1. A computer-implemented method of determining a reputation score for an agent, the method comprising: receiving multiple digital content items in a data processing system comprising one or more computers, each of the received digital content items being associated with a digital signature that uniquely associates the digital content item with a respective agent and an assertion made by the respective agent concerning the digital content item, the respective agent being one of multiple agents, each of the multiple agents being associated with at least one of the received digital content items; validating in the data processing system the associated digital signature for each of the received digital content items, and identifying, based upon the associated digital signature, the agent responsible for making the assertion concerning the received digital content item; and determining in the data processing system a reputation score for a first agent of the identified agents, wherein the reputation score for the first agent is a function of: a) for each of the digital content items associated by digital signature with the first agent and referenced by a digital content item associated with another of the identified agents, a reputation score of the another of the identified agents, and b) a reputation score of a second agent associated with a second digital content item that references one of the digital content items associated by digital signature with the first agent, the reputation score of the second agent based on a second assertion made by the second agent that specifies a role of the second agent with respect to the second digital content item; and wherein at least one of the digital content items associated by digital signature with the first agent is a first subset of a web page, the web page including at least a second subset of the web page that is not associated with the first agent, and the reputation score is not a function of the second subset of the web page.