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A polyalphabetic substitution cipher using a repeating keyword to shift letters via interwoven Caesar ciphers.
DESCRIPTION
The Vigenère cipher encrypts alphabetic text by shifting letters according to a repeating keyword over a tabula recta. This polyalphabetic approach reduces simple frequency analysis by varying substitutions across positions. Its strength depends on key length and unpredictability; short, repeating keys leak periodic patterns. Classical attacks like Kasiski examination and the Friedman test estimate key length and enable partitioned analysis. Vigenère remains a foundational teaching example in cryptography.
ALTERNATIVE DEFINITIONS
Wikipedia
The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword.
Stresses interwoven Caesar ciphers and keyword-driven shifts.
Encyclopaedia Britannica
Vigenère cipher, polyalphabetic substitution cipher in which the letters of the plaintext are shifted by a number of positions determined by a repeating keyword.
Highlights repeating keyword determining shifts in a polyalphabetic system.
RELATED TERMS
Multi-Alphabet Substitution Autokey Cipher Tabula Recta Kasiski Examination Friedman Test Caesar Cipher Plaintext Ciphertext
COMPETING TERMS
Gronsfeld Cipher Beaufort Cipher
KNOWLEDGE AREAS
TARGET AUDIENCE
Cybersecurity Engineer Security Architect Software Developer
EXTERNAL SOURCES
- Wikipedia - Vigenère cipher
- Wolfram MathWorld - Mathematical description and properties.
- Practical Cryptography - Tutorial and cryptanalysis of Vigenère.
- Khan Academy - Intro lesson on Vigenère cipher.
TOP QUESTIONS
How does the Vigenère Cipher use the tabula recta?
The tabula recta provides shifted alphabets; each plaintext letter is encrypted using the row tied to the keyword letter.
Category: Example
What makes the Vigenère Cipher stronger than Caesar?
It varies the shift per character using a keyword, reducing single-letter frequency patterns.
Category: Opinion
How is the Vigenère Cipher attacked?
Kasiski and Friedman methods estimate keyword length, enabling separate Caesar-style analysis of each position class.
Category: Risk
What is a common mistake when choosing Vigenère keys?
Using short, common words creates periodic leakage that eases cryptanalysis.
Category: Mistakes
Is Vigenère secure for modern use?
No. It’s educational only; modern cryptography like AES should be used for security.
Category: Relevancy
What is the difference between Vigenère and Beaufort ciphers?
They differ in how shifts are applied: Beaufort uses a subtraction-based variant producing a different tableau mapping.
Category: Example
How does key length affect Vigenère strength?
Longer, non-repeating keys reduce periodicity, complicating classical attacks.
Category: Opinion
What is an autokey variant of Vigenère?
Autokey extends the key with plaintext (or ciphertext), reducing periodicity but introducing other attack surfaces.
Category: Example
What is a practical learning exercise with Vigenère?
Implement tabula recta encryption, then write a Kasiski attack to estimate key length and recover plaintext.
Category: Tutorial