Foundations
Periodic Keys
How a repeating Key changes the substitution from one position to the next.
Also found as repeating key · key period · polyalphabetic
How the Key repeats
A repeating-Key cipher turns a short Key into a sequence of shifts or alphabet choices, then repeats that sequence across the text. When the Key starts over, the same relationships start over with it. This weakens whole-text letter frequencies, but creates clues about the Key length.Military Cryptanalysis, Part II
- Key
L E M O N L E M O N L E- Values
11 4 12 14 13 11 4 12 14 13 11 4- Phases
1 2 3 4 5 1 2 3 4 5 1 2
What repetition can reveal
Repeated ciphertext sequences and the gaps between them can suggest a period. Test a possible period by separating the text into Key positions. If the guess is right, each part may look more like a monoalphabetic sample. Short texts and accidental repetitions can give false leads, as can choosing the wrong language or alphabet.Military Cryptanalysis, Part II