# Tag Info

## Hot answers tagged tweakable-cipher

8

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 ...

6

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 ...

2

There are a couple of related concepts here: Tweakable blockciphers and format-preserving encryption (FPE). It turns out that tweakable blockciphers provide a very natural way of obtaining FPE, but they have other uses as well. As the blog discusses, sometimes we want, say, encrypted credit card numbers to themselves look like credit card numbers. That is, ...

1

First let's very precisely look at a tweakless blockcipher to fully understand it: A regular blockcipher $E_k(x)$ with blocksize $n$ and key size $k$ is a permutation of the input block. What do I mean with that? Let's first tackle the word permutation here. Often a permutation means re-arranging elements within a set. So the set of all permutations of ...

1

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 ...

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