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Estimate the key length of classical ciphers like the Vigenère cipher by analyzing repeated ciphertext sequences to assist in decryption.
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Overview
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Kasiski examination is a classical clue-finding method for periodic-key ciphers such as the Vigenère cipher. If a repeated plaintext fragment meets the same part of a repeating key, the encrypted text can contain a repeated group too. The distance between those occurrences may be a multiple of the key period. Several distances can therefore suggest candidate periods, but a repeated group can also occur by coincidence.
This analyzer keeps only English letters A–Z, converts them to uppercase, and measures positions in that filtered string. It searches three-letter groups and reports groups that recur, with the gaps between successive occurrences. The summary counts small factors found among those gaps and displays up to five candidate values. A factor is a lead to examine, not a recovered key length.
For a gap of 12 letters, for example, factors such as 2, 3, 4, 6, and 12 can all support candidate values. The tool does not calculate a greatest common divisor across every distance, test the resulting periods against letter frequencies, identify the cipher, or decrypt the message. It displays up to 20 repeated groups, ordered by recurrence.
Source: Michigan Technological University, Kasiski’s Method describes why repeated ciphertext fragments can have separations related to a repeating key period; classical Vigenère analysis; checked 2026-09-28.
Guide
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Use the encrypted letter sequence as the sample. Spaces, punctuation, digits, and non-English letters are discarded before positions and gaps are calculated.
Each row shows a recurring three-letter group and the distances between its consecutive appearances in the filtered text. A gap is measured in letters after filtering, not in the original character positions.
Look for a value that is supported by more than one observed gap, then compare it with other cryptanalysis evidence. The candidate list is limited to factors no greater than 20, so the absence of a value does not prove that a longer key is impossible.
A blank result can mean there are fewer than six A–Z letters after filtering, no repeated trigrams, or too little repeating structure for this method to expose. Try a longer ciphertext sample if one is available.
Use cases
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Students can compare visible repeated groups and their gaps, then explain why common factors suggest possible key periods rather than certain answers.
For a known classroom ciphertext, use the candidate values to decide which column groupings or frequency checks to investigate next.
Study whether separate repeated groups contribute compatible distances before settling on a likely period.
Q&A
Find concise answers to common questions and confusing cases.
No. It is a factor count from observed gaps. A true period or a multiple may be plausible, while accidental repetitions and short samples can create misleading candidates.
The input is reduced to A–Z letters before analysis. Distances count positions in that cleaned letter sequence, so removed characters do not add to a gap.
The cleaned text must contain at least six letters, and it must include a repeated three-letter group. A short or non-periodic sample may have no repeated groups to report.
No. It reports repeated groups, distances, and a short list of factor candidates. You would need separate analysis to assess a cipher and recover any plaintext.
Notes
Review scope, result limitations, and important precautions before use.
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