# CSE4303 Introduction to Computer Security (Exam Review)

## Details

Time and location

– In class exam – Thursday, 3/5 at 11:30 AM
– What is allowed:
    - One 8.5” X 11” paper of notes, single-sided only, typed or hand-written

Topics covered:

– Security fundamentals
– TCP/IP network stack
– Crypto fundamentals
– Symmetric key cryptography
– Hash functions
– Asymmetric key cryptography

## Security fundamentals

### Defining security

- Understand principles of security analysis
  - The security of a system, application, or protocol is always relative to
    - A set of desired properties
    - An adversary with specific capabilities ("threat model")

### Key security concepts

C.I.A. triad:

- Integrity: Prevent unauthorized modification of data, and/or detect if modification occurred.
  - ARP poisoning (ARP spoofing)
  - Authentication codes
- Confidentiality: Prevent unauthorized parties from learning the contents of data (in transit or at rest).
  - Packet sniffing / eavesdropping
  - Data encryption
- Availability: Ensure systems and data are accessible to authorized users when needed.
  - Denial-of-Service (DoS) / Distributed DoS (DDoS)
  - Rate limiting + traffic filtering (often with DDoS protection/CDN)

Other security goals:

- Authenticity: identity of an entity (issuer of info/message) is verified
- Anonymity: identity of an entity remains unknown
- Non-repudiation: messages can't be denied or taken back (e.g. online transaction commitments)

### Modeling attacks

Common components:

- System being attacked (usually a model, with assumptions and abstractions)
- Threat model 
- Attack surface: what can be attacked
  - Open ports and exposed services
  - Public APIs and their parameters
  - Web endpoints, forms, cookies
  - File system permissions
  - Hardware interfaces (USB, JTAG)
  - User roles and privilege boundaries
- Attack vector: how the attacker attacks
  - SQL injection via POST /login
  - Phishing to steal credentials, then SSH login
  - Buffer overflow in a network daemon
  - Cross-site scripting through a comment field
  - Supply-chain poisoning of a dependency
- Vulnerability: what the attacker can do
- Exploit: how the attacker exploits the vulnerability
- Damage: what the attacker can do
- Mitigation: mitigate vulnerability
- Defense: close vulnerability gap

Importance of correct modeling

- Attack-surface awareness guides defenses
  - E.g. pre-Covid-19 vs. post-Covid attack surface of company servers
- Match resources to expected threat actors
  - "Script kiddie": individual or group running off-the-shelf attacks
    - Caveat: off-the-shelf attacks can still be quite powerful! Metasploit, Shodan, dark web market.
  - "Insider attack": employee with access to internal machines/networks
  - "Advanced Persistent Threat (APT)": nation-state level resources and patience
  - All these threats have different motivations, require different defenses/responses!
- Reevaluate often
  - Threat capabilities change over time

## TCP/IP network stack

Local and interdomain routing

- TCP/IP for routing and messaging
- BGP for routing announcements

Domain Name System

- Find IP address from symbolic name (cse.wustl.edu)

### Layer Summary

Application: the actual sending message
Transport (TCP, UDP): segment
Network (IP): packet
Data Link (Ethernet): frame

### Types of Addresses in Internet

- Media Access Control (MAC) addresses in the network access layer
  - Associated w/ network interface card (NIC)
  - 00-50-56-C0-00-01
- IP addresses for the network layer
  - IPv4 (32 bit) vs IPv6 (128 bit)
  - 128.1.1.3 vs fe80::fc38:6673:f04d:b37b%4
- IP addresses + ports for the transport layer
  - E.g., 10.0.0.2:8080
- Domain names for the application/human layer
  - E.g., www.wustl.edu

#### Routing and Translation of Addresses

(All of them are attack surfaces)

- Translation between IP addresses and MAC addresses
  - Address Resolution Protocol (ARP) for IPv4
  - Neighbor Discovery Protocol (NDP) for IPv6
- Routing with IP addresses
  - TCP, UDP for connections, IP for routing packets
  - Border Gateway Protocol for routing table updates
- Translation between IP addresses and domain names
  - Domain Name System (DNS)

### Summary for security

- Confidentiality
  - Packet sniffing
- Integrity
  - ARP poisoning
- Availability
  - Denial of service attacks
- Common
  - Address translation poisoning attacks (DNS, ARP)
  - Packet spoofing
- Core protocols not designed for security
  - Eavesdropping, packet injection, route stealing, DNS poisoning
  - Patched over time to prevent basic attacks
- More secure variants exist:
  - IP $\to$ IPsec (IPsec is )
  - DNS $\to$ DNSsec
  - BGP $\to$ sBGP

## Crypto fundamentals

- Well-defined statement about difficulty of compromising a system
  - ...with clear implicit or explicit assumptions about:
    - Parameters of the system
    - Threat model
    - Attack surfaces
- Example: "A one-time pad cipher is secure against any cryptanalysis, including a brute-force attack, assuming:
  - the key is the same length as the plaintext,
  - the key is truly random, and
  - the key is never re-used."

