The plaintext is 387.
Recall that encryption involved raising a message to the power of e mod n
and decryption involved raising a ciphertext to the power of d mod n.
And so to decrypt to ciphertext 903,
we just take it to the d power mod n and get 387,
which we can see in Python.
And, if we want to check our answer,
we can encrypt 387 using a public key
and we get 903--our ciphertext.
