanism adopted herein considers the "XML Namespaces" specifications[x], which define the requirements for combining multiple DTDs or parts of individual DTD into a single document. According to these specifications, an XML application can build digital signature support by referring explicitly to the elements defined in the Signature DTD. This is accomplished by associating a namespace prefix to the Signature DTD and qualifying Signature element names by means of this prefix. Association of a namespace prefix to a DTD shall be done by means of a xmlns attribute, which could appear in any element that either refers to or contains sub-elements that refer to elements of the DTD considered. A qualified name consists of a namespace prefix, a colon, and a name. Richard D. Brown [Page 8] INTERNET-DRAFT January 1999 Digital Signatures for XML ... ... ... ... [The XML Namespaces specifications are still work in progress at W3C. A final draft should be available soon, as the specifications entered "last call" September 16, 1998. There are still a few unknowns regarding combination of DTD and use of qualified names in an application DTD that inherits definitions from another DTD. The current paragraph will be developed, as clarifications will be obtained.] 4.2 Encapsulating Arbitrary Contents To facilitate encapsulation of arbitrary contents into an XML document, the Signature DTD defines a Package element. Quite similar to a MIME wrapper, this element provides for such things as content type and content encoding. (safe content) Though it addresses a similar purpose, the Package element specified by this draft proposal is radically different from the one given in the " XML Package" proposal[x]. Such decision has been driven by the possibility to leverage other element definitions of this DTD. However, the current definition might be amended in time if the Richard D. Brown [Page 9] INTERNET-DRAFT January 1999 Digital Signatures for XML Package proposal were to be adopted. 4.3 Implementing Endorsement Endorsement consists of signing another signature. To facilitate endorsement, the definition of the Signature element provides for an element identifier attribute, which can be used to target a Signature element from a Resource element. ... ... ... ... ... ... 4.4 Supporting Composite Documents Some protocols consist of the exchange of documents that result from the combination by addition or deletion of common information blocks. This proposal preserves verifiability of the origin and authenticity of these blocks of information as they are exchanged between parties. To facilitate creation and verifiability of composite documents, the current draft proposal has adopted an element, known as the Resources element, which consist of a collection of Resource elements. The authentication of the Resources element is sufficient for ensuring proper authentication of the blocks of information that it references, and verifiability is preserved when individual blocks of information are missing. Richard D. Brown [Page 10] INTERNET-DRAFT January 1999 Digital Signatures for XML ... ... ... ... ... ... The adoption of simple XML links as resource locators makes possible the authentication of composite documents. If IDREFs were used instead, it would have been impossible to ensure validity of partial documents - some IDREFs could have been left referencing non-embedded IDs. 4.5 Facilitating One-pass Processing Without further definitions, it would be impossible to determine which blocks of information require authentication and which algorithms need to be employed before interpretation of the Resource elements. These elements being generally located at the end of the document, this restriction would prevent computation of the digests during acquisition of the blocks of information. To facilitate one-pass processing, this specification uses another functionality offered by the namespaces proposal. This functionality provides for the definition of global attributes that may be used and Richard D. Brown [Page 11] INTERNET-DRAFT January 1999 Digital Signatures for XML recognized across multiple elements. This document specifies the dsig:eval global attribute, which could be used for identifying the blocks of information to be authenticated. This attribute shall reference a Digest Algorithms element, which should be declared before making use of the attribute. ... ... ... ... ... When encountering the dsig:eval global attribute on the Authenticated Block element, the XML parser is immediately aware of the requirement of computing the digest of this element. All the pieces of information necessary for such computation are provided by the Digest Algorithm element referenced by the attribute. 