11. A system comprising: one or more computing devices and one or more storage devices storing instructions that, when executed by the one or more computing devices, cause the one or more computing devices to perform operations comprising: receiving, from a first client device associated with a first user, a request for sharing a document with a second client device, wherein the document is electronically signed using a credential of the first user, and the credential identifies one or more privileges allocated to the first user; in response to receiving the request for sharing the document with the second client device, generating a code that allows access to the document; transmitting, to the first client device, the code that allows access to the document; receiving, from the second client device, a request to access the document, the request to access the document including the code that allows access to the document; based on the request to access the document, determining that a second user associated with the second client device is authorized to access the document; and communicating, to the second client device, a message including information about the document based on determining that the second user associated with the second client device is authorized to access the document.
20. The system of claim 11 , wherein determining that the second user associated with the second client device is authorized to access the document comprises: determining that the second user associated with the second client device is in possession of a physical copy of the document.