Consider the following challenge-response protocol for A to convince B that lie

Question

Consider the following challenge-response protocol for A to convince B that lie is indeed A. Here we are assuming that once a virtual circuit has been set up, the BG cannot alter messages in the middle. So the idea is that at the start when the virtual circuit is being net up, A has to convince B that he is indeed A. A and B share a secret value S. B sends A a nonce N. A takes the first 64 bits of N and treats this as a key A' for DES, and returns C = E_K (S) back to B. B checks whether D_K (C) = S and if it is, accepts that A is indeed A (because he knows the secret S). i. Suppose this protocol was Ix-inR used only once i.e. A has to convince B of who he is only one time. Once A has done this, this protocol is never going to be used again. Is this a Rood scheme? A. Give a YES/NO answer. B. If you said NO, explain your answer i.e. explain what you think is the single biggest weakness of the scheme. ii. Now suppose this protocol was being repeated many times i.e. A has to convince B of who he is repeatedly, and each time follows the above protocol, with B generating a different nonce each time. Is tills a good scheme? A. Give a YES/NO answer. B. If you said NO. explain your answer i.e. explain what you think is the single biggest weakness of the scheme.

Explanation / Answer

According to the assignment secret has been shared using the DES encryption. The A and B share same private key has K which are private between them (DES works by using the same key to encrypt and decrypt a message, so both the sender and the receiver must know and use the same private key.)

No, using an algorithm that too with the weak key of 56bits is not safe because DES algorithm is a combination of substitution and transposition.

The two techniques are repeatedly applied through 16 cycles or rounds. Plaintext is encrypted in 64-bit blocks.

The key is 64-bits, but 8 bits are used as check digits and don’t affect the encryption.

So the data is encrypted in 64-bit blocks using a 56-bit key.

The 56-bit key is cause for worry because trying 256 combinations is not as formidable a task as it once was used. DES uses 16 48-bits keys generated from a master 56-bit key

The weak keys are

01010101 01010101

– FEFEFEFE FEFEFEFE

– E0E0E0E0 F1F1F1F1

– 1F1F1F1F 0E0E0E0E

Yes the use of the same algorithm multiples times helps us to overcome the problem.

To address the weakness it has been applying the algorithm multiple times.

Double encryption uses two keys and works as follows:

Suppose we have any block cipher with a keysize of k and also that you have known plaintext/ciphertext pairs (P, C) and you are using the encryption.

E(k2, E(k1,m))

where D is the corresponding decryption algorithm.

First compute and store in a hash table E(ki , P) = Mi for all 2k keys ki . Then successively compute the decryption D(kj , C) = Mj for each key kj . As soon as you find Mj = Mi , you have a potential keypair (k2, k1) = (i , j).

Using two keys and three encryptions does strengthen the algorithm. The procedure is:

C = E(k1,D(k2, E(k1,m))).

This effectively doubles the key length to a 112-bit equivalent key, at the cost of more complexity in key generation and management.

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