A monoalphabetic substitution cipher is a type of substitution cipher where each letter in the plaintext is consistently replaced by a single, corresponding letter in the ciphertext. This “mapping” is defined by a substitution alphabet. Unlike polyalphabetic substitution ciphers, only one substitution alphabet is used throughout the entire encryption process.
How it works:
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Create a Substitution Alphabet: A key is created that defines the mapping between plaintext letters and ciphertext letters. This can be a simple shift (like the Caesar cipher) or a completely randomized mapping.
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Encryption: Each letter in the plaintext is replaced with its corresponding letter in the substitution alphabet.
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Decryption: The ciphertext is decrypted using the inverse of the substitution alphabet.
Example:
Let’s say our substitution alphabet is:
Plaintext: ABCDEFGHIJKLMNOPQRSTUVWXYZ Ciphertext: QWERTYUIOPASDFGHJKLZXCVBNM
To encrypt the message “HELLO”, we would replace each letter:
- H → S
- E → R
- L → D
- L → D
- O → L
Therefore, “HELLO” becomes “SRDDL”. To decrypt, we would reverse the process.
Strengths:
- Relatively simple to implement: Both encryption and decryption are straightforward.
Weaknesses:
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Highly susceptible to frequency analysis: The frequency of letters in the ciphertext directly reflects the frequency of letters in the plaintext language. Since letter frequencies in most languages are well-known (e.g., ‘E’ is the most common letter in English), an attacker can easily analyze the ciphertext frequencies to deduce the substitution alphabet.
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Easily broken with known plaintext attacks: If an attacker knows even a small portion of the plaintext and its corresponding ciphertext, they can deduce parts of the substitution alphabet.
Comparison with other ciphers:
Compared to polyalphabetic substitution ciphers (like the Vigenère cipher), monoalphabetic substitution is significantly weaker due to its reliance on a single substitution alphabet. Polyalphabetic ciphers use multiple alphabets, making frequency analysis much more difficult. Modern cryptographic techniques are far more secure than any classical substitution cipher.
Historical Significance: Monoalphabetic substitution ciphers represent an early form of encryption and are important historically for understanding the evolution of cryptography. They highlight fundamental concepts like substitution and the vulnerability of simple ciphers to cryptanalysis.