> It then asks for a decryption b' of c'. If you can do that, why not just ask this "oracle" for a decryption b of c, i.e. to decrypt the original ciphertext?

> It then asks for a decryption b' of c'. If you can do that, why not just ask this "oracle" for a decryption b of c, i.e. to decrypt the original ciphertext? Either way you're basically asking for a copy of the pad since you can trivially derive that from any plaintext/ciphertext pair. The idea that anyone would just give you the decryption of an arbitrary message of your choosing using their supposedly secure one-time pad is a bit bizarre. You've already broken the security rules of the OTP well before you get to the point of calculating b from b'.