5. Detailed Signature Syntax Though it is expected that Signature support will be primarily built into general XML applications by incorporating definitions of the Signature DTD into other XML applications, the Signature DTD defines the elements necessary to providing an XML alternative to binary signature syntaxes. 5.1 Namespace Attributes All the elements defined by the Signature DTD are explicitly bound to the XMLDSIG namespace by means of a dsig prefix. In order to make sure that every element could be individually imported by other XML applications, the element definitions given hereinafter Richard D. Brown [Page 12] INTERNET-DRAFT January 1999 Digital Signatures for XML systematically declare a fixed xmlns:dsig attribute. Recall that many XML applications, presumably including namespaces- sensitive ones, fail to require validating processors. For correct operation with such applications, namespaces declarations must be also provided either directly or via default attributes declared in the internal subset of the DTD. 5.2 dsig:eval Global Attribute As mentioned previously, this draft proposal specifies a dsig:eval global attribute that could be used for identifying a block of information to be authenticated. This attribute shall refer to a Digest Algorithms element, which should be declared before making use of the attribute. The XML Namespaces specifications do not explicitly provide for declaration of global attributes. Distinguishing between global attributes and element attributes exists only in the prose description of such attributes. An essential property of global attributes consists nonetheless of the uniqueness of their name that is independent of the elements where they are defined. The definition of elements that could be subject to authentication may define the dsig:eval attribute as follows: Recall that the namespace prefix that is bound to the XMLDSIG namespace shall be defined before being employed. However, such definition may occur in the element that defines the dsig:eval attribute. The reader shall notice that the terminology "dsig:eval" is inappropriate and used solely for illustrative purposes. This simply means that the name of this attribute is hash and it belongs to the XMLDSIG namespace (whatever prefix is used). Richard D. Brown [Page 13] INTERNET-DRAFT January 1999 Digital Signatures for XML 5.3 Uniform Resource Names To prevent potential name conflicts in the definition of the numerous type qualifiers considered herein, this specification uses Uniform Resource Names [RFC 2141]. Nonetheless, the current draft proposal leverages established standards such as MIME types by providing unambiguous mapping conventions. A complete list of proposed URNs is given in appendix. This list is temporary and will be submitted for approval to the authors or promoters of the algorithms and data types referenced by these URNs. 5.4 Document The Document element constitutes the outermost envelope of an XML document that conforms to the Signature DTD. The definition of this element has been intentionally kept simple and is intended to provide an XML alternative to the ASN1 data types Authenticated Data and Signed Data defined by CMS[x] and PKCS7[x] binary syntax standards. It is expected that this simple, though complete, definition will help the adoption of this proposal and facilitate the production of conformant implementations by a plurality of providers. This definition does not preclude however the definition of more sophisticated constructions to be adopted by particular XML applications. Such applications may either redefine the Document element or promote their own DTD, which shall be partially constructed of elements defined by the Signature DTD. The definition given above has been deemed sufficient for implementing the following functionality provided by CMS or PKCS7: Authentication of arbitrary contents: This may be done by adequate encapsulation and encoding of the arbitrary contents into the Package element, which shall be further authenticated by means of a Signature element. -- Detached signature: This may be done by means of a Signature element that refers to a resource external to the document. -- Authentication versus signature: The distinction between authentication and signature only depends upon the algorithms Richard D. Brown [Page 14] INTERNET-DRAFT January 1999 Digital Signatures for XML being employed for computation of the "signature" value. -- Plurality of recipients: This consists of the insertion of a plurality of Signature elements, each making use of recipient- dependent keying material. -- Plurality of signers: This consists of the insertion of a plurality of Signature elements, each making use of originator- dependent keying