Network Security Chapter 8

Confidentiality, Authentication, Message Integrity, and Access and Availability

To ensure confidentiality, where only the sender and intended receiver can understand the message contents

Alice is the sender, Bob is the receiver, and Trudy is an intruder who might intercept, delete, or add messages

Web browser/server for electronic transactions, on-line banking client/server, DNS servers, routers exchanging routing table updates, etc.

To confirm the identity of the sender and receiver to each other

Trudy is an intruder who might intercept, delete, or add messages during transmission

intercepting messages

taking over an ongoing connection by removing the sender or receiver and inserting oneself in place

symmetric key crypto uses a single key for encryption and decryption, while public-key crypto uses a public key for encryption and a secret key for decryption

to protect data from unauthorized access

public-key crypto allows for secure key exchange over an insecure channel

encryption is the process of converting plaintext into ciphertext, while decryption is the process of converting ciphertext back into plaintext

Each letter of the plaintext is converted to its number, then the value for the key is added, and the resulting number is converted back to a letter.

The key value is added to the plaintext number to get the ciphertext, and subtracted from the ciphertext to get the plaintext back.

The Caesar Cipher is a monoalphabetic rotation cipher, where each letter of the plaintext is rotated forward three times to encrypt.

In symmetric key cryptography, Bob and Alice share the same key, whereas in asymmetric key cryptography, they do not.

Each character can be represented using 8, 16, 24, or 32 bits, depending on the system.

DES is the Data Encryption Standard, a US encryption standard that uses a 56-bit symmetric key and 64-bit plaintext input.

The problem is how Bob and Alice agree on the key value without actually exchanging it.

You subtract the key value from the ciphertext to get the plaintext back.

Through a publicly accessible repository or directory

The private decryption key can be used with the encryption algorithm to encrypt messages

Authentication

Monoalphabetic cipher

Plaintext is in lowercase letters, and ciphertext is in uppercase letters

To make attacking the ciphertext more difficult

Shift cipher

Brute force

The DES Challenge demonstrates that a 56-bit-key-encrypted phrase can be decrypted using brute force in 4 months, highlighting the limitations of DES encryption.

The DES operation involves an initial transformation, followed by 16 identical rounds of function application, each using different 48 bits of the key, and finally a final transformation.

Using three keys sequentially in 3-DES encryption makes it more secure by increasing the difficulty of brute force decryption.

AES encryption processes data in 128-bit blocks, whereas DES processes data in 64-bit blocks, and AES uses 128, 192, or 256-bit keys, whereas DES uses 56-bit keys.

In block cipher chaining, each input block is XORed with the previous block of cipher text, producing a unique cipher text for each block.

The initial cipher block is transmitted in clear to the receiver, allowing them to decrypt the message.

If cipher block chaining were not used, the same input block 'HTTP/1.1' would produce the same cipher text, making it vulnerable to attacks.

The 16 rounds of function application in DES encryption ensure that each input bit affects all output bits, making it more secure.