Kasiski Test

Tool Introduction

The Kasiski Test is an online cryptanalysis tool designed to identify the key length of polyalphabetic substitution ciphers, such as the Vigenère cipher. By analyzing repeating sequences of letters in the ciphertext, it estimates the most probable key length, providing important clues for subsequent cryptanalysis. Users simply input the ciphertext to be analyzed, and the tool will output the Kasiski test results, helping you understand the ciphertext structure and proceed with deeper cryptanalysis. This method is one of the key steps in Vigenère cipher analysis, effectively narrowing down the key search range.

How to Use

1. Copy and paste the ciphertext to be analyzed into the "Text Content" input box. Please ensure that the input text is pure ciphertext, without any additional formatting or descriptions.
2. After confirming that the content in the input box is correct, click the execute button (e.g., "Start Analysis" or "Submit").
3. The tool will perform a Kasiski test analysis on the input ciphertext and display the analysis results in the results area.

Input Parameter Description:

• Text Content (content): Required, a multi-line text box for entering the ciphertext to be subjected to the Kasiski test. It is recommended to input sufficiently long ciphertext to obtain more accurate analysis results, supporting uppercase and lowercase letters, numbers, and common punctuation marks as ciphertext content.

Output Result Format:

• The output results will be displayed in a text area (textarea), usually containing a list of repeating sequences and their interval distances, as well as factor analysis results of possible key lengths inferred from these distances. The specific output format may vary depending on implementation details, but it is generally designed to help users identify the most common distance factors, which are potential key lengths.

Frequently Asked Questions

• Q: What is the principle of the Kasiski Test?
• A: The Kasiski Test works by finding repeating sequences of letters in the ciphertext and calculating the distances between these sequences. If the ciphertext is encrypted by a polyalphabetic substitution cipher (like the Vigenère cipher), and the repeating sequences in the plaintext happen to align with the key's cycle, then the distance between them could be a multiple of the key length. By analyzing the greatest common divisor or the most common factors of all these distances, the key length can be effectively estimated, providing important clues for breaking the Vigenère cipher.
• Q: What input formats are supported?
• A: The tool supports pure text format for ciphertext input and does not accept images, documents, or other non-text formats. Please ensure that the input content is the ciphertext to be analyzed, without irrelevant metadata or descriptions.
• Q: What is the format of the output results?
• A: The output results are in plain text format, typically listing detected repeating sequences, their occurrences, calculated interval distances and their factors, and finally providing one or more suggestions for the most probable key lengths.
• Q: Does ciphertext length affect the test results?
• A: Yes, ciphertext length significantly affects the accuracy of the Kasiski Test. The longer the ciphertext, the more obvious the statistical repeating patterns, and the more reliable and accurate the test results. For overly short ciphertext, it may not be possible to detect enough repeating sequences, or the detected repeating sequences may be coincidental, leading to inaccurate results. It is recommended to use ciphertext of several hundred characters or more for testing.

Notes

• Input Data Requirements: Please ensure that the input is pure text ciphertext, without special characters or formatting other than spaces and punctuation, to avoid affecting the accuracy of the analysis results. Non-alphabetic characters may be ignored or treated as delimiters, depending on the tool's implementation.
• Ciphertext Length Recommendation: To obtain more accurate test results, it is recommended to input sufficiently long ciphertext. Generally, the longer the ciphertext, the more meaningful the detected repeating patterns, the higher the reliability of the Kasiski test results, and the better it can help you accurately estimate the key length of the Vigenère cipher.
• Results Are Not Absolute: The Kasiski Test provides an estimate or a list of possibilities for the key length, not a definitive final answer. In practical cryptanalysis, it is usually necessary to combine other techniques (such as the index of coincidence, frequency analysis) for further verification to confirm the most probable key length.
• Accidental Repetitions: In short ciphertexts, some sequence repetitions may be accidental rather than due to encryption patterns. Therefore, analysis results for short ciphertexts should be treated with caution, and attempts should be made to find longer repeating sequences to improve accuracy.