Security for Web Services and Service Oriented Architectures
Bhavani ThuraisinghamThe University of Texas at Dallas
February 18, 2011
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Acknowledgement Professors Elisa Bertino and Lorenzo Martino;
Purdue University for much of the information and charts on web services security standards and digital identity management
[email protected] [email protected]
Others: Dr. Frederica Pacci; University of Milan for ideas obtianed when
serving on her thesis committee on reserach in web services security
Prof. I-Ling Yen and Wei-She; University of Texas at Dallas for collaboration on web services security and the delegation model
Book by Thomas Erl on Service Oriented Architectures, Prentice Hall, 2005
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Objective and Scope
The objective of this course is to provide an overview of the significant developments in SOA and Web Services Security Standards as well as directions for future developments
Current work on SOA security is focusing mainly on access control as well as confidentiality and integrity.
Solutions proposed for systems to address intrusion detection, denial of service and infrastructure attacks, insider threat analysis including data mining techniques for security applications are beyond the scope of this course.
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Outline
SOA and Web services: Overview SOA and Web services security: Overview WS-Security and WS-* Security
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Service Oriented Architecture (SOA) http://en.wikipedia.org/wiki/Service-oriented_architecture
Service Oriented Architecture (SOA) is an architectural style that guides all aspects of creating and using business processes, packaged as services, throughout their lifecycle, as well as defining and provisioning the IT infrastructure that allows different applications to exchange data and participate in business processes loosely coupled from the operating systems and programming languages underlying those applications
SOA represents a model in which functionality is decomposed into distinct units (services), which can be distributed over a network and can be combined together and reused to create business applications
These services communicate with each other by passing data from one service to another, or by coordinating an activity between two or more services.
SOA concepts makes software development flexible and extensible Service oriented analysis is becoming key to modeling and analyzing software The concepts of Service Oriented Architecture are often seen as built upon, and
the evolution of, the older concepts of distributed computing and modular programming
While object-orientation views the world as a collection of objects, service orientation views the world as a collection of services
SOA is technology independent; however it is commonly realized using web services
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Web service definition
“A Web Service is a software system designed to support interoperable machine-to-machine
interaction over a network. It has an interface described in a machine-processable format
(specifically WSDL). Other systems interact with the Web service in a manner prescribed by its
description using SOAP messages, typically conveyed using HTTP with an XML serialization in
conjunction with other Web-related standards.”
Source: http://www.w3.org/TR/ws-arch/
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SOA
Service requestor
Service providers
UDDI
Publish ServicesQuery
Request
Answer
Response
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Web Services (WS) Framework An abstract (vendor neutral) existence defined by standards organizations
and implemented by (proprietary) technology platforms Core building blocks that include web sercices, service descriptions and
messages A communication agreement centered around service descriptions and
WSDL A messaging framework comprised of SOAP technology concepts A service description registration and discovery architecture sometimes
realized through UDDI A well defined architecture that supports messaging patterns and
compositions A second generation of web services extensions (also known as WS-*
specifications) continually broadening its underlying feature-set Concepts in WS-* include: Message Exchange Patterns (MEP), Service
Activity, Coordination, Atomic Transaction, Business Activities, Orchestration (WS-BPEL), Choreography (WS-CDL)
Reference: Service Oriented Architecture, Thomas Erl, Prentice Hall, 2005
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Standardization bodies related to Web Services
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SOA Security
Our approach is to implement SOA through web services; therefore SOA security essentially is about web services security
Three core specifications WS-Security, XML-Signature, XML-Encryption WS*-Security is the second generation of technologies for SOA
security Single sign-on (SSO) is a form of centralized security
mechanism that complements the WS-Security extensions Related specifications for SOA security
WS-Security, WS-SecurityPolicy, WS-Trust, WS-SecureConversation, WS-Federation, XACML, Extensibe Rights Markup Language, XML Key Management, XML, Signature, SAML, .NET Passport, Secure Socket Layer, WS-I Basic Security Profile
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Basic Components of SOA Security
Identification For service requestor to acces a secure service provider it must first provide
information that expresses its origin or owner. This is referred to as making a claim
Authentiaction A message being delivered to a receipient must prove that the message is in
fact from the sender that it claims Authorization
Once authenticated, the receipient of a message may need to determine what the requestor is alowed to do
Singe sign on It is supported by SAML, .NET Passport and XACML
Confidentiality and Integrity Confidentiality is concerned with protecting the privacy of the message
content, Integrity ensures that the message has not been altered Transport level and Message level security
Transport level securiy is provided by SSL (securing HTTP), message level confidentiality and integrity are provied by XML-Encryption and XML-Signature.
