This is XOR encryption, with the key
The following python script will decrypt the cyphertext:
plaintexthex=hex(int(cyphertextb16, 16) ^ int(keyb16, 16))
This produces the decrypted plaintext, which is:
ET Phone Home!!!!!!!!!!!!!!
In response to the question in the comment below about how I went about reverse-engineering this:
First, I did this to show that questions like this are pointless, because you can use a cipher like XOR to make any ciphertext decrypt to any plaintext you want, using a carefully chosen key.
As to how I chose the key: If you have A xor B = C then, the following is also true:
A xor C = B
C xor B = A
So, I made the given ciphertext A, and I made my wanted plaintext ('ET Phone Home!!!!!!!!!!!!!!') B, and used the first equation above to find the key, C.
This can be done using the following python script:
plaintext='ET Phone Home!!!!!!!!!!!!!!'
key=hex( int(plaintextb16, 16) ^ int(cyphertextb16, 16) )
Sure enough, 'decrypting' the given cyphertext by XORing it with the key above produces the wanted plaintext, as shown in the first script above.