CSEC 701 Cryptography Review

one time pad

perfect cipher

block cipher

plaintext divided into blocks of a preset size, using subs and transp

short key problem

use double DES using two keys and encrypt twice

AES

block cipher 128 bits, key size 128, 192, and 256

mode

a technique for applying the cipher by mixing its outputs to hide ciphertext patterns (ECB and CBC)

ECB

each block encrypted independently, identical blocks mean same ciphertext

CBC

message blocks linked otgethor, uses IV to start process

CTR

no chaining

RC4

RSA security, stream cipher psuedo one time pad

hash function

a transformation of a message of arbitrary length into a fixed length value, produces a unique value that corresponds to the data entered

good hash function

changes the hash output radically for a tiny chnage in input

hash functions

NOT encryption, used for error detection or checking integrity of a file, file comparison, and membership query

hash collision

duplicate hashes form same input, h =f (M), about 50% chnace to find collisions

birthday attack

finding at least two people having the same birthday, chance of collision

level of safety

number of trials have to try before finding a collision, possible outcomes 2^n (=H), ex: H=2^160 : 2^80

why is collision bad

can allow attackers to create twin documents and effectively alter the original document 

dangers of hash functions

they are non-cryptographic,

one way hash

easy to generate code but computationally infeasible to find data mapping to speciifc hash vaue (impractical to reverse), collision free

weak collision

not broken, given M, find M’ that has the same hash value

strong collision

broken, find a apir fo M that have the same hash value

MD5

one way hash function, 128 bit message, broken

SHA

one way hash function, 160bit message digest

message authentication

protect integrity of message, validate identity of originator, non-repudiation of origin, with or without message encryption

with message encyrption

where confidentiality is not needed, slow encryption

MAC

small block appended to the message, problem with altering, must use encryption to generate a MAC, encryption is slow, must encrypt only hash 

HMAC

faster that block cipher, need to add a key

entropy

level or amount of randomness

asymmetric key

use public and private key

public key algorithms

DH, RSA, and EC

RSA

public key scheme, security due to cost of factorng large numbers

magin in the finite field

based on an exponentiation in a finite field over integers modulo a prime

RSA algorithm calculate

select p and q values (prime, secret), calculate n=p*q (public), calculate (n)=(p-1)(q-1) (secret), select e, determine d such that d*e=1 mod (n)

DH key exchnage

based on difficulty of computing discrete logarithms, NOT encryption algorithm,

discrete logarithm 

find the smallest k for ex: 3^k =5 mod 7

ECDH

uses additive groups of points on an elliptuc curve, group operation is denoted by +

elliptic curve

y² = x³ + ax + b

ECC

harder to solve than DH, requirs less memory, less power consumption 

symmetric encryption

faster, better for bulk transfers, key management problem

asymmetric encryption

solves key distrubution issue, has computational complexity

session key

a symmetric key used for encrypting messages during a communication session, two step process 

digital signature

cryptographic implementation of electronic signature, not digitized signature, helps prove, authenticate, and verifies by 3rd party, senders private key is used

features of digital signature 

authenticity, unforgeability property, nonresuability, repudiation 

DSS

using SHA, contains the RSA method and ECDSA method,

digital certificate

public key combined with owners name, binds individuals identitty to a public key

public key challenges 

hwo to tell if public key is fake

3rd party endoresement

use public key, notary(CA) encrypts certificate with his private key, 3rd aprtys private key is used

digital certificate format

X.509, widely used standard

PKI

use of digital certificates and the endoresement process

certificate repository 

a centralized directory that can be accessed 

certificate based threats

forging false certificate, false sense of security, stolen certificates

COMODO incident

fraudulent certificates issued fpr 8 domains

MITM attack with hash collision 

an MITM attacker presents a fake certificate to the victim 

what encryption key used most common in RSA

65537 or 2^16 +1