In this video, we will discuss two important techniques in cryptography: monoalphabetic and polyalphabetic substitution ciphers. We will also discuss the Vigenere cipher, which is a type of polyalphabetic substitution cipher.
Monoalphabetic substitution ciphers
In a monoalphabetic substitution cipher, each plaintext letter is always substituted by the same ciphertext letter. For example, in a Caesar cipher, each letter is shifted three positions down the alphabet.
Monoalphabetic substitution ciphers are relatively easy to break because the frequency distribution of the letters in the ciphertext is the same as the frequency distribution of the letters in the plaintext. This means that an attacker can use frequency analysis to guess the ciphertext letters.
Polyalphabetic substitution ciphers
In a polyalphabetic substitution cipher, each plaintext letter is substituted by a different ciphertext letter, depending on the position of the letter in the plaintext. This makes it much more difficult to break the cipher using frequency analysis.
The Vigenere cipher is a type of polyalphabetic substitution cipher that uses a keyword to determine the ciphertext letters. The keyword is repeated over and over again, and each letter of the plaintext is substituted by the letter of the ciphertext that corresponds to the current position of the keyword.
The Vigenere cipher is a more secure cipher than a monoalphabetic substitution cipher, but it is still vulnerable to certain attacks. For example, if an attacker knows the keyword, then they can break the cipher.
Conclusion
Monoalphabetic and polyalphabetic substitution ciphers are two important techniques in cryptography. The Vigenere cipher is a type of polyalphabetic substitution cipher that is more secure than a monoalphabetic substitution cipher, but it is still vulnerable to certain attacks.
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