material. Content Description DigestAlgorithms: This element has been made mandatory whenever the document embeds the contents to be authenticated. This element specifies the algorithms to be used for computation of the digest of the Package element, thus enabling one-pass processing. Package: This element is used for enveloping and encoding of the contents to be authenticated. Whenever employed, this element shall make use of the dsig:eval global attribute to refer to the Digest Algorithms element described above. Signatures: This element consists of a collection of Signature elements. Certificates: This element consists of a collection of Certificate elements, which may be required by a given key management infrastructure. The definition of collection elements (i.e. Certificates and Signatures) for the sole purpose of grouping similar sub-elements has been adopted for facilitat- ing DOM manipulations. 5.5 DigestAlgorithm The purpose of the DigestAlgorithm element consists of specifying the algorithm to be employed in the computation of a message digest. This element is used either as a standalone element for enabling one-pass processing or as a sub-element of the Digest element. Content Description Richard D. Brown [Page 15] INTERNET-DRAFT January 1999 Digital Signatures for XML Algorithm: Algorithm and parameters to be used for Computation of the digest value. Attributes Description id: Element identifier that is used on standalone element for enabling reference by the dsig:eval global attribute. 5.6 Algorithm The current draft proposal has adopted a unique Algorithm data type. Though noticeably different from its ASN1 counterpart, this data type serves a similar purpose and provides for the definition of algorithm-specific parameters. The most noticeable difference with ASN1 consists of the assimilation of sub-algorithms as parameters of the primary algorithm. Content Description Parameter: The contents of an Algorithm element consists of an optional collection of Parameter elements which are specified on a per algorithm basis. Attributes Description type: The type of the algorithm expressed as a Uniform Resource Name. 5.7 Parameter A Parameter element provides the value of a particular algorithm parameter, Whose name and format have been specified fo rthe algorithm considered. Richard D. Brown [Page 16] INTERNET-DRAFT January 1999 Digital Signatures for XML Content Description ANY: The contents of a Parameter element consists of ANY valid Construct, which is specified on a per algoritm per parameter basis. Attributes Description type: The type of the parameter expressed as a free form string, whose value is specified on a per algorithm basis. 5.8 Package The Package element enables encapsulation of an arbitrary content into an XML document. Behaving like a MIME wrapper, the Package element provides for such things as content type identification and content encoding. Content Description ContentInfo: Type qualifier for the content. Value: Content value. Attributes Description id: Element identifier that could be used for referencing this element from a Resource element. Richard D. Brown [Page 17] INTERNET-DRAFT January 1999 Digital Signatures for XML 5.9 ContentInfo The purpose of the ContentInfo element is to describe a given content such that a receiving user agent can deal with the data in an appropriate manner. Attributes Description type: Type of the content expressed as a Universal Resource Name. subtype: Optional sub-classing of the content type. 5.10 Value Content Description PCDATA: Content value after adequate encoding. Attributes Description encoding: This attribute specifies the decoding scheme to be employed for recovering the original byte stream from the content of the element. The current draft proposal recognizes the following two schemes: none: the content has not been subject to any particular encoding. This does not preclude however the use of native XML encoding such as CDATA section or XML escaping. base64: The content has been encoded by means of the base64 encoding scheme. Richard D. Brown [Page 18] INTERNET-DRAFT January 1999 Digital Signatures for XML 5.11 Signatures The Signatures element consists of a collection of Signature elements. As mentioned in a previous paragraph, this element has been defined for the purpose of facilitating DOM manipulations. Content Description Signature: A collection of Signature elements. 