The Vigenère cipher held up because it broke the one tool that cracked every cipher before it: frequency analysis.

Written By Morgan Burns

The Vigenère cipher held up because it broke the one tool that cracked every cipher before it: frequency analysis.

**Why simple substitution ciphers fail:** In English, "E" shows up far more than "Z." Swap every letter for another one consistently, and that frequency pattern survives the swap — so codebreakers just count letters and match them to known language statistics.

**What Vigenère changed:** Instead of one substitution alphabet, it uses a repeating keyword to pick a different shift for each letter. "ATTACKATDAWN" with key "LEMON" becomes a cipher where the same plaintext letter can map to different ciphertext letters depending on position. That smears out the frequency pattern almost flat, defeating the standard attack. For 300 years (roughly 1553 to the mid-1800s), nobody found a reliable way around this — it earned the nickname "le chiffre indéchiffrable," the indecipherable cipher.

**The crack, when it finally came:** The weakness was the *repeating* key. If you can guess the key length, you can split the ciphertext into that many interleaved streams, each one a simple shift cipher again — vulnerable to frequency analysis after all.

- Charles Babbage worked out the technique in the 1850s but never published it

- Friedrich Kasiski published it openly in 1863, looking for repeated letter sequences in the ciphertext to guess the key length

- Longer, non-repeating keys (eventually leading to one-time pads) closed this gap for good

Want me to walk through an actual worked example — encoding a message and then cracking it with the Kasiski method?

Let's use a short message and a repeating key so the pattern is easy to spot.

**Encoding**

Plaintext: `CRYPTOISSHORTCRYPTOISSHORT`

Key: `ABCD` (repeated: `ABCDABCDABCDABCDABCDABCDAB`)

Each letter shifts by the key letter's position (A=0, B=1, C=2, D=3):

```

Plain: C R Y P T O I S S H O R T C R Y P T O I S S H O R T

Key: A B C D A B C D A B C D A B C D A B C D A B C D A B

Cipher: C S A S T P K V S I R U T D T A P U R L S T U R T U

```

So the ciphertext is `CSASTPKVSIRUTDTAPURLSTURTU`.

**Cracking it — Kasiski examination**

Step 1: Look for repeated sequences in the ciphertext. Scanning, you'd spot `T` and other fragments repeating — with a longer real message, trigrams like `TDT` or `PUR` would recur because the plaintext repeats every 7 letters (`CRYPTOISSHORT` is 13 letters, but with real ciphertext, patterns tend to show at multiples of the key length).

Step 2: Measure the distance between repeats. If a 3-letter sequence reappears 20 letters later, the key length likely divides 20 (so 1, 2, 4, 5, 10, or 20).

Step 3: Do this for several repeated sequences, then take the **GCD of all the distances** — that narrows it down fast. If distances are 20, 12, and 8, the GCD is 4, so the key is probably length 4.

Step 4: Once you know the key length (say 4), split the ciphertext into 4 interleaved subsequences — every 4th letter starting at position 1, 2, 3, 4. Each subsequence was shifted by just *one* key letter the whole way through, so it's now a plain Caesar cipher.

Step 5: Run frequency analysis on each subsequence separately. The most common letter in each one probably corresponds to "E" (or another high-frequency letter), which tells you that column's shift — and therefore that letter of the key.

Do this for all 4 subsequences and you've recovered the full key, then decrypt everything.

The whole method hinges on one fact: **repetition leaks structure**. A short key repeating over a long message is basically guaranteed to produce these giveaway patterns — which is exactly why real security later moved to keys as long as the message itself (one-time pads).


