Declassified NSA Cryptography Document— NSTISSAM TEMPEST 2 91

This episode presents the National Security Agency’s National Telecommunications and Information Systems Security Advisory Memorandum No. 2‑91, titled “Compromising Emanations Analysis Handbook.” The document was declassified and approved for public release on September twenty‑fifth, twenty‑twenty‑four, under Freedom of Information Act case number five‑one‑five‑seven‑three. It is hosted in the public archive at the Black Vault. The memorandum was released through a Freedom of Information Act request, case number five‑one‑five‑seven‑three, filed by a private requester. The National Security Agency authorized declassification on September twenty‑fifth, twenty‑twenty‑four, citing the standard declassification authority for historical communications‑security material. The full text is available from the Black Vault at the URL https://archive.org/download/declassified-nsa-cryptography-document-nstissam-tempest-2-91/Declassified%20NSA%20Cryptography%20Document—%20NSTISSAM‑TEMPEST‑2‑91_djvu.txt. The document begins with a foreword. It reads: “National Security Telecommunications and Information Systems Security Advisory Memorandum (NSTISSAM) No. 2‑91, ‘Compromising Emanations Analysis Handbook,’ describes analysis concepts and techniques currently in use. This advisory memorandum supersedes NACSEM No. 5106, ‘Compromising Emanations Analysis Handbook,’ dated December 1971. This document contains communications‑security information. Access by contractor personnel is restricted to United States citizens holding final United States Government clearances. This document is not releasable to the Defense Technical Information Center per Department of Defense Instruction 5100.38. Representatives of the National Security Telecommunications and Information Systems Security Committee may obtain additional copies of this advisory memorandum from the Executive Secretariat, National Security Telecommunications and Information Systems Security Committee, National Security Agency, Fort George G. Meade, Maryland 20755‑6000. United States Government contractors are to contact their appropriate government agency or Contracting Officer Representative regarding distribution of this document. Vice Admiral W. O. Studeman, United States Navy” The memorandum then lists its table of contents. Highlights include: “Chapter 1 – Introduction  1‑1 General Purpose  1‑2 Comments and Recommendations Chapter 2 – The Fundamentals of TEMPEST Analysis  2‑1 General Properties of Information  2‑2 Types of Information Sources   (1) English Text   (2) Military Text   (3) Alpha‑numeric Data   (4) Cathode‑Ray Tube Displays Chapter 3 – Mathematics for Analysis  3‑1 Probability and Frequency  3‑2 Conditional Probability, Bayes Rules  3‑3 Continuous Distributions and the Normal Distribution Chapter 4 – Pre‑test Planning and Analysis  4‑1 Introduction  4‑2 Pre‑test Planning Procedures Chapter 5 – On‑site Analysis  5‑1 Introduction  5‑2 Initial Detection Methods  5‑3 Alternate Methods of Analysis” Further into Chapter 2, the memorandum defines the “TEMPEST Model” and outlines the degrees of analysis, including pre‑test analysis, signal detection, and signal recording. It enumerates components of detection systems, such as signal recorders, analog‑to‑digital converters, and signal averaging