Figuring out RSA Encryption from 1 encrypted and decrypted message

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Suppose that you have an encrypted message and a decrypted message (just one). M (the public key) and k (the exponent you raise each number to) are public. Does having one copy both version of a message give you enough information to decrypt other messages? I think the question boils down to finding the value of the euler phi of M which is not easy if you don't know its factorisation. I don't think there is a way to decrypt other messages but I'm not sure and am thankful for any help in advance.

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Take any message you like. Encrypt it. You now have in your possession both an encrypted message, and the decrypted message. Does that help you to decrypt other messages?