Web Services Security Information

WS-Security (Web Services Security, short WSS) is a flexible and feature-rich extension to SOAP to apply security to web services. It is a member of the WS-* family of web service specifications and was published by OASIS.

The protocol specifies how integrity and confidentiality can be enforced on messages and allows the communication of various security token formats, such as SAML, Kerberos, and X.509. Its main focus is the use of XML Signature and XML Encryption to provide end-to-end security.

Features

WS-Security describes three main mechanisms:

• How to sign SOAP messages to assure integrity. Signed messages provide also non-repudiation.
• How to encrypt SOAP messages to assure confidentiality.
• How to attach security tokens.

The specification allows a variety of signature formats, encryptions algorithms and multiple trust domains, and is open to various security token models, such as:

• X.509 certificates
• Kerberos tickets
• UserID/Password credentials
• SAML-Assertion
• Custom defined token

The token formats and semantics are defined in the associated profile documents.

WS-Security incorporates security features in the header of a SOAP message, working in the application layer.

These mechanisms by themselves do not provide a complete security solution for Web services. Instead, this specification is a building block that can be used in conjunction with other Web service extensions and higher-level application-specific protocols to accommodate a wide variety of security models and security technologies. In general, WSS by itself does not provide any guarantee of security. When implementing and using the framework and syntax, it is up to the implementor to ensure that the result is not vulnerable.

Key management, trust bootstrapping, federation and agreement on the technical details (ciphers, formats, algorithms) is outside the scope of WS-Security.

Use Cases

Transport Layer Security (Without WS-Security)

The typical SOAP use case with a communication between trusted peers (using HTTPS) does not need WS-Security at all. It is described in Alternative, and reduces complexity and improves performance.

End-to-end security

If a SOAP intermediary is required, and the intermediary is not or less trusted, messages need to be signed and optionally encrypted. This might be the case of an application level proxy at a network perimeter, that will terminate TCP connections.

Non-Repudiation

The standard method for non-repudiation is to write transactions to an audit trail, that is subject to specific security safeguards. However, if the audit trail is not sufficient, digital signatures may provide a better method to enforce non-repudiation. WS-Security can provide this.

Alternative transport bindings

Although almost all SOAP services implement HTTP bindings, in theory other bindings such as JMS or SMTP could be used; in this case end-to-end security would be required.

Reverse proxy/common security token

Even if the web service relies upon transport layer security, it might be required for the service to know about the end user, if the service is relayed by a (HTTP-) reverse proxy. A WSS-header could be used to convey the end user's token, vouched for by the reverse proxy.

Issues

• If there are frequent message exchanges between service provider and consumer, the overhead of XML SIG and XML ENC are significant. If end-to-end security is required, a protocol like WS-SecureConversation may reduce the overhead. If sufficient, use only encryption or signing, as the combination of both is significantly slower than the mere sum of the single operations. See Performance below.
• The merging of several XML-schemata like SOAP, SAML, XML ENC, XML SIG might cause dependencies on different versions of library functions like canonicalization and parsing, that are difficult to manage in an application server.

Performance

WS-Security adds significant overhead to SOAP-processing due to the increased size of the message on the wire, XML and cryptographic processing, requiring faster CPUs and more memory and bandwidth.

An evaluation in 2005 ^[1] measured 25 types of SOAP messages of different size and complexity processed by WSS4J with both WS-Security and WS-SecureConversation on a Pentium 4/2,8 GHz CPU. Some findings were:

• Encryption was faster than signing
• Encryption and signing together were 2-7 times slower than signing alone and produced significantly bigger documents.
• Depending on the type of message, WS-SecureConversation either made no difference or reduced processing time by half in the best case.
• It took less than 10 milliseconds to sign or encrypt up to an array of 100 kilo bytes, but it took about 100~200 to perform the security operations for SOAP.

Another benchmark in 2006^[2] resulted in this comparison:

History

Web services initially relied on the underlying transport security. In fact, most implementations still do^{[citation needed]}. As SOAP allows for multiple transport bindings, such as HTTP and SMTP, an SOAP-level security mechanism was needed. The lack of end-to-end security because of the dependence on transport security was another factor. Originally developed by IBM, Microsoft, and VeriSign, the protocol is now officially called WSS and developed via committee in Oasis-Open. In 2002, 2 proposals were submitted to the OASIS WSS Technical Committee: Web Service Security (WS-Security) and Web Services Security Addendum. As a result, WS-Security was published:

• WS-Security 1.0 was released on April 19, 2004
• Version 1.1 was released on February 17, 2006

Associated specifications

The following draft specifications are associated with WS-Security:

• WS-Federation
• WS-Privacy
• WS-Test

The following approved specifications are associated with WS-Security:

• WS-Policy
• WS-Trust
• WS-SecureConversation

See also

• .NET Web Services Enhancements
• List of Web service specifications (WS-*)
• SAML
• WS-I Basic Security Profile
• Web service
• X.509
• XACML
• XML Encryption
• XML firewall

Alternative

In point-to-point situations confidentiality and data integrity can also be enforced on Web services through the use of Transport Layer Security (TLS), for example, by sending messages over https. WS-Security however addresses the wider problem of maintaining integrity and confidentiality of messages until after a message was sent from the originating node, providing so called end to end security.

Applying TLS can significantly reduce the overhead involved by removing the need to encode keys and message signatures into XML before sending. A challenge in using TLS would be if messages needed to go through an application level proxy server, as it would need to be able to see the request for routing. In such an example, the server would see the request coming from the proxy, not the client; this could be worked around by having the proxy have a copy of the client's key and certificate, or by having a signing certificate trusted by the server, with which it could generate a key/certificate pair matching those of the client. However, as the proxy is operating on the message, it does not ensure end-to-end security, but only ensures point-to-point security.

External links

• OASIS Web Services Security TC (Contains links to download specification documents)
• WS-Security Specification (IBM)
• WS-I Basic Security Profile
• Web Services Security Documentation
• Web Service Security Patterns (Microsoft)
• WSS4J (WS-Security Java Implementation from Apache)
• Apache Rampart (WS-Security Java Implementation from Apache Axis2)
• WSIT Web Services Interoperability Technologies (WSIT) that enable interoperability between the Java platform and Windows Communication Foundation (WCF)
• python ws-security example

References

1. ^ Hongbin Liu, Shrideep Pallickara, Geoffrey Fox: Performance of Web Services Security
2. ^ Francois Lascelles, Aaron Flint: WS Security Performance. Secure Conversation versus the X509 Profile

Categories: Web service specifications | Computer security software | XML-based standards

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