Question

I was wondering whether RSA can be cracked given:

• The public key
• A plaintext of a known cipher text.
  In other words the decrypted message from an encrypted one (but without knowing the private key)
  I.E.: you have managed to find the plaintext of an already encrypted message using the public key, however, you DO NOT know the private key.

Answer 1

No. That’s called a known-plaintext attack, and there is no such attack known against RSA.