P-1: (a) Make a scheme in which two carriers are used, such that if only one cou

Question

P-1: (a) Make a scheme in which two carriers are used, such that if only one courier is intercepted, one can still guarantee perfect privacy. Use the One-time Pad.

       (b) Generalize the previous scheme (a) using n couriers, such that: if the enemy intercepts at maximum n-1 couriers, the scheme still guarantees perfect privacy.

Note: This is not a Schema (not a problem from database). This is a problem from Applied Cryptography course. To solve this problem, check the One-time pad how does it work.

Explanation / Answer

Explanation:

The main goal of cryptography is to maintain the privacy. Privacy can be maintained perfectly if the key that is used for encryption and decryption is not gained easily even if the algorithm is known to the Man In the Middle(MIM).

One way to achieve this is to choose the key randomly from the key domain. The Additive Cipher could be a strong cipher if the key can be changed in every encryption or decryption process.

One Time Pad:

The above mentioned idea is used in case of One Time Pad cipher. In this cipher the key size is same as the size of the plain text. The key is chosen randomly each time the plaintext is encrypted. Therefore, One Time Pad cipher is a perfect cipher because even if the MIM gets to know the key by Brute force attack, the same key will not be used for the next time.

Even though One Time Pad is perfect cipher, it is not possible to implement commercially. The receiver needs to know the key each time it receives a ciphertext.

Question P-1(a):

If we use One Time Pad cipher then even if the MIM gets one courier by intercepting the line and gets the key, the next courier can’t be decrypted by the same key as for One Time Pad cipher the key is chosen randomly each time. Therefore if one courier is intercepted, it is not the same for the next courier also.

Question P-1(b):

As a generalized case, id we are using n couriers and among them n – 1 is intercepted and the key is revealed to the MIM, nevertheless the nth courier poses security guarantee as to get the plaintext corresponding to the cipher text the MIM needs to get the key the same way he gets it for the previous n – 1 couriers which is not efficient.

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