lecturefour

encryption
    random number is really important
    one-time pad encryption:
        perfect secrecy
        XOR
    block ciphers: DES, AES same key reused for each block  
                    not impossible to break, very experience
                    AES: Rijndael algorithm
            encrypting a large message: 
                    ECB: encryt each one separately using the block cipher( if you know one of them, you can know another part)
                    CBC: split plaintext into block, XOR each block with the result previous( last cipherblock depends on entire plaintext, still not guarantee integrity)
                    CTR: random IV with the each split plaintext, not guarantee integrity, very fast, repeat-- one-time pad
                    encryption should be randomized or stateful
    known-plaintext attack:
    chosen-plaintext attack: 1.change the pin 2.eavesdrops, repeat for any pin value
    how to against that: 
    chosen-ciphertext attack:
    stream ciphers



!!condifentiality, integrity, authentication!!
encrypt+MAC
    MAC is deterministic: message are equal, verify MAC
    but attack: if msg is same, then MAC always the same, then attack
hash function 
one-way：hard to invert, SHA-1 is a common hash function
collision resistance: should be hard to find x!=x', that h(x) = h(x')

one-way not mean collision resistance
hashing is one-way
MD5 already attack successfully