PART 2: HOW THE SHIFT ACTUALLY WORKS
PART 2: HOW THE SHIFT ACTUALLY WORKS
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The trick behind the Vigenère cipher is simpler than its reputation
suggests. It's Caesar cipher's cousin, run on repeat with a twist.
A Caesar cipher shifts every letter by the same fixed amount — shift by 3,
and A becomes D, B becomes E, and so on. Easy to break, because the shift
never changes; frequency analysis walks right through it.
Vigenère's fix: instead of one shift amount, use a keyword, where each
letter of the keyword tells you the shift amount for that position in the
message (A=0, B=1, C=2, and so on). The keyword repeats for as long as the
message runs.
Take the plaintext CRYPTOISSHORT and the key ABCD, repeated as needed:
Plain: 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
Cipher: C S A S T P K V S I R U T
Notice "C" appears twice in the plaintext's first few letters, but nothing
about the ciphertext gives that away directly — the shifting changes
depending on position. That's the whole defense mechanism in one example:
the same letter, disguised differently depending on where it lands
relative to the key.
The longer and more unpredictable the key, the flatter the frequency
pattern becomes. Push this idea to its logical extreme — a truly random
key as long as the message itself, never reused — and you get the
one-time pad, the only cipher mathematically proven unbreakable. Vigenère
was a step on that road, just not the final one.
Part 3 covers what finally brought it down — and it wasn't frequency analysis at all.
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