PART 3: THE CRACK — WHY REPETITION IS THE ENEMY OF SECRECY
--------------------------------------------------------------
Every "unbreakable" cipher eventually meets its match, and Vigenère's came
from an angle nobody had aimed at directly: not the substitution itself,
but the repetition of the key.
A short keyword repeated over a long message is bound to line up with
itself at points, especially where common words or letter patterns show
up in the plaintext. When that happens, the same plaintext fragment gets
encrypted the same way twice, and that repeated sequence shows up in the
ciphertext, sitting at some fixed distance apart.
That's the opening. This method, formalized as the Kasiski examination
(after Friedrich Kasiski, who published it in 1863 — though Charles
Babbage had worked it out privately in the 1850s and never published),
works like this:
1. Scan the ciphertext for repeated sequences of three or more letters.
2. Measure the distance between each repeat.
3. Take the greatest common divisor of those distances — it's very
likely the length of the key, or a factor of it.
4. Once you know the key length, split the ciphertext into that many
interleaved subsequences. Each one was shifted by only a single,
constant amount — a plain Caesar cipher in disguise.
5. Run ordinary frequency analysis on each subsequence separately.
The most common letter in each is probably a disguised "E" (or
whatever's most frequent in the source language), which reveals
that shift — and one letter of the key.
Do this across all the subsequences, and the entire key falls out, letter
by letter. What looked unbreakable turns out to have been several small,
breakable ciphers wearing a trench coat.
The lesson cryptographers took from this outlived the cipher itself:
repetition leaks structure. Any pattern that repeats gives an attacker a
foothold, no matter how sophisticated the underlying idea. It's the reason
modern cryptography leans so heavily on randomness and non-repeating
keys — echoes of a 300-year-old cipher's exact undoing.
================================================
END OF SERIES
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