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Web Services Security: Requirements and Standards
Securing Web services mainly requires to:
provide facilities for securing the integrity and confidentiality of the messages and
ensure that the service acts only on requests in messages that express the claims required by policies
Role of Standards Providing a Web Services Security Framework that is an integral part
of the Web Services Architecture
The framework is a layered and composable set of standard
specifications
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WS-* security Standards framework
Transport level security SSL/TLS
Network level security IPSec
XML security XML Encryption
XML Signature
SOAP foundation
Message security
WS SecurityWS
SecureConversation
Reliable Messaging
WS ReliableMessaging
Security mgmt.
XKMS WS-Trust
XACML SAML
WS-Policy
Policy & Access Control
Identity Mgmt.
WS-federation Liberty SAML
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WS-* security standards implementations
Microsoft .NET Framework 2.0 / WSE3.0 WS-Security (OASIS 2004 standard), WS-Policy, WS-
SecurityPolicy, WS-Trust, WS-SecureConversation and WS-Addressing
SUN Web Services Interoperability Technology (WSIT)
IBM WebSphere
Open Software: The Apache Software Foundation Web Services Project (http://ws.apache.org/)
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XML EncryptionXML Encryption Syntax and Processing10 December 2002Status W3C Recommendation
Core standardGoals: provide confidentiality for applications that exchange structured data by
Representing in a standard way digitally encrypted resources separating encryption information from encrypted data, and
supporting reference mechanisms for addressing encryption information from encrypted data sections and vice-versa
providing a mechanism for conveying encryption key information to a recipient
providing for the encryption of a part or totality of an XML document
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XML Signature
XML-Signature Syntax and Processing
12 February 2002
Status: W3C Recommendation
Core standard: XML Signature is a building block for many web services security standards (e.g. XKMS and WS-Security)
Goals:
represent a digital signature as an XML element Processing rules for creating this XML element The signed data items can be of different types and
granularity (XML documents, XML Elements, files containing any type of digital data)
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Securing SOAP messagesWeb Services Security: SOAP Message Security 1.1 (WS-Security 2004)Status: Approved OASIS Standard Specification 1 February 2006
Goals: Provide single SOAP message integrity and confidentiality
Using existing digital signature, encryption, and security token mechanisms
Provide mechanisms for associating security tokens with message content (header and body blocks)
Extensibility (i.e. support multiple security token format)
the recipient can trust the content of the message and its sender
Security Token - a representation of security-related information (e.g. X.509 certificate, Kerberos tickets and authenticators, mobile device security tokens from SIM cards, username, etc.). Signed Security Token - a security token that contains a set of related claims (assertions) cryptographically endorsed by an issuer.
Examples: X.509 certificates and Kerberos tickets.
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What is WS-Security? WS-Security enhances SOAP messaging to provide
quality of protection through: message integrity, message confidentiality, and single message authentication.
These mechanisms can be used to accommodate a wide variety of security models and encryption technologies.
WS-Security also provides a general-purpose, extensible mechanism for associating security tokens with messages: No specific type of security token is required support for multiple security token formats
WS-Security describes how to encode binary security tokens( X.509 certificates and Kerberos tickets)
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WS-Policy
Web Services Policy 1.2 - Framework (WS-Policy) W3C Member Submission 25 April 2006
Status: public draft release for review and evaluation only Main goal: The WS-Policy and WS-PolicyAttachment aim to
offer mechanisms to represent the capabilities and requirements of Web services as Policies
Policy view in WS-Policy: A policy is used to convey conditions on an interaction between two
Web service endpoints. The provider of a Web service exposes a policy to convey conditions
under which it provides the service. A requester might use this policy to decide whether or not to use the
service.
