Goals of Compsec
Confidentiality |
Integrity |
Availability |
Types of Compsec Attacks
Interception:unauthorized party gets asset(theft, copying, surveillance) |
Interruption:asset unusable(DDos, deletion) |
Modification:asset changed(edit files,trapdoor,logic,virus) |
Fabrication:fake asset planted(computer,software,records) |
Basic Types of Crypto
Symmetric key:des,aes,blowfish,rc5,rc6 |
Asymmetric:rsa,el-gamal.elliptic curve(slower than sym) |
Secure hash:md5,sha1,sha256,ripemd |
Collision
Weak:difficult to find text with same hash as a random text |
Strong:difficult to find pairs of text with same hash |
Crypto Analysis
etaoinshrdlu |
bigrams,trigrams(the,and), |
index of coincidence(3.8% vs 6.6%) |
Types of Attacks on Crypto
ciphertext only, known plaintext, chosen plaintext, chosen ciphertext, dumpster diving, social engineering, threats/blackmail/torture/bribes |
Entropy
Entropy:info in message,(Ex, 3.6 bits for a month) |
Rate: R = log2 Z, where Z is the size of the alphabet |
Abs Rate:how much info, r=H(M)/N where M is an N-bit message. |
Redundancy: D = R – r! |
Unicity: amount of cipher needed to find plaintext U = H(K) / D! |
Block Ciphers
Diffusion, small changes cause large effects |
Confusion, statistics between key and cipher hidden |
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DES
Adopted by NIST 1976 (IBM Lucifer), NSA reduced key from 128 to 56 |
Feistel with additional initial permutation, 16 rounds, complex f, 48b subkeys |
32b >Expand and permute> 48b (Kn) > Substitute (using S boxes) > 32 bits > Permutation |
Bruteforce in <24 hours in '96, double DES still too small (2^57) |
3DES, slow, almost secure? nsa backdoors? |
AES
NIST '96, replace DES, secure 50-100y, faster des, variable key size, block ciph |
MARS,RC6,Rijndael,serpent,twofish(key dependent sbox) |
blowfish:64b block, fast, still secure, used ssh and openbsd |
rijndael doesnt use constant but theyre good pseudorandom, infinite, public no trapdoors |
Round: swap state using sbox, cyclic shift each state row, invertible trans each row, XOR state by round key |
Modes
ECB: all blocks encrypted independently, identical blocks encrypted identically! |
CBC: each block is used next block, more secure |
OBC: feedback independent of plaintext, can parallelize |
CTR:nonce+counter instead of feedback, very parallel, stream is safe |
XEX:efficient,fast,parallel, Cs,j = EK (PS,j ⊕ X) ⊕ X where X = EK (S) ⊗ αj |
Side Channel Cryptanalysis
Detect power use, time delay, radiation |
Diffie-Hellman
agree on q = large prime, a = random generator |
A gets random X sends a^X mod q to B |
B gets random Y sends a^Y mod q to A |
Each one calculates (aX)Y mod q = K |
RSA
E(M) = M mod n, D(C) = C mod n |
n = p × q p, q are prime |
d is relatively prime to (p - 1)(q - 1) |
e × d ≡ 1 mod ((p - 1)(q - 1)) |
hard to factor, getting easier, quantum comp is risk |
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