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Students of Communication, study E-Commerce as an auxiliary subject. these are the key points discussed in these Lecture Slides of E-Commerce : Cryptographic Terminology, Message, Before Encoding, After Encoding, Cipher Text, Plain Text, Information, Convert, Decryption Algorithm, Decode Message
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Electronic Commerce (WS-02/03) 4-
Plain text: the message before encoding. Cipher text: the message after encoding. Key: information needed to convert from plain text to cipher text (or vice-versa). Function: the encryption or decryption algorithm used, in conjunction with key, to encode or decode message. Key distribution service: trusted service which hands out keys.
Electronic Commerce (WS-02/03) 4-
Encrypting data prevents unauthorised access and modification to the data (i.e. prevents eavesdropping and tampering). If encrypted data can only be decrypted with a matching key, this can be used to prove sender’s identity (i.e prevents masquerading). Likewise, it can be used to ensure that only intended recipients can use the data. Two main ways: secret key & public key.
Electronic Commerce (WS-02/03) 4-
Sender and recipient exchange keys through some secure, trusted, non-network based means Sender encodes message using function and sends, knowing that only the holder of key (the intended recipient) can use it Recipient decodes message and knows that only sender could have generated it Message can be captured but is of no use
Electronic Commerce (WS-02/03) 4-
AB Given Message M‘ = {M} KAB For all k oFor all M
Electronic Commerce (WS-02/03) 4-
Recipient generates key pair. Public key is published by trusted service. Sender gets public key, and uses this to encode message. Receiver decodes message. Replies can be encoded using sender’s public key from the trusted distribution service. Message can be captured but is of no use.
Electronic Commerce (WS-02/03) 4-
n n+3 n+2 n+1 XOR E(K, M) n-3 n-2 n- plaintext blocks ciphertext blocks Initialization vector required (e.g., timestamp)
Electronic Commerce (WS-02/03) 4-
Main Ideas o Confusion (XOR, circular shifing, ...) o Diffusion (transposition of plaintext block portions) Cyptographic Algorithms o DES (Data Encryption Standard, 1981, 56bit)
