quartz/content/notes/24-network-security.md

title	aliases	tags
24-network-security		cosc203 lecture

what is it?

• confidentiality
  • only sender and reciver should understand message contents
  • encryption
• authentication
  • sender and reciever want to confirm identity of each other
• message integrity
  • sender and reciever want to ensure message not altered (in transit, or afterwards) without detection
• access and availablility
  • services must be accessible and availble to users

sender and recieves:

• any king of onnline communication

what can trudy to

• eavesdrop
• insert messages into connection
• impersonation: fake (spoof) source address (or any field)
• hijacjking "take over" ongoing connection by removing sender or ereciever, inserting himself in place
• denial of service: prevent others from using a service (e.g., by overloading it)

terminology

• m: plaintext message
• K_{A}(m): ciphertext, encrypted with key K_{A}
• m = K_{B}(K_{A}(m))
• 
• key: secret data used to encrypt and decrypt messages

Symmetric key crypto

bob and alice share the same key K

• e.g., key is knowing a substitution pattern in mono alphabetic substitution cipher
• substiution cipher
  • map each letter to a different letter
  • key is a mapping from a set of 26 letters to another set of 26 letters
  • not secure: easy to decrypt using patterns etc

DES: data encryption standard

• data is split into blocks of 64 bits
• each block encrypted using 56-bit key
• blocks are chained together
  • encryption of current block is based on the previous block
• 56-bit symmetric key, 64 kit plaintext input
• not very secure: short key- only 56 bits - less than a day to brute force
  • no known good analytic attack
  • 3DES: encrypt 3 times with 3 different keys: more secure

AES: advanced encryption standard

• larger key 128, 192 or 256
• 128-bit blocks
• brute force taking 1 sec on DES takes 149 trillion years for AES

Public key crypto

symmetric requires sharing of key

process

• sender and reciever do not share secret key

• public key known to all

• pricate key known ony to reciever

• use public key to encrypt

• use private key to decrypt

public key reqs

• 

RSA

• popular public key encruption algorithm
• how to generate keys
  • choose two large prime numbers (1024 bits each)
  • compute n=pq, z=(p-1)(q-1)
  • choose e (with e<n) that has no common factors with z (e, z, are "relatively prime")
  • choose d such that ed-1 is exactly divisible by z. (i.e., ed mod z = 1)
  • public key is (n, e) private key in (n, d)
• encrypt message m (<n)
  • c = m^e\mod n
• decrypt recieved c
  • c = m^e\mod n

Authentication of devices

digital signature