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### Summary

### Summary

Cryptology, for millennia a "secret science," is rapidly gaining in practical importance for the protection of communication channels, databases, & software. Beside its role in computerized information systems (public key systems), more & more applications within computer systems & networks are appearing, which also extend to access rights & source file protection. The first part of this book treats secret codes & their uses - cryptography. The second part deals with the process of covertly decrypting a secret code - cryptanalysis - where in particular advice on assessing methods is given. The book presupposes only elementary mathematical knowledge. Spiced with a wealth of exciting, amusing, & sometimes personal stories from the history of cryptology, it will also interest general readers. Decrypted Secrets has become a standard book on cryptology. The new edition has been revised & extended in many details.

### Table of Contents

Part I Cryptography | p. 1 |

1 Introductory Synopsis | p. 8 |

1.1 Cryptography and Steganography | p. 8 |

1.2 Semagrams | p. 9 |

1.3 Open Code: Masking | p. 12 |

1.4 Cues | p. 16 |

1.5 Open Code: Veiling by Nulls | p. 18 |

1.6 Open Code: Veiling by Grilles | p. 22 |

1.7 Classification of Cryptographic Methods | p. 24 |

2 Aims and Methods of Cryptography | p. 25 |

2.1 The Nature of Cryptography | p. 25 |

2.2 Encryption | p. 31 |

2.3 Cryptosystems | p. 33 |

2.4 Polyphony | p. 35 |

2.5 Character Sets | p. 37 |

2.6 Keys | p. 40 |

3 Encryption Steps: Simple Substitution | p. 42 |

3.1 Case V^{{(1)}} \longrightarrow W (Unipartite Simple Substitutions) | p. 42 |

3.2 Special Case V \longleftrightarrow V (Permutations) | p. 44 |

3.3 Case V^{{(1)}} \longrightarrow W^m (Multipartite Simple Substitutions) | p. 51 |

3.4 The General Case V^{{(1)}} \longrightarrow W^{{(m)}} , Straddling | p. 53 |

4 Encryption Steps: Polygraphic Substitution and Coding | p. 56 |

4.1 Case V^2 \longrightarrow W^{{(m)}} (Digraphic Substitutions) | p. 56 |

4.2 Special Cases of Playfair and Delastelle: Tomographic Methods | p. 62 |

4.3 Case V^3 \longrightarrow W^{{(m)}} (Trigraphic Substitutions) | p. 66 |

4.4 The General Case V^{{(n)}} \longrightarrow W^{{(m)}} : Codes | p. 66 |

5 Encryption Steps: Linear Substitution | p. 78 |

5.1 Self-reciprocal Linear Substitutions | p. 80 |

5.2 Homogeneous Linear Substitutions | p. 80 |

5.3 Binary Linear Substitutions | p. 84 |

5.4 GeneralLinear Substitutions | p. 84 |

5.5 Decomposed Linear Substitutions | p. 85 |

5.6 Decimated Alphabets | p. 88 |

5.7 Linear Substitutions with Decimaland Binary Numbers | p. 89 |

6 Encryption Steps: Transposition | p. 91 |

6.1 Simplest Methods | p. 91 |

6.2 Columnar Transpositions | p. 95 |

6.3 Anagrams | p. 98 |

7 Polyalphabetic Encryption: Families of Alphabets | p. 101 |

7.1 Iterated Substitutions | p. 101 |

7.2 Shifted and Rotated Alphabets | p. 102 |

7.3 Rotor Crypto Machines | p. 105 |

7.4 Shifted Standard Alphabets: Vigenère and Beaufort | p. 114 |

7.5 Unrelated Alphabets | p. 118 |

8 Polyalphabetic Encryption: Keys | p. 126 |

8.1 Early Methods with Periodic Keys | p. 126 |

8.2 `Double Key' | p. 128 |

8.3 Vernam Encryption | p. 129 |

8.4 Quasi-nonperiodic Keys | p. 131 |

8.5 Machines that Generate Their Own Key Sequences | p. 132 |

8.6 Off-Line Forming of Key Sequences | p. 143 |

8.7 Nonperiodic Keys | p. 144 |

8.8 Individual, One Time Keys | p. 148 |

8.9 Key Negotiation and Key Management | p. 151 |

9 Composition of Classes of Methods | p. 155 |

9.1 Group Property | p. 155 |

9.2 Superencryption | p. 157 |

9.3 Similarity of Encryption Methods | p. 159 |

9.4 Shannon's `Pastry Dough Mixing' | p. 160 |

9.5 Confusion and Diffusion by ArithmeticalOperations | p. 166 |

9.6 DES and IDEA | p. 170 |

10 Open Encryption Key Systems | p. 179 |

10.1 Symmetric and Asymmetric Encryption Methods | p. 180 |

10.2 One-Way Functions | p. 182 |

10.3 RSA Method | p. 189 |

10.4 Cryptanalytic Attack upon RSA | p. 191 |

10.5 Secrecy Versus Authentication | p. 194 |

10.6 Security of Public Key Systems | p. 196 |

