Block ciphers
Block ciphers
operation modes
ECB
weak
codebook attack
attacks
codebook
small plaintext blocks
known plaintext
matching ciphertext
like...
gigantic lookup table!
AES
Rijndael
key scheduler
substitution boxes
random permutation
very non-linear
usually...
byte-sized
fixed-size lookup table
very fast!
key
selection
truly random?
entropy
<2^64 bits = breakable
English?
need > 32 chars
avalanche effect
1-bit input change
completely different output
strictly...
50% bits changed on average
Blowfish
key derives s-boxes
identical rounds
mathematically non-linear
can't simplify
involution
reversible
self-inverting
independent of function
XOR
y = Fk(a ^ b) ^ a
x = Fk(a ^ b) ^ b
z = Fk(a ^ b) ^ b ^ Fk(Fk(a ^ b) ^ a ^ Fk(a ^ b) ^ b) => z = Fk(a ^ b) ^ b ^ Fk(a ^ b) => z = b
function F
highly non-linear
very fast