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118 lines
3.7 KiB
Markdown
118 lines
3.7 KiB
Markdown
---
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title: "24-network-security"
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aliases:
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tags:
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- cosc203
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- lecture
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---
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# what is it?
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- confidentiality
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- only sender and reciver should understand message contents
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- encryption
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- authentication
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- sender and reciever want to confirm identity of each other
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- message integrity
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- sender and reciever want to ensure message not altered (in transit, or afterwards) without detection
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- access and availablility
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- services must be accessible and availble to users
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sender and recieves:
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- any king of onnline communication
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what can trudy to
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- eavesdrop
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- insert messages into connection
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- impersonation: fake (spoof) source address (or any field)
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- hijacjking "take over" ongoing connection by removing sender or ereciever, inserting himself in place
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- denial of service: prevent others from using a service (e.g., by overloading it)
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terminology
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- m: plaintext message
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- $K_{A}(m)$: ciphertext, encrypted with key $K_{A}$
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- m = $K_{B}(K_{A}(m))$
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- 
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- key: secret data used to encrypt and decrypt messages
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# Symmetric key crypto
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bob and alice share the same key K
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- e.g., key is knowing a substitution pattern in mono alphabetic substitution cipher
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- substiution cipher
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- map each letter to a different letter
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- key is a mapping from a set of 26 letters to another set of 26 letters
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- not secure: easy to decrypt using patterns etc
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DES: data encryption standard
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- data is split into blocks of 64 bits
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- each block encrypted using 56-bit key
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- blocks are chained together
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- encryption of current block is based on the previous block
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- 56-bit symmetric key, 64 kit plaintext input
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- not very secure: short key- only 56 bits - less than a day to brute force
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- no known good analytic attack
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- 3DES: encrypt 3 times with 3 different keys: more secure
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AES: advanced encryption standard
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- larger key 128, 192 or 256
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- 128-bit blocks
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- brute force taking 1 sec on DES takes 149 trillion years for AES
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# Public key crypto
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symmetric requires sharing of key
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process
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- sender and reciever do not share secret key
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- public key known to all
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- pricate key known ony to reciever
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- use public key to encrypt
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- use private key to decrypt
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public key reqs
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- 
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RSA
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- popular public key encruption algorithm
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- how to generate keys
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- choose two large prime numbers (1024 bits each)
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- compute $n=pq, z=(p-1)(q-1)$
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- choose e (with e<n) that has no common factors with z (e, z, are "relatively prime")
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- choose d such that ed-1 is exactly divisible by z. (i.e., ed mod z = 1)
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- public key is (n, e) private key in (n, d)
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- encrypt message m (<n)
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- $c = m^e\mod n$
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- decrypt recieved c
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- $m = c^d\mod n$
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- magic
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- $m = (m^e \mod n)^d \mod n$
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- where $c = m^e\mod n$
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# Authentication of devices
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- AP2.0 athenticate based on IP: bad because can spoof IP
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- AP3.0 put secret password into each packet: bad because trudy can get a message and see the password
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- replay attack
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- can find password without decrypting
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- AP4.0 avoid replay attack:
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- nonce: number R used only once-in-a-lifetime
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- prove alice "live", Bob sends alice nonce, R
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- alice must return R, encrypted with shared key
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- 
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- problems
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- must choose a key in a secure way before they communicate
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- AP5.0: use nonce but with public key techniques
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- problem: man in the middle attack: trudy poses as alice to bob, and as bob to alice
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- 
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- need a better way to get public key
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- Digital signatures
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- analogous to hand-signatures
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- sender digitally signs a document
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- verifiable, nonforgeable
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- simple digital signature
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- bob signs with his private key: creating signed message
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-
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# digital signature
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