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Well, whether it is a secure tweakable block cipher depends on how resistant (E,D) are to related key attacks; that's not a standard assumption for block ciphers. For example, this would not be a secure tweakable block cipher with 3DES; because every 8th bit is ignored, the attacker can effectively test the value of the 7 adjacent bits (except for the 7 ...

A block cipher is a family of permutations where the key selects a particular permutation from that family. With a tweakable bockcipher both key and tweak are used to select a permuation. So tweak and key are pretty similar. The main difference are the security and performance requirements for a tweak: Changing a key can be expensive, changing a tweak ...

To expand / generalize @poncho's reply, given a block cipher $(E,D)$ with keylength $n$, you can make a new one $(E',D')$ with key length $n+1$, which ignores the last bit of the key and just runs $(E,D)$. If $(E,D)$ is a secure PRP, then so is $(E',D')$. But plugging $(E',D')$ into the OP's construction does not yield a secure tweakable block cipher. To see ...