5.12 Signature The Signature element constitutes the core of this specification. It is comprised of two sub-elements. The first one is a set of attributes, known as the Manifest, which actually constitutes the authenticated part of the document. The second sub-element consists of the signature value. Content Description Manifest: A set of attributes that actually constitutes the authenticated part of the document. Value: Encoding of the signature value. Attributes Description id: Element identifier that could be used for referencing the Signature element from a Resource element when implementing endorsement. Richard D. Brown [Page 19] INTERNET-DRAFT January 1999 Digital Signatures for XML 5.13 Manifest The Manifest element consists of a collection of attributes that specify such things as a unique reference to the resource being authenticated and an indication of the keying material and algorithms to be used. Content Description Resource: Unique and unambiguous reference to the resource being authenticated. Attributes: Optional element that consists of a collection of complementary attributes to be authenticated. OriginatorInfo: Element that provides dentification and keying material information related to the originator. RecipientInfo: Optional element that provides identification and keying material information related to the recipient. KeyAgreementAlgorithm: Optional element that indicates the algorithm to be used for establishment of a one-time session key. SignatureAlgorithm: Algorithm to be used for computation of the signature value. 5.14 Resource The Resource element consists of a unique and unambiguous reference to a resource being authenticated. It is comprised of a resource locator, a fingerprint, and optionally a content-type qualifier. Richard D. Brown [Page 20] INTERNET-DRAFT January 1999 Digital Signatures for XML Content Description ContentInfo: Content type qualifier. Locator: Locator value that contains either a URI [RFC 2396], a fragment identifier, or both. Notice that making use of a fragment identifier for a document content other than XML is out of the scope of this draft proposal and may lead to inconsistent results. Digest: Fingerprint of the resource. 5.15 Locator The Locator element consists of simple XML link [XLink]. This element allows unambiguous reference to a resource or fragment of a resource. Attributes Description xml:link Required XML link attribute that specifies the nature of the link (simple in this case). href: Locator value that may contains either a URI [RFC 2396], a fragment identifier, or both. 5.16 Digest The Digest element consists of the fingerprint of a given resource. This element is constructed of two sub-elements. This first one Richard D. Brown [Page 21] INTERNET-DRAFT January 1999 Digital Signatures for XML indicates the algorithm to be used for computation of the fingerprint. The second element consists of the fingerprint value. Content Description DigestAlgorithm: Algorithm to be used for computation of the fingerprint. Value: Encoding of the fingerprint value. 5.17 Attributes The Attributes element consists of a collection of complementary attributes, which shall be included in the authenticated part of the document. Content Description Attribute: Collection of Attribute elements. 5.18 Attribute The Attribute element consists of a complementary piece of information, which shall be included in the authenticated part of the document. Though the current draft proposal defines well-known attributes, this element has been defined primarily for enabling some level of customization in the signature element. An Attribute element consists of a value, a type, and a criticality. Richard D. Brown [Page 22] INTERNET-DRAFT January 1999 Digital Signatures for XML Content Description ANY: The actual value of an attribute depends solely upon its type. Attributes Description type: Type of the attribute. critical: Boolean value that indicates if the attribute is critical (true) or not (false). A recipient shall reject a signature that contains a critical attribute that he does not recognize. However, an unrecognized non-critical attribute may be ignored. Signing-time Attribute Standard attribute that could be used for specifying the time at which the originator purportedly performed the signature process. This attribute content shall be given as a Date element (cf. element description). The type identifier of this attribute is given in the URN appendix. 