### Common roles in cryptography

Alice and Bob: Sender and receiver

Eve: Adversary that can see but not create any packets

Mallory: Man in the middle, can create and modify packets

The message M is called the **plaintext**.

Alice will convert plaintext M to an encrypted form using an
encryption algorithm E that outputs a **ciphertext*- C for M.

#### Cryptography goals

Confidentiality:

- Mallory and Eve cannot recover original message from ciphertext

Integrity:

- Mallory cannot modify message from Alice to Bob without detection by Bob

#### Threat models

- Attacker may have (with increasing power):
  - a) collection of ciphertexts (ciphertext-only attack)
  - b) collection of plaintext/ciphertext pairs (known plaintext attack: KPA)
  - c) collection of plaintext/ciphertext pairs for plaintexts selected by the attacker (chosen plaintext attack: CPA)
  - d) collection of plaintext/ciphertext pairs for ciphertexts selected by the attacker (chosen ciphertext attack: CCA/CCA2)

## Symmetric key cryptography

### Classical cryptography

Techniques: substitution and transposition

- Substitution: 1:1 mapping of alphabet onto itself
- Transposition: permutation of elements (i.e. rearrange letters)

- Caesar cipher: rotate each letter by k positions (k is fixed)
- Vigenère cipher: If length of key is known, split letters into groups based on index within key and do frequency analysis within groups

> The three steps in cryptography:
>
> - Precisely specify threat model
> - Propose a construction
> - Prove that breaking construction under threat mode will solve an underlying hard problem

#### Perfect secrecy

Ciphertext attack reveal no "info" about plaintext under ciphertext only attack

Def: A cipher $(E, D)$ over $(K, M, C)$ has perfect secrecy if

- $\forall m_0, m_1 \in M$ $(|m_0| = |m_1|)$ and $\forall c \in C$,
  - $\Pr[E(k, m_0) = c] = \Pr[E(k, m_1) = c]$ where $k \leftarrow K$

#### XOR One-time pad (perfect secrecy)

Assumptions:

- Key is as long as message
- Key is random
- Key is never re-used

In practice, relax this assumption gets "Stream ciphers"

### Stream cipher

- Use pseudorandom generator as keystream for xore encryption (security is guaranteed by pseudorandom generator)

Security abstraction:

1. XOR transfers randomness of keystream to randomness of CT regardless of PT’s content
2. Security depends on G being “practically” indistinguishable from random string and “practically” unpredictable
3. Idea: shouldn’t be able to predict next bit of generator given all bits seen so far

#### Semantic security

- $(E, D)$ has semantic secrecy if $\forall m_0, m_1 \in M$ $(|m_0| = |m_1|)$,
  - $\{E(k, m_0)\} \approx_p \{E(k, m_1)\}$ where $k \leftarrow K$
- ...and the adversary exhibits $m_0, m_1 \in M$ explicitly

The advantage of adversary is defined as the probability of distinguishing $E(k, m_0)$ from $E(k, m_1)$.

#### Weakness for stream ciphers

- Week pseudorandom generator
- Key re-use
- Predicable effect of modifying ciphertext or decrypted plaintext.

### Block cipher

View cipher as a Pseudo-Random Permutation (PRP)

#### Pseudorandom permutation

- PRP defined over $(K, X)$:
  - $E: K \times X \to X$
  - such that:
    1. There exists an "efficient" deterministic algorithm to evaluate $E(k, x)$.
    2. The function $E(k, \cdot)$ is one-to-one.
    3. There exists an "efficient" inversion algorithm $D(k, y)$.

- i.e. a PRF that is an invertible one-to-one mapping from message space to message space

#### Security of block ciphers

Intuition: a PRP is secure if: a random function in $Perms[X]$ is indistinguishable from a random function in $SF$ (real random permutation function)

The adversarial game is to let adversary decide $x$, then we choose random key $k$ and give $E(k,x)$ and real random permutation $Perm(X)$ to let adversary decide which is which.

