Documents cannot be read by anyone other than the receiver; for instance, in the United States it is a violation of federal law for anyone other than the receiver to open mail. However, exceptions do exist, such as postcards, which can be read by the postman for the purpose of identifying the sender and receiver. For mail contained within an envelope, there are legal provisions in some jurisdictions allowing the recording identities.[13] The privacy of correspondence is guaranteed by the Mexican Constitution, and is alluded to in the European Convention of Human Rights[14] and the Universal Declaration of Human Rights.[13] According to the laws in the relevant jurisdiction, correspondence may be openly or covertly opened or the contents determined via some other method, by the police or other authorities in some cases relating to their relevance to an alleged or suspected criminal conspiracy, although black chambers (largely in the past, though there is apparently some continuance of their use today) opened and open letters extralegally. Military mail to and from soldiers on active deployment is more often subject to strict censorship. International mail and packages are subjects to customs control.