Mumbai university > Electronics and telecommunication Engineering > Sem 7 > Data compression and Encryption

Marks: 4

Years: May 2016

Decrypted message is same as plain text because:

1. Key distribution problem:

• Private Key systems suffer from the key distribution problem. In order for a secure communication to occur, the key must first be securely sent to the other party. An unsecure channel such as a data network cannot be used.

• Courier or other secure means are typically used. Public key systems do not suffer from this problem because of their use of two different layers. Messages are encrypted with a public key and decrypted with a private key. No keys need to be distributed for a secure communication to occur.

1. Public key cryptosystems:

• A user wishing to exchange encrypted messages using a public key cryptosystem would place their public encryption procedure E in a public file. The user’s corresponding decryption procedure D is kept confidential.

• Rivest, Shamir and Adleman provide four properties that the encryption and decryption procedures have three important steps:

a. Deciphering the enciphered form of a message M yield M. That is D (E(M)) = M.

b. E and D are easy to compute.

c. Publicly revealing E does not reveal an easy way to compute D. As such only the user can decrypt messages which were encrypted with E. Likewise, only the user can compute D efficiently.

d. Deciphering a message M and then enciphering it results in M. That is E (M) = C.

• As Rivest, Shamir and Adleman point out, if a procedure satisfying property c. is used, it is extremely impractical for another user to try to decipher the message by trying all possible messages until they find one such that E (M) = C.

• A function satisfying properties a – c is called a trap-door one way function. It is called one way because it is easy to compute in one direction but not the other. It is called trap door because the inverse functions are easy to compute once private trap door information is known.

1. The public key cryptosystem encryption and decryption process:

Suppose user A wants to send a private message M to user B

• User A gets User B’s public key from some public source.

• User A encrypts message M using B’s public key. This produces a cipher text message C.

• Cipher text message C is sent over some communication channel.

Upon receipt, user B decrypts message C using their private key. This results in the original message M.