Debate on whether the NSA spies domestically on US persons appears to be "yes" according to USSID 18, dated July 23, 1993, which was obtained by the National Security Archive a while back, for which we offer an HTML: http://cryptome.org/nsa-ussid18.htm Parts previously redacted concerning domestic surveillance are now revealed, among them these provisions for acquiring and retaining indefinitely domestically acquired encipherments: [Quote] (2) Domestic communications reasonably believed to contain technical data base information may be retained for a period sufficient to allow a thorough exploitation and to permit access to data that are, or are reasonably believed likely to become, relevant to a current or future foreign intelligence requirement. Sufficient duration may vary with the nature of the exploitation. (S-CCO) a. In the context of a cryptanalytic effort, maintenance of technical data bases 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. (S-CCO) b. In the case of communications that are not enciphered or otherwise thought to contain secret meaning, sufficient duration is one year unless the Deputy Director for Operations, NSA, determines in writing that retention for a longer period is required to respond to authorized foreign intelligence or counterintelligence requirements. (S-CCO) [End quote] Again, these sections were censored in versions of USSID 18 previously made public, a 1980 version here: http://cryptome.org/nsa-ussid18-80.htm While the quoted material is a small part of the 52-page document, variations on it are repeated more than once, and seems to be the one exception to the requirement to avoid domestic interceptions and to destroy any that are inadvertently acquired. The classification (S-CCO) is not explained but some think it perhaps indicates material limited to the UK/USA agreement and/or the Echelon partners. A better answer is welcomed.