Maybe this one was a bit too tough. Aura Guardian solved the first step, Math Wolf was well on his way.
The solution is as follows:
The poem is written by Tennyson. He talked to Mr. Bacon it would seem. This hints to a Bacon cipher. In a Bacon cipher, we construct the alphabet using A's and B's, or sometimes 1's and 0's.
The poem has strings of 5 characters written in a red and blue color. They replace those A's and B's. Decoding the Bacon Cipher with RED = A and BLUE = B, it gives us the following text:
Note the text "WITHCAESAR" at the end of this string. Since only the last bit makes sense, on we had to wade. And never become sceptical, as did the Light Brigade.
The Caesar Cipher is the rotation of letters. Every letter in the ciphertext gets shifted by the value n of the rotation, denoted by ROT(n). This means there are 25 options after that.
Decoding our ciphertext with ROT(6) and leaving out the 'WITHCAESAR' part, we obtain:
In which the last part reads 'THEN WITH MONOALPHABETICAL SUBSTITUTION AND LASTLY'
So we have to use monoalphabetical substitution. Usually, these substitutions are done with a keyword to construct a new alphabet. Using the key 'THELIGHTBRIGADE' we obtain the following substitution-alphabet:
Decoding our previously obtained ciphertext with this substitutio-alphabet, while leaving out 'THENWITHMONOALPHABETICALSUBSTITUTIONANDLASTLY' gets us
And note the last bit says 'VIGENERE'.
Decoding our message according to the Vigenere Cipher with key 'THELIGHTBRIGADE' gives us
So, by putting it all together, the plaintext reads:
The answer is cannonfire. This sentence was first encrypted wi Vigenere, then with monoalphabetical substitution and lastly with Caesar.
And that solves the riddle.
Edited 7/12/2020 15:12:49