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  78. Whispers from the Stars — Blake Edwards — https://books2read.com/u/4X19rL

  79. Whispers of the Old World — Steven Jacobs — https://books2read.com/u/bz89g2

  80. William Tell — Liam Conrad — https://books2read.com/u/49AM6M

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  1. Alien Magic — William Benjamin — https://books2read.com/u/4EdvBl

  2. All The Flowers In The Rainbow — Ronald Bartholomew — https://books2read.com/u/4ArqDN

  3. Blackwood Mountain's Secret — Jack Wilson — https://books2read.com/u/bxxWWv

  4. Bomb Iran — Jack Wilson — https://books2read.com/u/mqjV66

  5. Choke Point — Garrison West — https://books2read.com/u/3LPpd7

  6. Commander in the Chief House — James Essex — https://books2read.com/u/mdZpRw

  7. Cyber Dracula on the Moon — Stephen Jacobs — https://books2read.com/u/47KzBE

  8. Draconicum Astralis — Frank Jackson — https://books2read.com/u/38NWqB

  9. Dragon Beast 2: The Global Resonance — Orion Graves — https://books2read.com/u/mgMPo0

  10. Dragon Beast: Magic of the Machine — Orion Graves — https://books2read.com/u/31ND1W

  11. Dull World — Jack Wilson — https://books2read.com/u/mgDRgv

  12. Echoes of Eternity — Frank Jackson — https://books2read.com/u/3kXX7g

  13. Echoes of the Machine — Blake Edwards — https://books2read.com/u/bO5Gjo

  14. Elysium: The AI Conundrum — Buddi T: GAL — https://books2read.com/u/brdvvM

  15. Enhancing Cyber Culture — Stan Bradley — https://books2read.com/u/4N5O7x

  16. Enhancing Tech Theory — T. T. Samuels — https://books2read.com/u/3112aa

  17. Existential Reflections — Jean Francois Enrique — https://books2read.com/u/bwgPgP

  18. From Khartoum to Kharg Island — Liam Conrad — https://books2read.com/u/mB0Qkv

  19. Galaxy Outlaws — Jack Wilson — https://books2read.com/u/bQEeVP

  20. He Who — Morgan Burns — https://books2read.com/u/mZGQ92

  21. Heady Days — Barnaby Finch — https://books2read.com/u/mZ6JxE

  22. History in Time: Middle East — Neville St Claire — https://books2read.com/u/4DvDlg

  23. I'm You — Morgan Burns — https://books2read.com/u/3L7ljJ

  24. I(You!) — Morgan Burns — https://books2read.com/u/m0DwlV

  25. Introduction to Algorithms — Harold Morrison — https://books2read.com/u/m08kOl

  26. Magic Aliens — Marvin Hamner — https://books2read.com/u/3G7EqK

  27. Memory Days — Yug Gohan — https://books2read.com/u/4DvOQr

  28. Mirror of Memories — Steven Jacobs — https://books2read.com/u/bwALVv

  29. Neon Sanctuary — Marvin Hamner — https://books2read.com/u/b6glaZ

  30. Ocean Moon — Rane Corvus — https://books2read.com/u/3LPpdX

  31. Peaks — Robin Crystal — https://books2read.com/u/mYlZRY

  32. Portals — Chester Craig — https://books2read.com/u/4EOrRO

  33. Practical Mysticism I — Brent Newman — https://books2read.com/u/b5yp17

  34. Practical Mysticism II — Brent Newman — https://books2read.com/u/3L7QpM

  35. Practical Mysticism III — Brent Newman — https://books2read.com/u/4XpxK9

  36. Practical Mysticism IV — Brent Newman — https://books2read.com/u/4EV2Wz

  37. Practical Mysticism V — Brent Newman — https://books2read.com/u/479q77

  38. Practical Mysticism VI — Brent Newman — https://books2read.com/u/b5yKgG

  39. Quade's Cosmos — Blake Edwards — https://books2read.com/u/brjVoY

  40. Raid Island — Frank Jackson — https://books2read.com/u/bQ8Kx6

  41. Silicon Cross — Rane Corvus — https://books2read.com/u/mvAWee

