I'm looking to accomplish a Enc-then-Mac approach using AES-CTR and ECBC-MAC
Now ECBC-MAC requires 2 distinct keys, one for the MAC, and one to encrypt the Tag.
If I do this: Use AES256-CTR for the encryption. Then split the AES256 key into 2, and use each 128 bit key to do AES-128-CBC-MAC and Encrypt the Tag using AES-128.
Is this semantically secure? Or do I definitely need 3 distinct keys?