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Cipher Systems - E-Commerce - Lecture Slides, Slides of Fundamentals of E-Commerce

Aligarh Muslim UniversityFundamentals of E-Commerce

Students of Communication, study E-Commerce as an auxiliary subject. these are the key points discussed in these Lecture Slides of E-Commerce : Cipher Systems, Terminology, Plaintext, Ciphertext, Digits, Encrypted Data, Unencrypted Data, Intended Receivers, Cryptosystem, Encrypting Messages

Typology: Slides

2012/2013

Uploaded on 07/29/2013

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Terminology
• Unencrypted data is called plaintext.
• Encrypted data is called ciphertext.
• A key is a string of digits that acts as a password.
• Only the intended receivers should have the key
that transforms the ciphertext into plaintext.
• A cipher or cryptosystem is a technique or
algorithm for encrypting messages.
• Cryptographic ciphers have a long history.
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Terminology

  • Unencrypted data is called

plaintext

  • Encrypted data is called

ciphertext

• A

key

is a string of digits that acts as a password.

  • Only the intended receivers should have the key

that transforms the ciphertext into plaintext.

  • A

cipher

or

cryptosystem

is a technique or

algorithm for encrypting messages.

  • Cryptographic ciphers have a long history.

Early cipher systems

  • Ciphers were used as far back as the ancient

Egyptians. Text was encrypted by hand.

  • The two main types of ciphers were used:

–^

Substitution cipher

: Every occurrence of a

given letter is replaced by a different one.Example: “a” by “b”, “b” by “c”, etc.“Uftujoh, uftujoh”

-^

Transposition cipher

: The ordering of the

letters is shifted to form new words.Example: Plaintext = exampleCiphertext = eape xml

Knowledge needed

  • Someone can know the details of an encryption

algorithm and yet not be able to decipher anencrypted message without the key.

  • The resistance of the encrypted message depends

on the size, in terms of bits, of the key used inthe encryption procedure. The longer the key,the more computing power and time it takes tobreak the code.Example: 128-bit encryption systems.

Types of cryptosystems There are two main types of cryptosystems:• Private-key cryptography

Also known as

symmetric

or

secret-key

encryption, it uses a single key to bothencrypt and decipher the message.

-^ Public-key cryptography

Also known as

asymmetric

encryption, it uses

a public key to encrypt messages and a privatekey to decipher messages.

Key distribution center

  • A key distribution center shares a different key

with each user in the network.

  • When Alice and Bob want to communicate, they

obtain a session key from the KDC.

  • They communicate using the session key.• If Chris wants to communicate with Alice, they

obtain a new session key, improving security.

  • If the KDC is compromised, the security of the

entire network is at risk.

DES

  • Data Encryption Standard (DES) is a 56-bit

private-key encryption algorithm developedby the NSA and IBM in the 1950s.

  • Cryptoanalysts no longer believe that 56-bit

keys are secure. See

Cracking the 56-bit DES

system

  • The current standard is to use Triple DES, three

DES systems in a row, each with its own key.

  • Work is underway on the Advanced Encryption

Standard (AES).

Authentication

  • If a customer sends a message to a merchant using

the merchant’s public key, the customer knowsthat only the merchant can decipher the message.

  • Similarly if the customer sends a message using the

customer’s private key, the merchant can decipherit using the customer’s public key thus identifyingthe customer.

  • Both together give two way authentication.

Example: Merchant to customer– First encode using the customer’s public key.– Use the merchant’s private key on the result.