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XACML eXtensible Access Control Markup Language 2 (XACML)
Version 2.0 OASIS Standard, 1 Feb 2005
Status: approved OASIS Standard within the OASIS Access 12 Control TC.
XACML is a general-purpose access control policy language for managing access to resources
It describes both a policy language and an access control decision request/response language
Fine access control grained control
Access control based on subject and object attributes
Consistent with and building upon SAML
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XACML – Key Aspects General-purpose authorization policy model and
XML-based specification language XACML is independent of SAML specification Triple-based policy syntax: <Object, Subject, Action> Negative authorization is supported Input/output to the XACML policy processor is clearly
defined as XACML context data structure Input data is referred by XACML-specific attribute
designator as well as XPath expression Extension points: function, identifier, data type, rule-
combining algorithm, policy-combining algorithm, etc. A policy consists of multiple rules A set of policies is combined by a higher level policy
(PolicySet element)
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XACML data flow model
Source: oasis-access_control-xacml-2.0-core-spec-os
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XACML Protocol
Policy
Enforcement Point (PEP)
Policy
Decision Point (PDP)
Policy
Access Point (PAP)
Policy
Information Point (PIP)
XACMLRequest/Response
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XACML Protocol When a client makes a resource request upon a server, the PEP is charged with
AC In order to enforce AC policies, the PEP will formalize the attributes describing
the requester at the PIP and delegate the authorization decision to the PDP Applicable policies are located in a policy store, managed by the PAP, and
evaluated at the PDP, which then returns the authorization decision Using this information, the PEP can deliver the appropriate response to the
client
XACML Request Subject Object Action
XACML Response Permit Permit with Obligations Deny NotApplicable (the PDP cannot locate a policy whose target matches the
required resource) Indeterminate (an error occurred or some required value was missing)
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XACML Protocol1. The Policy Administration Point (PAP) creates
security policies and stores these policies in the appropriate repository.
2. The Policy Enforcement Point (PEP) performs access control by making decision requests and enforcing authorization decisions.
3. The Policy Information Point (PIP) serves as the source of attribute values, or the data required for policy evaluation.
4. The Policy Decision Point (PDP) evaluates the applicable policy and renders an authorization decision.
Note: The PEP and PDP might both be contained within the same application, or might be distributed across different servers
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XACML policy A Policy has four main components:
A target A rule-combining algorithm identifier A set of rules Obligations
The Rule is the elementary unit of a policy Main components of a rule:
A target An effect: permit or deny A condition
Policy Language A policy target specifies a set of:
Resources Subjects Actions Environment
to which it applies
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Security Assertion Markup Language (SAML) Developed by the OASIS XML-Based Security Services
Technical Committee (SSTC) Status: SAML V2.0 OASIS Standard specification set was
approved on 15 March 2005 Main goal: authentication and authorization
promote interoperability between disparate authentication and authorization systems
How: defining an XML-based framework for communicating security and
identity information (e.g., authentication, entitlements, and attribute) between computing entities
using available different security infrastructures (e.g., PKI, Kerberos, LDAP, etc)
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SAML basic concepts Assertions: The core concept
SAML Authority: a system entity that makes SAML assertions (also called Identity Provider – IdP – and Asserting Party)
Service Provider: a system entity making use of SAML assertions
Relying Party: a system entity that uses received assertions (named also SAML requester)
SAML Bindings: Bindings describe exactly how the SAML protocol maps onto the transport protocols.
