1. A computer-implemented method comprising: identifying, by a data processing apparatus, repurposable information in an electronic document, the repurposable information being less than all of the content in the electronic document; defining one or more repurposing constraints for the repurposable information, the repurposing constraints identifying an authorized destination for the repurposable information; and receiving a digital signature for the repurposable information and the one or more repurposing constraints with a digital signature, the digital signature authenticating the repurposable information, the digital signature verifying that the repurposable information has not been altered since it was signed, the digital signature being maintained by the authorized destination when the repurposable information is repurposed.