I have the following set-up:
Bob has an RSA Cryptosystem with a large modulus $n$ for which the factorization cannot be found in a reasonable amount of time. Alice sends a message to Bob by representing each alphabetic character as an integer between $0$ and $25$ (that is, $A ↔ 0$, $B ↔ 1$, etc.), and then encrypting each residue modulo 26 as a separate plaintext character.
I want to figure out how Oscar can easily decrypt a message in this case. However, I don't really understand how the encryption works here.
For RSA we have
But I don't understand what $b$ would be in this case and I don't fully understand what 'then encrypting each residue modulo 26 as a separate plaintext character' implies. Would this mean that each character is encrypted as