The U.S. National Security Agency (NSA) can retain communications of U.S. citizens or residents potentially indefinitely if those communications are encrypted, according to a newly leaked secret government document.The document describes the procedures used by the NSA to minimize data collection from U.S. persons and is one of two documents published Thursday by U.K.-based newspaper The Guardian. The documents date from July 2009, were signed by U.S. Attorney General Eric Holder and were approved by the U.S. Foreign Intelligence Surveillance Court, the newspaper reported.The documents state that the NSA is not allowed to intentionally target persons known to be located in the U.S., but describe several provisions under which the agency is allowed to retain, or share with other U.S. agencies, communications of U.S. persons that were acquired inadvertently. These include cases when the data is likely to contain foreign intelligence, information on criminal activity or is encrypted.According to the document describing data collection "minimization procedures," foreign communications between a U.S. person and a party located outside of the U.S. that was collected during data acquisitions authorized under the Foreign Intelligence Surveillance Act (FISA) can be retained for "cryptanalytic, traffic analysis, or signal exploitation purposes."The retention of such communications is permitted for "a period sufficient to allow a thorough exploitation" and to permit access to data reasonably believed to be or become relevant to current or future foreign intelligence requirements."In the context of a cryptanalytic effort, maintenance of technical data base requires retention of all communications that are enciphered or reasonably believed to contain secret meaning, and sufficient duration may consist of any period of time during which encrypted material is subject to, or of use in, cryptanalysis," the document says.Encrypted domestic communications between two U.S. persons that were collected inadvertently can also be retained and targeted for cryptanalysis if they are reasonably believed to contain "technical data base" information or information needed to assess a communications security vulnerability.This suggests that if the NSA believes that a particular encrypted communication contains valuable technical or cybersecurity information, it can keep it indefinitely and attempt to decrypt it, regardless of whether it involves U.S. persons or not.The new revelations come at a time when U.S. government officials are pressed to provide answers about the extent of NSA's surveillance efforts inside the U.S., following reports that the agency is collecting phone records and electronic communications from U.S.-based companies.