Suppose that Alice and Bob have access to two secure block ciphers, say, Cipher
ID: 3792112 • Letter: S
Question
Suppose that Alice and Bob have access to two secure block ciphers, say, Cipher A and Cipher B, where Cipher A uses a 64-bit key, while Cipher B uses a 128-bit key. Alice prefers Cipher A, while Bob wants the additional security provided by a 128-bit key, so he insists on Cipher B. As a compromise, Alice proposes that they use Cipher A, but they encrypt each message twice, using two independent 64-bit keys. Assume that no shortcut attack is available for either cipher. Is Alice's approach as secure as Bob's?Explanation / Answer
Yes, the approach by Alice is as secure as that of Bob beacuse while encrypting the given block twice using two different 64 bit keys, 2 decrypting algos needs to be apploed to decryptg that message. The more the number of rounds taken in encrypting the message the more the number of keys are required and hence making the data more secure. In this as well the data is encrypted teice with the two independent 64 bit keys. Therefore one can easily trust that data to be as secure as that encrypted using one 128 bit key.