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SAML assertions
An assertion is constituted by one or more statements made by a SAML authority
Different kinds of assertion statement that can be created by a SAML authority: Authentication: The specified subject was authenticated
by a particular means at a particular time. Attribute: The specified subject is associated with the
supplied attributes. Authorization decision statements: the specified
subject is entitled to do a specified action
“Martino authenticated with a password at 9:00am”
“Bill is an account manager with a $1000 spending limit per one-day travel”
“John Doe” is permitted to buy a specified item
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SAML entities
SAML RequesterSAML Requestera system entity that
uses received assertions
Service ProvidersService Providers a a system entity making use
of SAML assertions
SAML AuthoritySAML Authoritymakes SAML assertions SAML assertions
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SAML and XACML
Source: Security Assertion Markup Language (SAML) V2.0 Technical Overview Working Draft 08, 12 September 2005
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SAML & Federated Identity
SAML addresses one key aspect of identity management: how identity information can be communicated from one domain to another
SAML 2.0 will be the basis on which Liberty Alliance builds additional federated identity applications (such as web service-enabled permissions-based attribute sharing).
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Summary Points SOA concept based on service orientation is now a
significant method for software development and promotes extensibility and flexibility; Service oriented analysis has now become a standard way to model software
Web Services is just one way to realize SOA Security for SOA is crucial as SOA is being used in
numerous sectors; since web services realize SOA, web services security is critical
SOA and SOA Security Standards are being developed by W3C and OASIS; WS-Security, WS*-Security Framework, and XACML are some of the key standards
SOA security currently focuses mainly on access control. SOA-specific techniques to address intrusion detection, denial of service and insider threat analysis need attention
Appendix
Bhavani ThuraisinghamThe University of Texas at Dallas
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Securing the network traffic: SSL/TLS and IPsec Secure Socket Layer SSL and Transport Layer Security are
used to provide transport level security for web services applications. Security features:
authentication data integrity data confidentiality
SSL/TLS enables point-to-point secure sessions.
IP security (IPsec) security features secure sessions with host authentication data integrity data confidentiality
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WS-Policy: Policy model Policy:
A potentially empty collection of policy alternatives. Alternatives are not ordered
Policy Alternative: A potentially empty collection of policy assertions. An alternative with zero assertions indicates no behaviors.. Alternatives are mutually exclusive (exclusive OR)
Policy Assertion: Identifies a a requirement (or capability) of a policy subject. Assertions indicate domain-specific (e.g., security, transactions)
semantics and are expected to be defined in separate, domain-specific specifications
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WS-Policy example<wsp:Policy> <wsp:ExactlyOne> <wsse:SecurityToken> <wsse:TokenType>wsse:Kerberosv5TGT </wsse:TokenType> </wsse:SecurityToken> <wsse:SecurityToken> <wsse:TokenType>wsse:X509v3 </wsse:TokenType> </wsse:SecurityToken> </wsp:ExactlyOne></wsp:Policy>
Which security token we want to use among the various tokens such as Kerberos and X509
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WS-Policy WS-Policy:
is an extensible model for expressing all types of domain-specific policy models: transport-level security, resource usage policy, even end-to-end business-process level policy. It Define basic policy, policy statement, and policy assertion models. WSPolicy is also able to incorporate other policy models such as SAML and XACML
WS-PolicyAssertions: Defines a few generic policy assertions
WS-Policy Attachment: Defines how to associate a policy with a service, either by directly
embedding it in the WSDL definition or by indirectly associating it through UDDI
WS-SecurityPolicy: Defines security policy assertions corresponding to the security claims
defined by WS-Security: message integrity assertion, message confidentiality assertion, and message security token assertion
The only policy assertions standardized so far are those defined in WS-SecurityPolicy (specific assertions that describe how messages are secured) and WS-PolicyAssertions.
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WS-Security mechanisms and considerations
Mechanism(s) Mechanisms for message integrity: digital signatures and certificates Mechanism for confidentiality: encryption (XML Encryption)
Digital signatures alone do not provide message authentication. To prevent replay attack (one can record a signed message and resend it), digital signatures must be combined with timestamps or sequence numbers to ensure the uniqueness of the message.
When digital signatures are used for verifying the identity of the sending party, the sender must prove the possession of the private key. One way to achieve this is to use a challenge-response type of protocol.
