Nonce, a randomly generated token
Nonce is a randomly generated, cryptographic token used to prevent the theft of user name tokens used with SOAP messages. Nonce is used with the basicauth authentication method.
Without nonce, when a user name token is passed from one machine to another machine using a non-secure transport, such as HTTP, the token might be intercepted and used in a replay attack. The same key might be reused when the username token is transmitted between the client and the server, which leaves it vulnerable to attack. The user name token can be stolen even if you use XML digital signature and XML encryption.
To help eliminate these replay attacks, the <wsse:Nonce> and <wsu:Created> elements are generated within the <wsee: usernameToken> element and used to validate the message. The request receiver or response receiver checks the freshness of the message to verify the difference between when the message is created and the current time falls within a specified time period. Also, WAS verifies that the receiver has not processed the token within the specified time period. These two features are used to lessen the chance that a user name token is used for a replay attack.
See AlsoUsername token element
Configuring nonce for the application level
Configuring nonce for the server level
Securing Web services using basicauth authentication