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A polyalphabetic substitution cipher that extends the key by appending plaintext characters to the key stream.
DESCRIPTION
The autokey cipher enhances the Vigenère method by using plaintext to extend the key stream, increasing variability and spreading letter frequencies. While stronger than simple Vigenère, autokey remains vulnerable to known-plaintext attacks and statistical methods. It is historically significant in classical cryptography and illustrates trade-offs between key management and security. Modern cryptography does not employ autokey for practical protection due to its weaknesses.
ALTERNATIVE DEFINITIONS
Wikipedia
The autokey cipher is a polyalphabetic substitution cipher in which a plaintext is used as part of the key.
States the defining property: plaintext extends the key.
Practical Cryptography
The Autokey cipher is a polyalphabetic substitution cipher that uses the plaintext to extend the key.
Plainly emphasizes the use of plaintext to extend the key stream.
RELATED TERMS
Polyalphabetic Cipher Kasiski Examination Frequency Analysis Cryptanalysis Playfair Cipher Substitution Keystream Classical Cryptography
COMPETING TERMS
RELATIONS
KNOWLEDGE AREAS
TARGET AUDIENCE
Cybersecurity Engineer Security Architect Software Developer
EXTERNAL SOURCES
- Wikipedia - Autokey cipher
- Wikipedia - Overview and history of autokey
- Crypto StackExchange - Community Q&A on autokey
TOP QUESTIONS
How does the Autokey Cipher work?
It starts with a short key and appends plaintext to extend the keystream, then applies Vigenère-style substitution.
Category: Concept
Is the Autokey Cipher secure today?
No. It is vulnerable to known-plaintext and statistical attacks, and should not be used for modern security needs.
Category: Risk
How does Autokey differ from Vigenère?
Autokey uses plaintext to extend the key, increasing variability compared to repeating a fixed key in Vigenère.
Category: Comparison
What is a simple attack against Autokey?
A known-plaintext attack can recover the initial key and thereby decrypt the message.
Category: Example
What lessons does Autokey teach cryptographers?
Key reuse and predictable keystreams are dangerous; strong ciphers avoid correlations with plaintext.
Category: Insight
When was the Autokey Cipher used?
It appeared in classical cryptography periods, preceding modern symmetric and public-key methods.
Category: History
What is a secure modern alternative to Autokey?
Use authenticated encryption like AES-GCM with strong keys and nonces.
Category: Alternatives
Is the Autokey Cipher related to one-time pads?
Conceptually, both use a keystream; however, autokey is predictable, while one-time pads are truly random and secure.
Category: Misunderstanding
How can I experiment with Autokey safely?
Use it as an educational exercise only, never for securing data or communications.
Category: BestPractices