The combination of signing and encryption over a common data item may introduce some cryptographic vulnerability: For example, encrypting digitally signed data, while leaving the digital
signature in the clear, may allow plain text guessing attacks
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WS-Security request example1 <soap:Envelope> 2 <soap:Header> 3 <ws:Security> 4 <ws:BinarySecurityToken id="X509token" ValueType="X.509"> 5 sdfOIDFKLSoidefsdflk … 6 </ws:BinarySecurityToken> 7 <ds:Signature> 8 <ds:Reference> 9 <ds:Ref URI="#PO"/> 10 </ds:Reference> 11 <ds:SignatureValue>akjsdflaksf</ds:SignatureValue> 12 <ds:KeyInfo> 13 <ws:BinarySecurityTokenReference URI="#X509token"/> 14 </ds:KeyInfo> 15 </ds:Signature> 16 </ws:Security> 17 </soap:Header> 18 <soap:Body> 19 <po:PurchaseOrder ID="PO"/> 20 </soap:Body> 21 </soap:Envelope>
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WS-SecureConversation Conversations focus on the public processes in which the participants of a Web
service engage; WSCL is Web Services Conversation Language.
Web Services Secure Conversation Language (WS-SecureConversation) February 2005 Status: revised public draft release provided for review and evaluation only
Main goal: provide secure communication across one or more messages. Extends WS-Security mechanisms Allows to authenticate a series of SOAP messages (conversation)
by establishing and sharing between two endpoints a security context for a message conversation using a series of derived keys to increase security.
The security context is defined as a new token type that is obtained using a binding of WS-Trust This allows for exchange in a potentially more efficient way keys or new key
material
Security Context A security context is an abstract concept that refers to an established authentication state and
negotiated key(s) that may have additional security-related properties. A security context token (SCT) is a representation of that security context abstract concept,
which allows a context to be named by a URI and used with WS-Security.
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Security policies for Web Services
The concept of Policy: Guiding principles and procedures
Security policy might mean different things to different people:
Firewall filtering rules
Access control policy
Privacy policy
Standards for Web Services Policies WS-Policy
XACML
XACML profile for Web Services
Approaches: “specialized” models & languages vs. one-size-fits-all framework
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XACML Profile for Web-Services OASIS XACML Profile for Web-Services XACML Working draft
04, 29 Sep 2003
Status: working draft
Main goal: extending XACML to deal with the specific characteristics of Web services
Two main extensions to XACML: define in a precise way the various aspects to which a security policy
applies to, for example for distinguishing the security policy that must be applied to the message level from the access control policy applied to a Web service or to an operation of the Web service
use of the policy combination mechanisms defined in XACML in order to combine the preference/requirements policy of the Web service client with the access control policy of the Web service provider
Note: XACML profile is not getting as much attention as it used to
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SAML profiles Defines constraints and/or extensions of the core protocols
and assertions in support of the usage of SAML for a particular application.
Achieve interoperability. Stipulates how particular statements are communicated
using appropriate protocol messages over specified bindings.
E.g. Web Browser SSO Profile specifies how SAML authentication assertions are communicated using the Authentication Query and Response messages over a number of different bindings in order to enable Single Sign-On for a browser user
By agreeing to support a particular SAML profile (as opposed to the complete specification set), parties who wish to exchange SAML messages have a much simpler job of achieving interoperability.
