commit 121923f6e4caac1a943d096d64f0e2b6ce4afdf6
parent add351fc55ee77cbaa21eb4cdc500710113fa6cc
Author: Alex Balgavy <alex@balgavy.eu>
Date: Fri, 16 Oct 2020 17:57:03 +0200
Updated CNS notes
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diff --git a/content/computer-network-security/Lecture 15_ Cryptography.md b/content/computer-network-security/Lecture 15_ Cryptography.md
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+title = 'Lecture 15: Cryptography'
+template = 'page-math.html'
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+
+# Lecture 15: Cryptography
+
+Allows secure comms between parties when an attacker can intercept/modify their messages.
+Goals:
+- confidentiality: message between parties can't be read
+ - in apps, ideal is end-to-end encryption (only sender and recipient have key)
+- integrity: message between parties can't be changed without them finding out
+ - in apps, digital signatures (not valid if message or sender changes)
+- authentication: attacker can't impersonate a party
+- non-repudiation: a party can't deny that they sent a message
+ - use asymmetric digital signatures
+
+Terms:
+- plaintext: readable text to be transmitted
+- ciphertext: unreadable text actually sent
+- encryption: converting plaintext into ciphertext
+- decryption: recovering plaintext from ciphertext
+
+Kerckhoffs' Principle: when encrypting, separate algorithm from key. Assume attacker knows algorithm, keep key secret
+
+Caesar cipher: shift letters in alphabet by a fixed amount
+- attacks: bruteforce, frequency analysis (some letters more common in natural languages)
+
+Cryptographic hashes:
+- hash function maps message (arbitrary length) to hash (fixed length), should be computationally hard to reverse
+- often used to identify message, because it's hard to find message with same hash
+- properties:
+ - pre-image resistance: given hash, hard to find message that hashes to it
+ - second pre-image resistance: given message, it's hard to find another message with the same hash
+ - collision resistance: hard to find any two messages that have the same hash
+- well-known types: old and insecure (MD5, SHA-1), SHA-2, SHA-3
+
+Random numbers:
+- CPUs are deterministic so can't generate truly random numbers
+- Pseudo-random number generator (PRNG) computes number from seed, which is updated for every generated number
+
+Blockchain:
+- Bitcoin: decentralised digital currency using blockchain
+- each block of transactions contains hash of previous block, so can't change past transactions without changing later ones
+- real chain is what more than 50% of users agree on
+
+Perfect encryption:
+- never reuse the key
+- key is perfectly random
+- no information is leaked from ciphertext
+
+## Symmetric cryptography
+Uses single key to encrypt and decrypt.
+Properties:
+- provides confidentiality
+- signature schemes provide integrity and authentication
+
+One-time pad:
+- assume we have message and key both with n bits
+- encrypt: ciphertext = msg ⨁ key
+- decrypt: msg = ciphertext ⨁ key
+- holds because ciphertext ⨁ key = msg ⨁ key ⨁ key = msg ⨁ 0
+- provides perfect encryption because doesn't leak any information (0 bit could be 0⨁0 or 1⨁1 with equal probability)
+- breaks if key is reused
+
+Stream ciphers:
+- use PRNG to generate key for one-time pad
+- initial seed becomes real key
+
+Block ciphers:
+- divide data in blocks
+- perform computation using key to map plain block into cipher block
+
+Cipher block chaining:
+- make each block depend on the previous one
+- for first block, use initialization vector instead (doesn't have to be secret, shouldn't be reused)
+
+Padding:
+- if message sizes not multiple of block size, must be padded
+- don't add zeros, because impossible to recover original if it ends with zeros
+- instead, pad with bytes containing padding length
+
+Signatures:
+- message authentication code is signature for message
+- combine key with message and apply hash function
+- key needed to generate and to validate code
+- provides integrity and authentication, but not non-repudiation (any verifier can sign)
+
+## Asymmetric cryptography
+Uses two keys: anyone encrypts with public key, only owner decrypts with private key
+
+RSA:
+- sender generates public and private key together
+- public key available to anyone, private key kept secret
+- mathematically not viable to derive d from e
+- to generate key, generate large number _m_, public key _e_, private key _d_ such that: $(n^{e})^{d} = n \enspace (\text{mod $m$})$ for any _n_
+- relies on difficulty of writing _m_ as product of prime number factors
+- special random padding
+- only works for data that fits inside key size
+- signatures:
+ - to sign, compute hash of message and encrypt hash with private key
+ - to verify, compute hash of message and decrypt with signer's public key, then compare with encrypted hash received from signer
+
+## Key management
+Symmetric: key distribution center:
+- one trusted party which shares symmetric keys with all others
+- Kerberos is a system for key communication, but need constant access to center and is single point of failure
+- possible to establish key without sending any revealing info (e.g. Diffie-Hellman Key Exchange)
+
+Asymmetric:
+- Certification Authority (CA)
+ - issues certificates, keeps revocation list of certificates that may be compromised
+ - X5.09 certificates containing e.g. identity of holder, public key of holder, expiration date, signature from CA
+ - verifies identities of users
+ - typically company that must be trusted
+- Self-signed certificate:
+ - signed with own private key
+ - can be made by anyone so not trusted by default
+
+## SSL and HTTPS
+SSL (Secure Sockets Layer) ensures crypto protection for network connection on top of TCP (aka TLS)
+Goals:
+- end-to-end encryption, Diffie-Hellman to create shared key
+- authenticates using X5.09 certificate (prevents MITM attack)
+- typically doesn't authenticate client
+
+## Password storage
+Hash password when storing.
+
+Bruteforce solutions:
+1. Use salted hash:
+ - when storing password, generate random string and concatenate with password before hashing
+ - store salt with hash.
+2. Use slow hash: e.g. apply hash 1000 times
+
diff --git a/content/computer-network-security/_index.md b/content/computer-network-security/_index.md
@@ -18,3 +18,5 @@ title = "Computer and Network Security"
- [Lecture 12: advanced exploitation](lecture-12-advanced-exploitation)
- [Lecture 13: advanced exploitation 2](lecture-13-advanced-exploitation-2)
- [Lecture 14: Intrusion Detection Systems (IDS)](lecture-14-intrusion-detection-systems-ids)
+- [Lecture 15: Cryptography](lecture-15-cryptography)
+