5.19 Date The Date element consists of a constrained ISO 8601:1998 date and time value. Attributes Description value: Identification data value. Date Format Richard D. Brown [Page 23] INTERNET-DRAFT January 1999 Digital Signatures for XML The current draft proposal requires date values to be expressed according to the following pattern: YYYY '-' MM '-' DD 'T' hh ':' mm [':' ss ['.' f+]]('+' | '-') hhmm YYYY: four-digit year MM: two-digit month (01=January, etc.) DD: two-digit day of the month (01-31) hh: two digits of hour (00-23) mm: two digits of minute (00-59) ss: two digits of second (00-59) optional f: digit(s) of fractions of second - optional zzzz: four digits of amount of offset from UTC expressed in hour (00-11) and minute (00-59) For example "1994-11-05T16:15:02.031-0500" denotes November 5, 1994, 4:15:02 pm and 31 milliseconds, US Eastern Standard Time. 5.20 OriginatorInfo The OriginatorInfo element is used for providing identification and keying material information for the originator. Content Description ANY: Identification and keying material information may consist of ANY construct. Such a definition allows the adoption of application-specific schemes. However, implementations that comply with the current DTD MUST be able to recognize and process the elements Identifier and IssuerAndSerialNumber defined below. 5.21 RecipientInfo The RecipientInfo element is used for providing identification and keying material information for the recipient. This element is used either for enabling recognition of a Signature element by a given recipient or when determination of the authentication key consists of the combination of keying material provided by both the recipient and the originator. Richard D. Brown [Page 24] INTERNET-DRAFT January 1999 Digital Signatures for XML Content Description The content of this element is similar to the one defined for the Originator (cf. OriginatorInfo element descrition). 5.22 Identifier The Identifier element enables identification between parties that benefit from a prior relationship. The actual meaning and content of this element is left to the parties. Attributes Description value: Identification data value. 5.23 IssuerAndSerialNumber The IssuerAndSerialNumber element identifies a certificate, and thereby an entity and a public key, by the distinguished name of the certificate issuer and an issuer-specific certificate serial number. Attributes Description issuer: Distinguished name of the issuing certification authority. number: Issuer-specific certificate serial number. Richard D. Brown [Page 25] INTERNET-DRAFT January 1999 Digital Signatures for XML 5.24 SignatureAlgorithm The SignatureAlgorithms element is used for specifying the algorithms to be used for computation of the signature value. Content Description Algorithm: Algorithm and parameters to be used for computation of the signature value. 5.25 Certificates The Certificates element consists of a collection of Certificate elements. The Certificate elements contained in this element are intended to be sufficient to make chains from the originator credential(s) to a recognized "certification authority" for all the recipients. However, this element may contain more Certificate elements than necessary or, alternatively, less than necessary if it is known that recipients have an alternate means of obtaining necessary certificates. Content Description Certificate: A collection of Certificate elements. 5.26 Certificate The Certificate element may be used for either providing the value of a digital certificate or specifying a location from where it may be retrieved. Richard D. Brown [Page 26] INTERNET-DRAFT January 1999 Digital Signatures for XML Content Description IssuerAndSerialNumber: Unique identifier of this certificate. This element has been made mandatory is order to prevent unnecessary decoding during validation of a certificate chain. This feature also helps certificates caching, especially when the value is not directly provided. Value: Encoding of the certificate value. The actual value to be encoded depends upon the type of the certificate. Locator: XML link element that could be used for retrieving a copy of the digital certificate. The actual value being returned by means of this locator depends upon the protocol being used. Attributes Description type: Type of the digital certificate. This attribute is specified as a Universal Resource Name (cf. Certificate Supplement). 5.27 Integer The Integer element is a primary data type that is used in the definition of algorithm parameters. Attributes Description value: Value of the element given according to the format given below. Richard D. Brown [Page 27] INTERNET-DRAFT January 1999 Digital Signatures for XML Integer Format The current specification requires integer values to be expressed According to the following pattern: ['+'|'-'] n+ For example, +128, -35635, and 64535 are valid integer values. 5.28 Real The Real element is a primary data type that is used in the definition of algorithm parameters. Attributes Description value: Value of the element given according to the format given below. Real Format The current specification requires real values to be expressed according To the following pattern: ['+'|'-'] n+ ['. ' f+]['E' ('+'|'-') ee] For example, 12, -12.34, +12.34E-01, and +0.5 are valid real numbers. 