How hard is to figure out the ENCRYPTION key given message and clear text?

Question

How feasible is to brute force the ENCRYPTION key if you have

• the clear-text message
• the encrypted-version of the same message
• you know that Rijandel is the encryption algorithm
• you know that the salt is 2 bytes long

Answer 1

The attack you describe is called a "known-plaintext attack". It was commonly used to break older encryption schemes, most notably Enigma in the second world war.

AES/Rijndael is not susceptible to known-plaintext attacks.

Further reading:

Wikipedia: Known-plaintext attack

Why is AES resistant to known-plaintext attacks?