11 Encryption Security | p. 197 |

11.1 Cryptographic Faults | p. 197 |

11.2 Maxims of Cryptology | p. 205 |

11.3 Shannon's Yardsticks | p. 210 |

11.4 Cryptology and Human Rights | p. 211 |

Part II Cryptanalysis | p. 217 |

12 Exhausting Combinatorial Complexity | p. 220 |

12.1 Monoalphabetic Simple Encryptions | p. 221 |

12.2 Monoalphabetic Polygraphic Encryptions | p. 222 |

12.3 Polyalphabetic Encryptions | p. 225 |

12.4 GeneralRemarks on Combinatorial Complexity | p. 227 |

12.5 Cryptanalysis by Exhaustion | p. 227 |

12.6 Unicity Distance | p. 229 |

12.7 Practical Execution of Exhaustion | p. 231 |

12.8 Mechanizing the Exhaustion | p. 234 |

13 Anatomy of Language: Patterns | p. 235 |

13.1 Invariance of Repetition Patterns | p. 235 |

13.2 Exclusion of Encryption Methods | p. 237 |

13.3 Pattern Finding | p. 238 |

13.4 Finding of Polygraphic Patterns | p. 242 |

13.5 The Method of the Probable Word | p. 242 |

13.6 Automatic Exhaustion of the Instantiations of a Pattern | p. 247 |

13.7 Pangrams | p. 249 |

14 Polyalphabetic Case: Probable Words | p. 251 |

14.1 Non-Coincidence Exhaustion of Probable Word Position | p. 251 |

14.2 Binary Non-Coincidence Exhaustion of Probable Word Position | p. 254 |

14.3 The De Viaris Attack | p. 255 |

14.4 Zig-Zag Exhaustion of Probable Word Position | p. 263 |

14.5 The Method of Isomorphs | p. 264 |

14.6 Covert Plaintext-Cryptotext Compromise | p. 270 |

15 Anatomy of Language: Frequencies | p. 271 |

15.1 Exclusion of Encryption Methods | p. 271 |

15.2 Invariance of Partitions | p. 272 |

15.3 Intuitive Method: Frequency Profile | p. 274 |

15.4 Frequency Ordering | p. 275 |

15.5 Cliques and Matching of Partitions | p. 278 |

15.6 Optimal Matching | p. 284 |

15.7 Frequency of Multigrams | p. 286 |

15.8 The Combined Method of Frequency Matching | p. 291 |

15.9 Frequency Matching for Polygraphic Substitutions | p. 297 |

15.10 Free-Style Methods | p. 298 |

15.11 Unicity Distance Revisited | p. 299 |

16 Kappa and Chi | p. 301 |

16.1 Definition and Invariance of Kappa | p. 301 |

16.2 Definition and Invariance of Chi | p. 304 |

16.3 The Kappa-Chi Theorem | p. 306 |

16.4 The Kappa-Phi Theorem | p. 307 |

16.5 Symmetric Functions of Character Frequencies | p. 309 |

17 Periodicity Examination | p. 311 |

17.1 The Kappa Test of Friedman | p. 312 |

17.2 Kappa Test for Multigrams | p. 313 |

17.3 Cryptanalysis by Machines | p. 314 |

17.4 Kasiski Examination | p. 320 |

17.5 Building a Depth and Phi Test of Kullback | p. 326 |

17.6 Estimating the Period Length | p. 329 |

18 Alignment of Accompanying Alphabets | p. 331 |

18.1 Matching the Profile | p. 331 |

18.2 Aligning Against Known Alphabet | p. 335 |

18.3 Chi Test: Mutual Alignment of Accompanying Alphabets | p. 339 |

18.4 Reconstruction of the Primary Alphabet | p. 344 |

18.5 Kerckhoffs' Symmetry of Position | p. 346 |

18.6 Stripping off Superencryption: Difference Method | p. 351 |

18.7 Decryption of Code | p. 354 |

18.8 Reconstruction of the Password | p. 354 |

19 Compromises | p. 356 |

19.1 Kerckhoffs' Superimposition | p. 356 |

19.2 Superimposition for Encryptions with a Key Group | p. 358 |

19.3 In-Phase Superimposition of Superencrypted Code | p. 373 |

19.4 Cryptotext-Cryptotext Compromises | p. 376 |

19.5 A Method of Sinkov | p. 381 |

19.6 Cryptotext-Cryptotext Compromise: Doubling | p. 388 |

19.7 Plaintext-Cryptotext Compromise: Feedback Cycle | p. 402 |

20 Linear Basis Analysis | p. 412 |

20.1 Reduction of Linear Polygraphic Substitutions | p. 412 |

20.2 Reconstruction of the Key | p. 413 |

20.3 Reconstruction of a Linear Shift Register | p. 414 |

21 Anagramming | p. 417 |

21.1 Transposition | p. 417 |

21.2 Double Columnar Transposition | p. 420 |

21.3 Multiple Anagramming | p. 420 |

22 Concluding Remarks | p. 423 |

22.1 Success in Breaking | p. 424 |

22.2 Mode of Operation of the Unauthorized Decryptor | p. 429 |

22.3 Illusory Security | p. 434 |

22.4 Importance of Cryptology | p. 435 |

Appendix: Axiomatic Information Theory | p. 438 |

Bibliography | p. 448 |

Index | p. 451 |

Photo Credits | p. 471 |