5.29 Keyword The Keyword element is a primary data type that is used in the definition of algorithm parameters. Richard D. Brown [Page 28] INTERNET-DRAFT January 1999 Digital Signatures for XML Attributes Description value: Value of the element given as a free form string. 5.30 Resources The Resources element consists of a collection of Resource elements. Though inaccessible from the Document element of the Signature DTD, this element is available to more sophisticated constructs that make use of composite documents. Content Description Resource: A collection of Resource elements. Attributes Description id: Element identifier that could be used for referencing this element from a Resource element. 6. Supported Algorithms This specification uses a unique Algorithm data type. Though noticeably different from its ASN1 counterpart, this data type serves a similar purpose and provides for the definition of algorithm- specific parameters. The most noticeable difference with ASN1 consists of the assimilation of sub-algorithms as parameters of the primary algorithm. In other words, where ASN1 recognizes an algorithm of the type AlgxWithAlgy (i.e. DsaWithSha1) the current specifications recognize Algx with an Richard D. Brown [Page 29] INTERNET-DRAFT January 1999 Digital Signatures for XML Algy parameter. Such a recursive construct is expected to facilitate integration with cryptographic toolkits. 6.1 Digest Algorithms This specificaiton contemplates two kinds of digest algorithms: Surface string digest algorithms: These algorithms do not have any particular knowledge about the content being digested and operate on the raw content value. Changes in the surface string of a given content affect directly the value of the digest being produced. Canonical digest algorithms: These algorithms have been tailored for a particular content type and produce a digest value that depends upon the core semantics of such content. Changes limited to the surface string of a given content do not affect the value of the digest being produced. 6.2 Key Agreement Algorithms A key-agreement algorithm consists of a function that is used for deriving a one-time session key from a given master key. Usage of one-time session keys prevents some kinds of attacks that require a large volume of cipher-text to be produced with a given key. Key-agreement algorithms shall not be mistaken with key-exchange algorithms, which may be implicitly employed for computation of a master key that results from the combination of keying material provided by the parties involved in an exchange. In other words, parties provided with credentials such as Diffie-Hellmann-based certificates shall establish the value of the master key by means of the key exchange algorithm and may further derive a one-time session key from this master key by means of a key-agreement algorithm. A similar procedure is recommended when making use of a message authentication code and a shared secret. 6.3 Key Exchange Aglorithms A key-exchange algorithm consists of a function that is used for deriving a one-time session key from a given master key. (TBD) Richard D. Brown [Page 30] INTERNET-DRAFT January 1999 Digital Signatures for XML 6.4 Signature Algorithms This specification abusively uses the terminology of 'digital signature' for qualifying indifferently digital signature and message authentication codes. Thus, the signature algorithms contemplated herein include public key digital signature algorithms such as DSA and message authentication codes such as HMAC [RFC 2104]. 6.5 SHA1 Surface string digest algorithm designed by NIST and NSA for use with the Digital Signature Standard. This algorithm produces a 160-bit hash value. This algorithm does not require any parameter. 6.6 DOM-HASH XML canonical digest algorithm proposed by IBM Tokyo Research Laboratory and documented in the DOMHASH proposal[x]. This algorithm operates on the DOM representation of the document and provides an unambiguous means for recursive computation of the hash value of the nodes that constitute the DOM tree. This algorithm has many applications such as computation of digital signature and synchronization of DOM trees. However, because the hash value of an element is computed from the hash values of the inner elements, this algorithm is better adapted to small documents that do not require one-pass processing. As of today, this algorithm is limited to the contents of an XML document and, therefore, does not provide for authentication of the internal or external subset of the DTD. Also, there is no explicit support for XML Namespaces. The DOM-HASH algorithm requires a single parameter, which shall consist of a surface string digest algorithm