Final answer:
Vigenère decryption involves reversing the cipher's encryption process with the keyword, subtracting the keyword letter position from the ciphertext letter position, and taking the modulo to obtain the original message.
Step-by-step explanation:
The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. Decrypting a message requires reversing the encryption process, which involves aligning the keyword with the ciphertext and performing a reverse Caesar cipher for each letter. If we were to decrypt using a keyword, for each letter in the ciphertext (c), we would find the position of the corresponding keyword letter in the alphabet, subtract that from the position of the ciphertext letter, and then find the letter in the alphabet that matches the resulting number, essentially shifting back each letter by the amount specified by the keyword. To handle wrapping past the letter 'A', we would take the result of the subtraction modulo 26 (the number of letters in the English alphabet).
To explain with a simple rule similar to the one you provided: suppose we wanted to decrypt the ciphertext 'DAHKK' with the keyword 'LEMON'. The letter 'D' is shifted back by the amount corresponding to 'L' and so on for each subsequent letter in the corresponding positions. Note that this is different from the rule that you provided, which changes the first letter to 'm' and then changes the first vowel to 'a', as that rule pertains to simply transforming words rather than using a cipher.
Therefore, the Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword.