337: cryptography & hashing

week 3

cryptography important point

“cryptography fails at the system + human layer, not just algorithm level”

crypto quote

“cryptographic security is determined by the weakest component in the system — not just the strength of the algorithm alone”

what is cryptography

the art of secret writing

cryptography

plaintext —-encryption—→ ciphertext —-decryption—→ plaintext

plaintext

the original message (msg/data which are in their normal, readable form)

encryption

encoding the contents of the message in such a way that hides its contents from outsiders

ciphertext

the encoded/encrypted message

decryption

the process of retrieving the plaintext back from the ciphertext

key

sequence of characters or bits used by an algorithm to encrypt or decrypt a message (key = secret value)

is it okay for the algorithm to be public?

yes, algorithm without the key doesn’t help unmangle the msg

should the crypto algorithms used for military secrets be public?

no

ceaser cipher

given a key, move each letter [key] places to the right to encrypt and [key] places to the left to decrypt

break ceaser cipher (long method)

try all possible keys

public key AKA

asymmetric cryptography

private key AKA

symmetric cryptography

public key: how many keys?

one public key and one private key

private key: how many keys?

one secret key

asymmetric crypto public key role

use reciever’s key to encrypt

asymmetric crypto private key role

use receivers keys to decrypt

which is slower to compute?

asymmetric crypto

symmetric cryptography private key role

use same key to encrypt and decrypt (both sender and receiver know what the key is)

key distribution problem

challenge of securely sharing secret keys between parties in symmetric cryptography without an eavesdropper intercepting them

stream cipher (symmetric crypto)

encrypt bit-by-bit for smooth, fast encryption good for data in-transit

how does stream cipher work?

cipher generates a keystream (random letters) and adds it to the plaintext

block cipher (symmetric crypto)

plaintext is sliced into 128 bit chunks and encrypts those chunks one at a time

hybrid cryptography

public key used to establish temporary shared key. symmetric (so secret key) used for remainder

are hash functions random

no, they are always the same when given the same input

are hash inputs fixed size?

yes, they are always one fixed length (depending on which hashing algorithm you use)

hash input term

message

hash output term

message digests

hashes are one-way functions

meaning you cannot decrypt like encryption algorithms

hashes return _ if the message is even number of digits/letters

0

hashes return _ if the message is odd number of digits/letters

1

avalanche effect

even the smallest change causes a huge change in the output (ex. T vs t)

cryptographic hash

designed for data integrity and security, resisting reversal and collision attacks. so more security reasons

hashing security goal

integrity

encryption security goal

confidentiality

digital signature

guarantees that the contents of a message have not been altered in transit

which security goal is digital signature

non-repudiation

what kind of encryption do digital signatures use

asymmetric cryptography

how: recipient uses their private key to view, sign, etc so obvi they can’t deny

collision attack

occurs when an attacker finds two different messages that hash to the same value

how to conduct collision attack

very difficult and requires generating a separate algorithm

what does hashing algorithm MD stand for

message digest

how does MD work

use a secure method to compress the file and generate a computed output of a specified number of bits

MD2

takes a data of any length and produces a hash output of 128 bits

MD4

was subject to collision attack but was fast. not is use anymore and if is shouldn’t be

MD5

creates a 128-bit hash of a message of any length and segments the message into 512-bit blocks (similar to MD4 but slightly slower and more secure)

what does hashing algorithm SHA stand for

secure hash algorithm

SHA-3

latest and most secure

block method

splitting your input into chunks and repeatedly mixing each chunk into a running value with heavy scrambling math until one fixed-size final hash comes out