Question

Explain why it is impractical to use an ideal block cipher for real-world encryption.

Answer 1

Explanation :

When cryptographers create algorithms, they usually provide some argument that the algorithm is secure. They need to start the argument with some set of assumptions. For example, the in public-key cryptography, they may begin with the assumption that factoring large numbers is hard.

Many algorithms use use a block cipher as a building block. The arguments that these algorithms are secure need to make some (mathematical) assumption about the block cipher in question in order to get started. Normally, the assumption goes something like this: "If the encryption key is chosen at random, then an attacker who does not know the key cannot distinguish between the block cipher and a truly random permutation, even using a chosen-plaintext attack." (This is an informal statement of the pseudo-random permutation [PRP] assumption.)

But the PRP assumption isn't always applicable. Sometimes block ciphers are used in ways where the key is either not random or not secret (we'll see an example in a bit). In these cases, we need to make some other assumption about the block cipher's security properties.

Ideal cipher model :

This is where the ideal cipher model comes into play. In the ideal cipher model, we just pretend the block cipher is a random permutation for every key. Furthermore, we treat these permutations as independent. We assume that if an attacker wants to know what happens when a block is encrypted under a given key, he has to go to the work of computing it himself. He can't infer anything about the output by encrypting other blocks, or the same block under a different key.