#### Block cipher constructions: Feistel network

Forward network:

![Feistel network](https://notenextra.trance-0.com/CSE4303/Feistel_network.png)

- Forward (round $i$): given $(L_{i-1}, R_{i-1}) \in \{0,1\}^n \times \{0,1\}^n$,
  - $L_i = R_{i-1}$
  - $R_i = L_{i-1} \oplus f_i(R_{i-1})$

- Proof (construct the inverse):
  - Suppose we are given the output of round $i$, namely $(L_i, R_i)$.
  - Recover the previous right half immediately:
    - $R_{i-1} = L_i$
  - Then recover the previous left half by undoing the XOR:
    - $L_{i-1} = R_i \oplus f_i(R_{i-1}) = R_i \oplus f_i(L_i)$
  - Therefore each round map is invertible, with inverse transformation:
    - $R_{i-1} = L_i$
    - $L_{i-1} = f_i(L_i) \oplus R_i$
  - Applying this inverse for $i=d,d-1,\ldots,1$ recovers $(L_0,R_0)$ from $(L_d,R_d)$, so the whole Feistel network $F$ is invertible.

- Notation sketch (each wire is $n$ bits):
  - Input: $(L_0, R_0)$
  - Rounds:
    - $L_1 = R_0,\ \ R_1 = L_0 \oplus f_1(R_0)$
    - $L_2 = R_1,\ \ R_2 = L_1 \oplus f_2(R_1)$
    - $\cdots$
    - $L_d = R_{d-1},\ \ R_d = L_{d-1} \oplus f_d(R_{d-1})$
  - Output: $(L_d, R_d)$

## Hash functions

## Asymmetric key cryptography

## Appendix for additional algorithms and methods

### Feistel network (used by several items below)

A **Feistel network*- splits a block into left/right halves and iterates rounds of the form $(L_{i+1},R_{i+1})=(R_i, L_i\oplus F(R_i,K_i))$, so decryption reuses the same structure with subkeys in reverse order.

Feistel-based here: **DES, 3DES, CAMELLIA, SEED, GOST 28147-89 (and thus GOST89MAC uses a Feistel block cipher internally).**

### A) Cipherlist *filters / set operations- (not crypto primitives)

These don’t implement encryption or authentication; they just include/exclude suites.

- **COMPLEMENTOFDEFAULT*- — (selection) picks suites in `ALL` that are not enabled by default (notably RC4/anonymous, depending on build).
- **ALL*- — (selection) all suites except `eNULL`, in a default preference order (OpenSSL-defined ordering).
- **COMPLEMENTOFALL*- — (selection) suites excluded from `ALL` (currently `eNULL`).
- **HIGH / MEDIUM / LOW*- — (selection) groups suites by effective key strength class (OpenSSL policy buckets).
- **TLSv1.2 / TLSv1.0 / SSLv3*- — (selection) restricts to suites whose *minimum supported protocol version- is at least that value.
- **SUITEB128 / SUITEB128ONLY / SUITEB192*- — (selection) enforces “Suite B”-style constraints: only very specific ECDHE-ECDSA-AES-GCM suites and curves/hashes.
- **CBC*- — (mode selector) selects suites using **CBC mode*- for symmetric encryption (confidentiality only unless paired with a MAC).

---

### B) “No encryption” / “no authentication” flags

- **eNULL, NULL*- — **encryption/decryption: none**; **cipher method: N/A**; core idea: the record payload is not encrypted at all (plaintext).
- **aNULL*- — **authentication: none*- (no peer authentication); **cipher method: N/A**; core idea: uses anonymous key agreement (no cert/signature), enabling MITM.
- **ADH / AECDH*- — **authentication: none**; **cipher method: N/A**; core idea: anonymous (EC)DH establishes a shared secret but without identity binding → MITM-friendly.

---

### C) Key exchange and authentication selectors (not symmetric encryption, not MAC)

These describe *how keys are negotiated- and/or *how the peer is authenticated*, not whether payload is a block/stream cipher.

#### RSA / DH / ECDH families

- **kRSA, RSA*- — (key exchange) the premaster secret is sent encrypted under the server’s RSA public key (classic TLS RSA KX).
- **aRSA, aECDSA, aDSS, aGOST, aGOST01*- — (authentication) the server identity is proven via a certificate signature scheme (RSA / ECDSA / DSA / GOST).
- **kDHr, kDHd, kDH*- — (key exchange) *static- DH key agreement using DH certificates (obsolete/removed in newer OpenSSL).
- **kDHE, kEDH, DH / DHE, EDH / ECDHE, EECDH / kEECDH, kECDHE, ECDH*- — (key exchange) *ephemeral- (EC)DH derives a fresh shared secret each handshake; “authenticated” variants bind it to a cert/signature.
- **aDH*- — (authentication selector) indicates DH-authenticated suites (DH certs; also removed in newer OpenSSL).