such as SHA1. Example Richard D. Brown [Page 31] INTERNET-DRAFT January 1999 Digital Signatures for XML 6.7 XHASH XML canonical digest algorithm proposed by GlobeSet and documented in the XHASH proposal[x]. This algorithm has been inspired by the DOM- HASH proposal, but operates closer to the surface string of the document. Elements and attributes are subject to formalization in a way quite similar to the one proposed by DOM-HASH - XML delimiters are represented by binary values and entities are replaced by their actual values. However, formalization happens as elements are acquired. Furthermore, this algorithm has been tailored for explicit support of the XML Namespaces and it takes into account some specifics of this specification (e.g. dsig:eval attribute). The XHASH algorithm makes use of two parameters. The first one consists of a surface string digest algorithm such as SHA1. The second one, optional, may be used for specifying how non-significant SPACE characters shall be handled by default. Actually, the XML Specifications define the xml:space attribute that could be used for specifying if non-significant SPACE characters are to be preserved. However, possible values for this attribute are limited to 'default' and 'preserve'. Thus, there is no known way to explicitly specify that non-significant SPACE characters should be discarded. Example 6.8 PKCS12-PBE Key-agreement algorithm proposed by RSA Laboratories and documented in PKCS12[x]. This algorithm is a generalization of the PBE algorithm defined in PKCS5[x] and provides for the generation of symmetric keys and other cryptographic parameters from an established password. This algorithm requires three parameters. The first one consists of a one-way hash function (i.e. SHA1), the second one of a random string (salt), and the last one of an iteration count. Richard D. Brown [Page 32] INTERNET-DRAFT January 1999 Digital Signatures for XML Example Abkirjegks123qwgtawd456g47 6.9 HMAC Generalities [RFC 2104] Example 6.10 DSA 6.11 RSA Richard D. Brown [Page 33] INTERNET-DRAFT January 1999 Digital Signatures for XML 6.12 ECDSA 7. Uniform Resource Names Generalities [RFC 2141] Content-type URNs Leveraging MIME [RFC 2046] types. Other content-types Algorithm URNs Certificate Type URNs 8. Certificate Supplement Locator Protocols (HTTP, LDAP) Expected Formats Richard D. Brown [Page 34] INTERNET-DRAFT January 1999 Digital Signatures for XML 9. Conformance Requirements TBD 10. Examples The URN given in the following examples are purely illustrative and, therefore, shall not be used as reference material. 10.1 Signature DTD - Embedded Content > > abncjflf311257gghn6mj2k134h64AANHdd12== > Richard D. Brown [Page 35] INTERNET-DRAFT January 1999 Digital Signatures for XML > bndWGryrt245u6t1dgURTIrr4ir5= xsqsfasDys2h44u4ehJDe54he5j4dJYTJ= Richard D. Brown [Page 36] INTERNET-DRAFT January 1999 Digital Signatures for XML xsqsfasDys2h44u4ehJDe54he5j4dJYTJ 10.2 Signature DTD - Detached Signature bndWGryrt245u6t1dgURTIrr4ir5= Richard D. Brown [Page 37] INTERNET-DRAFT January 1999 Digital Signatures for XML xsqsfasDys2h44u4ehJDe54he5j4dJYTJ= 10.3 Extended DTD - Domain-specific Attribute Richard D. Brown [Page 38] INTERNET-DRAFT January 1999 Digital Signatures for XML ]> > > bndWGryrt245u6t1dgURTIrr4ir5= > > bndWGryrt245u6t1dgURTIrr4ir5= > > Richard D. Brown [Page 39] INTERNET-DRAFT January 1999 Digital Signatures for XML bndWGryrt245u6t1dgURTIrr4ir5= xsqsfasDys2h44u4ehJDe54he5j4dJYTJ= 11. Signature DTD (Insert here) Richard D. Brown [Page 40] INTERNET-DRAFT January 1999 Digital Signatures for XML 12. Security Considerations The entirety of this document is concerned with a signature standard for XML. Richard D. Brown [Page 41] INTERNET-DRAFT January 1999 Digital Signatures for XML References [RFC 2046] - N. Freed & N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", November 1996. [RFC 2104] - H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", February 1997. [RFC 2141] - R. Moats, "URN Syntax", May 1997. [RFC 2396] - T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", August 1998. [Schneier] - Bruce Schneier, "Applied Cryptography: Protocols, Algorithms, and Source Code in C", 1996, John Wiley and Sons [XLink] - Eve Maler, Steve DeRose, "XML Linking Language (XLink)", [XML] - Tim Bray, Jean Paoli, C. M. Sperber-McQueen, "Extensible Markup Language (XML) 1.0", [...more to come] draft-ietf-trade-iotp-v1.0-protocol-*.txt - David Burdett Author's Address Richard D. Brown GlobeSet, Inc. 1250 Capital of TX Hwy. So. Building One, Suite 300 Austin, TX 78746 USA EMail: richard_dbrown@globeset.com Expiration and File Name This draft expires July 1999. Its file name is draft-brown-xml-dsig-00.doc. Richard D. Brown [Page 42]