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Use UsernameToken Nonce Values for Message Verification
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If you find yourself struggling to extract a unique piece of information from a message (using the SoapContext class), and the message includes a UsernameToken security token, then you can use a nonce-based token ID as a unique identifier. A nonce is simply a random cryptographic string that can be assigned as the ID
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Secure Web Services with WS-Security
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value for the UsernameToken security token. When the service receives a request message, it can extract the nonce value from the security token and cache the value for the duration of the request message. These ID values are part of the message signature and cannot be spoofed. And because they are nonce values, it is highly unlikely that two request messages will coincidentally share the same ID values. However, this could happen if you choose to rely on the auto-generated ID value for the security token. Again, the burden remains on the service to cache information on incoming request messages. But if you need to take this approach, then a nonce value is the simplest way to do so. Listing 6-15 shows how the client can assign a nonce value to a UsernameToken security token.
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Listing 6-15. Assigning a Nonce Value to a UsernameToken Security Token
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using Microsoft.Web.Services2.Security; using Microsoft.Web.Services2.Security.Tokens; SecurityToken token = new UsernameToken(username, passwordEquivalent, PasswordOption.SendHashed); // Assign a random nonce value to the security token Nonce objNonce = new Nonce(34); token.Id = objNonce.Value;
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You may be wondering why nonce values apply specifically to the UsernameToken security token. This is because other security tokens are more sophisticated and do not require the additional guarantee of uniqueness that a nonce value provides. A UsernameToken security token is, after all, simply a hashed username-password combination, and there is nothing inherently unique about this combination. Usernames and passwords can be duplicated between users much more easily than cryptographic values can, especially if a malicious client is intentionally using another client s credentials. If you use an alternate security token such as an X.509 certificate, then you are automatically afforded some protection because the client and the service are using credentials that are not easily discovered. However, as I pointed out with SSL, this does not provide protection against replay attacks. You cannot assume that authorized clients will by their nature avoid carrying out a replay attack. For example, consider a client that auto-generates Web service calls in batch mode. If this client were to experience a system error or breakdown in business logic, then it is conceivable that the client might generate duplicate request messages to the service. This is why you must tackle replay attacks at the message and service level. You cannot protect against replay attacks under the umbrella of a trusted relationship between client and service.
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Use Message Correlation and Sequence Numbers for Message Verification
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The key to preventing replay attacks is for the Web service to verify the uniqueness of incoming request messages. The WS-Addressing specification describes a GUID-based message ID that is one of several addressing headers that can be assigned to a SOAP message. WSE provides support for the WS-Addressing specification in general, and for addressing headers specifically. Once again, the burden is on the Web service to store message correlation information and to determine whether an incoming message has already been received. As with other kinds of identifiers, the message ID does not in and of itself prevent replay attacks, but it provides another simple, unique identifier for an incoming SOAP message.
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NOTE
Refer to 9 for more information on the WS-Addressing specification.
Another type of message identifier is the sequence number, which stamps a message with the sequential role that it plays in a business process. Sequence numbers are part of the WS-Reliable Messaging specification, and are designed to enable business orchestration, which refers to a business process or workflow that spans multiple components. In service-oriented architectures, sequenced messages are exchanged between multiple Web services, and the collective outcome represents the completion of the business workflow. Sequence numbers provide an additional advantage for preventing replay attacks because a message that contains a duplicate sequence number is automatically suspect. Sequence numbers alone do not ensure uniqueness, but they will in conjunction with a message ID.
Summary
This chapter has shown you how to use WS-Security to implement several types of security measures in SOAP messages, including the following: 1. Message authentication using security tokens based on usernamepassword combinations and X.509 certificates. Digital signatures on SOAP messages to detect message tampering. Encryption of SOAP messages (using asymmetric encryption) to protect the contents of a SOAP message from network sniffers.
2. 3.
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