This report describes the German Tunny encoding machine and tells the full story of how the British broke Tunny and decoded a steady stream of messages from Hitler and other members of the German High Command. It includes an account of Colossus, the world's first large-scale electronic digital computer, installed at Bletchley Park in December 1943.

The report was written at Bletchley Park in 1945 by three codebreakers, Jack Good, Donald Michie, and Geoffrey Timms.

This report was classified until the middle of 2000, when it was released to the Public Record Office, Kew (document reference HW 25/4 and HW 25/5).

The links in the table of contents below lead to a digital facsimile of the entire document.

The original document is Crown Copyright and is reproduced by permission of the Public Record Office.

• Photograph of Lorenz machine
• Photograph of Colossus
• Tony Sale's retype of the report

GENERAL REPORT ON TUNNY
With Emphasis on Statistical Methods

TABLE OF CONTENTS

Part 0

• Preface

Part 1 INTRODUCTION

• Contents
• German Tunny
• Cryptographic Aspects
• Machines
• Organisation
• Some Historical Notes

Part 2 METHODS OF SOLUTION

• Some Probability Techniques
• Statistical Foundations
• Machine Setting
• Rectangling
• Chi-breaking (from Cipher)
• Wheel-breaking (from Key)
• Cribs
• Language Methods

Part 3 ORGANISATION

• Mr Newman's Section
• Major Tester's Section
• Knockholt
• Registration and Circulation
• Tape-making and Checking
• Chi-breaking and Cribs
• Machine Setting
• Wheel-breaking (from Key)
• Language Methods

Part 4 EARLY METHODS AND HISTORY

• The First Break
• Early Hand Methods
• Testery Methods 1942-4
• Hand Statistical Methods

Part 5 MACHINES

• General Introduction
• Development of Robinson and Colossus
• Colossus
• Robinson
• Specialised Counting Machines
• Copying Machines
• Simple Machines
• Photographs

Part 6

• Raw Materials and Production with Plans of Tunny Links

Part 7 REFERENCE

• Glossary and Index
• Notation
• Bibliography
• Chronology

Part 8

• Conclusions

Part 9 APPENDICES

• 5202
• Motor Rectangles
• Thrasher
• QEP Research
• Mechanical Flags