#### PSK family

- **PSK*- — (keying model) uses a pre-shared secret as the authentication/secret basis.
- **kPSK, kECDHEPSK, kDHEPSK, kRSAPSK*- — (key exchange) PSK combined with (EC)DHE or RSA to derive/transport session keys.
- **aPSK*- — (authentication) PSK itself authenticates endpoints (except RSA_PSK where cert auth may be involved).

---

### D) Symmetric encryption / AEAD (this is where “block vs stream” applies)

#### AES family

- **AES128 / AES256 / AES*- — **encryption/decryption**; **block cipher**; core algorithm: AES is an SPN (substitution–permutation network) of repeated SubBytes/ShiftRows/MixColumns/AddRoundKey rounds.
- **AESGCM*- — **both encryption + message authentication (AEAD)**; **both*- (AES block cipher used in counter mode + auth); core algorithm: encrypt with AES-CTR and authenticate with GHASH over ciphertext/AAD to produce a tag.
- **AESCCM / AESCCM8*- — **both encryption + message authentication (AEAD)**; **both**; core algorithm: compute CBC-MAC then encrypt with CTR mode, with 16-byte vs 8-byte tag length variants.

#### ARIA family

- **ARIA128 / ARIA256 / ARIA*- — **encryption/decryption**; **block cipher**; core algorithm: ARIA is an SPN-style block cipher with byte-wise substitutions and diffusion layers across rounds.

#### CAMELLIA family

- **CAMELLIA128 / CAMELLIA256 / CAMELLIA*- — **encryption/decryption**; **block cipher**; core algorithm: Camellia is a **Feistel network*- with round functions plus extra FL/FL$^{-1}$ layers for nonlinearity and diffusion. *(Feistel: yes)*

#### ChaCha20

- **CHACHA20*- — **encryption/decryption**; **stream cipher**; core algorithm: ChaCha20 generates a keystream via repeated ARX (add-rotate-xor) quarter-rounds on a 512-bit state and XORs it with plaintext.

#### DES / 3DES

- **DES*- — **encryption/decryption**; **block cipher**; core algorithm: DES is a 16-round **Feistel network*- using expansion, S-boxes, and permutations. *(Feistel: yes)*
- **3DES*- — **encryption/decryption**; **block cipher**; core algorithm: applies DES three times (EDE or EEE) to increase effective security while retaining the **Feistel*- DES core. *(Feistel: yes)*

#### RC4

- **RC4*- — **encryption/decryption**; **stream cipher**; core algorithm: maintains a 256-byte permutation and produces a keystream byte-by-byte that is XORed with plaintext.

#### RC2 / IDEA / SEED

- **RC2*- — **encryption/decryption**; **block cipher**; core algorithm: mixes key-dependent operations (adds, XORs, rotates) across rounds with “mix” and “mash” steps (not Feistel).
- **IDEA*- — **encryption/decryption**; **block cipher**; core algorithm: combines modular addition, modular multiplication, and XOR in a Lai–Massey-like structure to achieve diffusion/nonlinearity (not Feistel).
- **SEED*- — **encryption/decryption**; **block cipher**; core algorithm: a 16-round **Feistel network*- with nonlinear S-box-based round functions. *(Feistel: yes)*

---

### E) Hash / MAC / digest selectors (message authentication side)

These are not “ciphers” but are used for integrity/authentication (often as HMAC, PRF, signatures).

- **MD5*- — **message authentication component*- (typically via HMAC, historically); **cipher method: N/A**; core algorithm: iterated Merkle–Damgård hash compressing 512-bit blocks into a 128-bit digest (now considered broken for collision resistance).
- **SHA1, SHA*- — **message authentication component*- (typically HMAC-SHA1 historically); **N/A**; core algorithm: Merkle–Damgård hash producing 160-bit output via 80-step compression (collisions known).
- **SHA256 / SHA384*- — **message authentication component*- (HMAC / TLS PRF / signatures); **N/A**; core algorithm: SHA-2 family Merkle–Damgård hashes with different word sizes/output lengths (256-bit vs 384-bit).
- **GOST94*- — **message authentication component*- (HMAC based on GOST R 34.11-94); **N/A**; core algorithm: builds an HMAC tag by hashing inner/outer padded key with the message using the GOST hash.
- **GOST89MAC*- — **message authentication**; **block-cipher-based MAC (so “block” internally)**; core algorithm: computes a MAC using the GOST 28147-89 block cipher in a MAC mode (cipher-based chaining). *(Feistel internally via GOST 28147-89)*