vault backup: 2022-10-20 12:35:02

content/notes/24-network-security.md

@@ -29,11 +29,61 @@ what can trudy to
 
 terminology
 - m: plaintext message
-- 
+- $K_{A}(m)$: ciphertext, encrypted with key $K_{A}$
+- m = $K_{B}(K_{A}(m))$
+- ![](https://i.imgur.com/6veueus.png)
+- 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
+- ![](https://i.imgur.com/DrH8hmU.png)
+
+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
+	- $c = m^e\mod n$ 
 
-# Public key
 
 # Authentication of devices