  42. Singularity — Harold Morrison — https://books2read.com/u/m279l1

  43. Skin to Sea — Brooks Miller — https://books2read.com/u/mZ9q0R

  44. Soot and Sapphire — Garth Toxo — https://books2read.com/u/bx2Yeo

  45. Souls Beneath — Zoules The Magnificent — https://books2read.com/u/mgQdyD

  46. Sovereign Intelligence — Jack Wilson — https://books2read.com/u/479LKR

  47. Sovereign Sync — Rane Corvus — https://books2read.com/u/3JzZVg

  48. Sovereign Sync II — Rane Corvus — https://books2read.com/u/3kRLK8

  49. Starman — Chad Winthrop — https://books2read.com/u/mKgZZ5

  50. System Restore — Emit Jugal — https://books2read.com/u/3ndwOx

  51. The 9/11 War — Liam Conrad — https://books2read.com/u/b66x8M

  52. The Aldous Huxley Compendium — Brent Newman — https://books2read.com/u/3LPQoD

  53. The Celestial Nomad — Russell Lavine — https://books2read.com/u/m2nqwj

  54. The Chrono Accord — Frank Jackson — https://books2read.com/u/mdBa1l

  55. The Cosmic Awakening — Stan Bradley — https://books2read.com/u/bPPvA7

  56. The Dream Navigator — William Benjamin — https://books2read.com/u/bpvXR6

  57. The Elaraeon — Garth Toxo — https://books2read.com/u/38A8wL

  58. The Fabric of the Hill — Preston Ashcroft — https://books2read.com/u/meYZxl

  59. The Forgotten Enclave — Lyle Davenport — https://books2read.com/u/bO2koA

  60. The Game of Win — Barnaby Finch — https://books2read.com/u/mYNrPM

  61. The Glass Garden — Jonathan Smith — https://books2read.com/u/3n9kpo

  62. The Good Ghost — Ronald Bartholomew — https://books2read.com/u/bzElEn

  63. The Hard Handover — Tomison Peters — https://books2read.com/u/meWP2g

  64. The Infinite Spiral — Steven Jacobs — https://books2read.com/u/499ddW

  65. The Mnemosyne Protocol — Harold Morrison — https://books2read.com/u/4AqgV0

  66. The Node Seven Sync — Rane Corvus — https://books2read.com/u/3GnWLn

  67. The Perpetual Twins — Ronald Bartholomew — https://books2read.com/u/3RENkp

  68. The Soot of Oakhaven — Garth Toxo — https://books2read.com/u/3REVpx

  69. The Temporals — Morgan Burns — https://books2read.com/u/m29EaG

  70. The Visionary — Preston Ashcroft — https://books2read.com/u/4jZ5wl

  71. The War That Rewrote the Middle East — Liam Conrad — https://books2read.com/u/3L7o95

  72. The Zeus Mandate — Russell Lavine — https://books2read.com/u/m0yVYP

  73. They Are Here — Yug Gohan — https://books2read.com/u/3RW5Zv

  74. Venus Rising — Marvin Hamner — https://books2read.com/u/38Nr76

  75. Verses Through the Ages — Morgan Burns — https://books2read.com/u/baENGP

  76. Visions Through Time — Lyle Davenport — https://books2read.com/u/mvAYVe

  77. War and Wisdom — Liam Conrad — https://books2read.com/u/mgMjwK

  78. Whispers from the Stars — Blake Edwards — https://books2read.com/u/4X19rL

  79. Whispers of the Old World — Steven Jacobs — https://books2read.com/u/bz89g2

  80. William Tell — Liam Conrad — https://books2read.com/u/49AM6M

  81. X-9 — The Robotic Van Helsing — Marvin Hamner — https://books2read.com/u/mvgwxX

  82. XOXO — Garth Toxo — https://books2read.com/u/4XWRp5

  83. Zephyrz — Garth Toxo — https://books2read.com/u/47L2PN

  84. Zooz — Frank Jackson — https://books2read.com/u/3y0WOZ

Morgan Burns
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THE VIGENÈRE CIPHER: A THREE-PART BLOG SERIES
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TRIFID CIPHER, CONTINUED: LAYERS 2 AND 3