Kasiski examination

Definition

The kasiski examination is a method used to break ciphers, specifically those involving polyalphabetic substitution, like the Vigenère cipher. It works by analyzing the ciphertext to find repeated sequences of characters and their distances apart, which can reveal the length of the key used in the encryption process. This technique is a fundamental part of cryptanalysis for classical ciphers and helps to expose weaknesses in their security.

5 Must Know Facts For Your Next Test

1. Kasiski examination was developed by Friedrich Kasiski in 1863 and is one of the earliest systematic methods for attacking polyalphabetic ciphers.
2. The technique involves identifying sequences of three or more characters that repeat in the ciphertext and calculating the distances between their occurrences.
3. By finding the greatest common divisor (GCD) of these distances, cryptanalysts can estimate the length of the cipher key.
4. Once the key length is known, it becomes easier to apply frequency analysis to each segment of the ciphertext corresponding to each character of the key.
5. This examination helps highlight vulnerabilities in classical ciphers and laid the groundwork for more advanced cryptanalytic techniques.

Review Questions

• How does the kasiski examination aid in determining the length of a key used in polyalphabetic ciphers?
  • The kasiski examination assists in finding the key length by analyzing repeated sequences of characters in the ciphertext and measuring the distances between these repetitions. By identifying multiple occurrences of these sequences, cryptanalysts can calculate their distances and find their greatest common divisor. This GCD offers a strong indication of the likely length of the key, which is crucial for breaking polyalphabetic ciphers like the Vigenère cipher.
• Discuss how frequency analysis can be applied after determining the key length using kasiski examination.
  • Once the key length is identified through kasiski examination, frequency analysis can be performed on segments of ciphertext that correspond to each character of the key. This allows cryptanalysts to analyze these segments independently, treating them as if they were encrypted with a monoalphabetic cipher. By evaluating letter frequencies within these segments, they can make educated guesses about possible substitutions used in the original plaintext, ultimately leading to its decryption.
• Evaluate the overall effectiveness of kasiski examination in breaking classical ciphers compared to modern cryptanalytic techniques.
  • The kasiski examination has been an effective tool for breaking classical ciphers, particularly those that utilize polyalphabetic substitution like the Vigenère cipher. However, its effectiveness diminishes against modern encryption methods that employ more sophisticated algorithms and key management techniques. While kasiski remains foundational in understanding classical cipher vulnerabilities, it illustrates how cryptanalysis has evolved alongside advancements in encryption technology, highlighting a shift toward more complex mathematical frameworks and computational approaches in modern cryptography.

Related terms

Polyalphabetic Cipher: A type of cipher that uses multiple substitution alphabets to encrypt the text, making it more resistant to frequency analysis compared to monoalphabetic ciphers.

Frequency Analysis: A technique used in cryptanalysis that studies the frequency of letters or groups of letters in ciphertext to identify patterns and infer the plaintext.

Vigenère Cipher: A method of encrypting alphabetic text by using a simple form of polyalphabetic substitution, which relies on a keyword to determine the shift for each letter.