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Policies and Policy Sets Policy
Smallest element PDP can evaluate Contains: Description, Defaults, Target, Rules, Obligations, Rule
Combining Algorithm Policy Set
Allows Policies and Policy Sets to be combined Use not required Contains: Description, Defaults, Target, Policies, Policy Sets, Policy
References, Policy Set References, Obligations, Policy Combining Algorithm
Combining Algorithms: Deny-overrides, Permit-overrides, First-applicable, Only-one-applicable
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Overview of the Policy Element
<Rule RuleId=“R2” Effect=“Deny”> <Target> <Resources> <Subjects> <Actions> <Condition></Rule>
<Policy> <Target> <Resources> <Subjects> <Actions> <RuleSet ruleCombiningAlgId = “DenyOverrides”> <Rule ruleId=“R1”> <Rule ruleId=“R2”> … <Obligations> <RuleSet></Policy>
<Rule RuleId=“R1” Effect=“Permit”> <Target> <Resources> <Subjects> <Actions> <Condition></Rule>
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XML Key Management Specification (XKMS 2.0) Version 2.0 5 April 2004
Status: W3C Candidate Recommendation
XKMS provides a Web-based interface to existing public key infrastructure (PKI)
XKMS specifies protocols for: Distributing Registering public keys
The protocol is suitable for use in conjunction with the standard for XML Signatures [XML-SIG] and companion standard for XML Encryption [XML-ENC].
The XML Key Management Specification (XKMS) defines two services: the XML Key Information Service Specification (X-KISS) and the XML Key Registration Service Specification (X-KRSS).
Standards for security management:
XKMS (XML Key Management Standard)
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XKMS services
XML Key InformationService (X-KISS)
XML Key RegistrationService (X-KRSS)
BOB
XKMS protocol
- locate a public key - validate a public key
- register - reissue - revoke - recover
ALICE
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Standards for security management: WS-TRUST
Security (confidentiality & integrity) is achieved through encryption, digital signatures and certificates
Ultimately, security depends on the secure management of cryptographic keys and security tokens: Key/security token issuance Key/security token transmission Key/security token storage Key/security token exchange
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WS-Trust Web Services Trust Language (WS-Trust) February 2005 Status: Initial public draft release provided for review and evaluation only Main goal: to enable the issuance and dissemination of credentials
among different trust domains WS-Trust defines extensions to WS-Security that provide:
Methods for issuing, renewing, and validating security tokens. Ways to establish, assess the presence of, and broker trust
relationships. Motivation: The recipient of a WS-Security-protected SOAP message
has three potential issues with the security token contained within the Security header: Format: the format or syntax of the token is not known to the recipient Trust -- the recipient may be unable to build a chain-of-trust from its
own trust anchors (e.g. its X.509 Certificate Authority, a local Kerberos KDC, or a SAML Authority) to the issuer or signer of the token
Namespace -- the recipient may be unable to directly comprehend the set of claims within the token because of syntactical differences
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WS-Trust: trust model
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WS-Trust: example
Fir
ewal
l
STSService
SOAP Gateway
Client Provider Web service
The Client uses X.509 certificate
The Provider
understands Kerberos certificate
WS-Security SOAP msg
NO previouos trust
relationship
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WS-* Security standards and security
WS-* security standard specifications address interoperability aspects
Each standard specification provides a specific section describing security threats that are not addressed by that specification
When using implementations of the specifications, the above warnings must be carefully analyzed
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WS-* Security standards and interoperability
Theory: The framework mandates for a layered approach every upper layer standard could/should re-use and
extend the specification of lower-layer standards. Practice:
Specifications issued by different bodies are not always compatible, but
Adherence to profiles improves interoperability Implementations of different vendors are not always
interoperable
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WS-* Security standards and performance
XML induces overhead Efficient ways of packaging and transmitting binary
data in SOAP messages are needed: XML-binary Optimized Packaging (XOP) SOAP Message Transmission Optimization Mechanism
(MTOM) Resource Representation SOAP Header Block (RRSHB)
Processing of WS-* security compliant messages require encryption/decryption and eventually signature management capabilities
XML accelerators and the XML firewalls try to solve those problems
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XML Accelerators and Firewalls Accelerators: A customized hardware and software performing the
following processing tasks: XML/SOAP parsing, XML schema validation, XPath processing and XSLT transformation functions
Firewalls: Also known as XML gateways: Perform functions of a XML accelerator Support WS-Security standard Additional functionalities:
content or metadata-based XML/SOAP filtering functions XML messages encryption/decryption at the message or element level XML signatures’ verification and XML message signing according to XML
Encryption standard Authentication and authorization functions (that in some XML appliance can
be based on local or on off-board repositories)