1 Public Key Encryption A Simple Case Uses

1. Public Key Encryption (A Simple Case) Uses two mathematically related digital keys n 1. Public key (widely distributed) 2. Private key (kept secret by owner) n Both keys used to encrypt and decrypt message n Once key used to encrypt message, same key cannot be used to decrypt message n Sender uses recipient’s public key to encrypt message; recipient uses his/her private key to decrypt it Copyright © 2010 Pearson Education, Inc. Slide 5 -1

Public Key Cryptography—A Simple Case Figure 5. 8, Page 290 Copyright © 2010 Pearson

Asymmetric/Public key Encryption n n In public key encryption mathematically algorithms produce the keys through one – way function. In one – way function an input can’t be derived from output. Example: it is easy to make scrambled eggs, but impossible to retrieve whole eggs from the scrambled eggs. Keys used in Asymmetric key encryption are 128 bit, 256 bit, 512 bit. Algorithm used in Asymmetric key encryption are RSA(Ron Rives, Adi Shamir and Len Adleman ) etc. Copyright © 2010 Pearson Education, Inc.

The RSA Algorithm – Key Generation (Just Concept) 1. 2. 3. 4. 5. 6. 7. Select p, q Calculate n = p x q Calculate Select integer e Calculate d Public Key Private key Copyright © 2010 Pearson Education, Inc. p and q both prime KU = {e, n} KR = {d, n}

The RSA Algorithm – Encryption/Decryption Just Concept n Plaintext: M<n n Ciphertext: C =

Example of RSA Algorithm (Just Concept) Copyright © 2010 Pearson Education, Inc.

Problem: public key encryption (a simple case) n n n In public key encryption some elements of security are missing. Although we can be quite sure the message was not understood or read by a third party (Message Confidentiality) But No guarantee the sender really is the sender (no authentication of message) v This mean the sender could deny ever sending the message (repudiation) And there is no assurance the message was not altered some how in transit. Example: message change to Buy Cisco@Rs. 25 from sell Cisco @Rs. 25. This suggests a potential lack of integrity in the system. Copyright © 2010 Pearson Education, Inc.