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Security

Computer Network Fundamentals

Learning Assistant Slides

Week 6

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eado.me/laform

2

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Tentative Plans

3

Security

Week 6

Week 9

Code Explore

Routing

Week 10

Final Review

(Today)

Code Explore

Security

Week 7

Routing

Week 8

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Project 2

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5

A

B

Internet

Unreliable

Reliable

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6

A

B

Reliable

C

Hello!

Hey there!

\x43\x98??

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7

A

C

B

Wait...you’re not B!

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Many different parts

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Reliable Data Transport

Key Exchange

Certificate Verification

Ciphers

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Almost like TCP + TLS 1.3

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Reliable Data Transport

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Project specifics

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stdin

stdout

Client

stdout

stdin

Server

Bidirectional

Similar to TCP, but:

Sequenced by packet; not by byte
No congestion control; window fixed to 20 packets
Single retransmission timer; on expire, resend smallest SEQ in window
Maximum of 2000 packets sent (no need to check for repeated SEQ)

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// transport.c

packet tosend[MAX_SEQ];

packet torecv[MAX_SEQ];

while (1) {

int base_seq = 1; // Last ACKed packet

int seq = 1; // Last sent packet

// Get data from peer

// Non-blocking; eado.me/cs118 > Socket Programming Tips

int bytes_recvd = recvfrom();

// Check if data from peer, then process

if (bytes_recvd > 0) process_data();

// Retransmit if needed

if (timer_set() && base_seq < seq) {

// Send packet with lowest SEQ that's unACKed

sendto(get_pkt(base_seq));

}

// Send packet if within window

if (packet_available() && seq < base_seq + CWND) {

sendto(get_pkt(seq - 1));

}

// Read from stdin

make_packet();

}

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Achieving reliability

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cwnd = 4 (fixed)

pkt 2 lost
pkt 3, 4 buffered
retransmit pkt 2 on RTO

How does the congestion window look after each step? What about the receiver buffer?

ACK 1

RTO

ACK 5

ACK 6

ACK 7

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Certificate Verification

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Elliptic Curve Cryptography

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y² = x³ + ax + b

Source: https://curves.xargs.org

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Elliptic Curve Cryptography

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How many times did we add the blue point to itself to get to the green point?

It’s hard to tell, right? This is a trapdoor function: easy to go in one direction, hard to go the other way.

Source: https://curves.xargs.org

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Elliptic Curve Cryptography

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Generator

Public Key

Private Key

#

Source: https://curves.xargs.org

P = kG

= 3

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Elliptic Curves in Finite Fields

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𝐅₂₉ (not actually; just for illustrative purposes)

Source: https://curves.xargs.org

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Delegating trust

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A

CA

B?

#

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Delegating trust

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A

CA

B?

Remember DNS?

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A

CR

root

edu

ucla

Why do we trust the root server?

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Project specifics

ECDSA: Elliptic Curve Digital Signature Algorithm

SHA-256: Hash function

Client is given: public key of certificate authority� Must derive: own public, private key

Server is given: private key, public key signed by certificate authority� Must derive: own public key

Goal: exchange public keys

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Project specifics

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client_hello

1. Verify pub key

2. Verify client nonce

3. Sign server nonce

1. Sign nonce

server_hello

client nonce

server nonce, pub key, CA’s sig, signed client nonce

key_exchange

Client

signed server nonce, pub key, signed pub key

Server

Both sides have each other’s public key

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// signing_nonce.c

int main() {

// Load private key from file and derive public key

load_private_key("../keys/priv_key.bin");

derive_public_key();

load_peer_public_key(public_key, pub_key_size);

// Generate nonce

unsigned char nonce[32];

generate_nonce(nonce, 32);

printf("Nonce: %.*s\n", 32, nonce);

// Sign nonce with private key

unsigned char sig[255];

size_t sig_size = sign(nonce, 32, sig);

printf("Signature: %.*s\n", sig_size, sig);

// Verify nonce with public key

int verified = verify(nonce, 32, sig,

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// key_deriv.c

int main() {

/* Server side */

// Load server private key from file

load_private_key("../keys/priv_key.bin");

// Load client public key (typically over the network)

char* der;

int size;

load_peer_public_key(der, size);

// Derive ECDH secret

derive_secret();

printf("Server side secret: %.*s\n", 32, secret);

/* Client side */

// Load client private key from file

load_private_key("../keys/client_priv_key.bin");

// Load server public key (typically over the network)

load_peer_public_key(der, size);

// Derive ECDH secret

derive_secret();

printf("Client side secret: %.*s\n", 32, secret);

return 0;

}

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Ciphers

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Slicing up data

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S

32 bytes

0101010111101010101011010101010111101011101010110111110110101010110001010000101010110010101010111010

...

?? bytes

16 bytes

...

Blocks

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AES-256-CBC (with PKCS #7 padding)

AES: Advanced Encryption Standard; encryption/decryption algorithm

256-bit (32 bytes): Key size; determines number of rounds used

CBC: Cipher Block Chaining; how to “stitch up” each block

PKCS: Public Key Cryptography Standards

#7: Cryptographic Message Syntax

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Cipher Block Chaining

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16 bytes

Initialization Vector

(16 bytes)

Cipher

S

AES

16 bytes

S

AES

Cipher

16 bytes

S

AES

Cipher

16 bytes

S

AES

Cipher

...

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What if my data isn’t 16 bytes?

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Source: https://node-security.com/posts/cryptography-pkcs-7-padding/

0d

a1

d3

b6

65

34

99

e4

cc

5d

32

b9

ff

3 bytes left

03

PKCS #7 Padding

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What if my data isn’t 16 bytes?

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Source: https://node-security.com/posts/cryptography-pkcs-7-padding/

0d

a1

d3

b6

65

34

99

e4

cc

5d

32

b9

ff

2 bytes left

PKCS #7 Padding

c3

02

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// cipher.c

int main() {

// Derive ECDH secret

derive_secret();

// Plaintext for encryption

unsigned char data[] = "Hello world!";

printf("Plaintext: %s\n", data);

// Encrypt plaintext

unsigned char cipher[255];

unsigned char iv[16];

size_t cipher_size = encrypt_data(data, sizeof(data), iv, cipher);

printf("Ciphertext: %.*s\n", cipher_size, cipher);

// Decrypt ciphertext

unsigned char decrypted_data[255];

size_t data_size = decrypt_cipher(cipher, cipher_size, iv, decrypted_data);

printf("Decrypted plaintext: %.*s\n", data_size, decrypted_data);

return 0;

}

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General Tips

Be on the lookout for starter code + guides at:

eado.me/cs118 > Miscellaneous > Security Programming Tips

Next week is our next Code Exploration. It’ll be a social deduction game, so you’re going to have to work together! We’ll be using the cryptographic primitives discussed today. Fingers crossed it’ll be in-person!

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