communications research group network security volkan cambazoglu adapted from: computer networking,...

Communications Research Group

Network SecurityVolkan Cambazoglu

Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

Outlook•Secure channel

•Principles of cryptography

•Authentication, Integrity

•Security at different layers

•Firewalls and Intrusion Detection


Communication Channels

•Assume always that a communication channel is insecure!

Source: Kurose Ross


Examples of Alice and Bob•E-commerce applications

- Amazon, Spotify, etc.

•Online banking applications- Swedbank, Nordea, etc.

•Online chat applications- Skype, Google chat, etc.

•DNS servers- Exchange messages about where a website is

located

•Routers- Exchange messages about routing tables (Routing

Information Protocol)Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

What can Trudy do?•Eavesdrop

- Sniff and record traffic between users (e.g. Alice and Bob)

• Insertion- Insert messages as if it comes from a specific user

(Alice/Bob)

•Modification- Alter messages going from a user (Alice) to the other

one (Bob)

•Deletion- Delete messages going from a user (Alice) to the

other one (Bob)

•Denial of service- Prevent users (Alice) from reaching an existing service

(provided by Bob)Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

Properties of Secure Communication•Confidentiality

- Only the receiver should understand the message content

•Authentication- Receiver should be able to confirm sender’s identity

• Integrity- Receiver should be able to check that the message is

not altered

•Availability- Receiver should be able to access services provided by

the senderAdapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

Information Security•Conceptually, the way information is recorded

has not changed dramatically over time. What has changed dramatically is the - ability to copy and alter information. - technological advancements- change from physical to digital

•Cryptography is the study of mathematical techniques related to aspects of information security such as - confidentiality- entity authentication- data integrity- data origin authentication


The Basic Idea

•Mathematical functions f(x) that are efficient to compute. No efficient algorithm is known for the inverse function.

•such as•Discrete Logarithm•Factorizing large numbers

f(x): efficient

f (x): hard-1

x f(x)


Kerkhoff’s Principle

An enemy knows the whole system including all transformations, but not the secret key(s).


Principles of Cryptography•Plaintext or cleartext

- has some meaning

•Ciphertext- unintelligible content

•Encryption algorithm- encrypt (plaintext) = ciphertext

•Decryption algorithm- decrypt (ciphertext) = plaintext


Encryption/Decryption•non-keyed

- no secret parameters- one-way functions- e.g. MD5

•secret key- two or more entities share some common secret values- encrypt and decrypt with the same secret- e.g. Caesar cipher, AES

•public key- no shared secret keys - one secret for encryption and another secret for

decryption- e.g. RSAAdapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

Symmetric Key Cryptography

ff ffm mc = f(m,k)

k k

ciphertextplaintext


Gaius Julius Cæsar•Shared secret encryption/decryption•Secret is a number to shift the alphabet

abcdefghijklmnopqrstuvwxyz

k = 3

defghijklmnopqrstuvwxyzabc


There will be a secret meeting in one of the Swedish cities. We obtained the ciphertext for it! Which city is it?

toorzkz

Gaius Julius Cæsar

abcdefghijklmnopqrstuvwxyz

uppsala


Attacks on Symmetric Key Cryptography•ciphertext-only: - statistical analysis (e,t most frequent)

- typical words (the, in, it, ...ing, etc.)

•known-plaintext- Uppsala, Alice, Bob, etc.

•chosen-plaintext- “the quick brown fox jumps over the lazy dog”


Improving Symmetric Key Cryptography•Monoalphabetic cipher

- Caesar cipher

•Polyalphabetic cipher- e.g. combine two Caesar ciphers for one word

•Block cipher- e.g. 3-bit block cipher (000:110, 001: 101, 010: 000, ...)

- DES: 64 bit input, 16 rounds of 48 bit key from 56 bit key, final permutation 64 bit output

- AES: 128 bit blocks, accepts different key lengths (128, 192, 256)

- brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AESAdapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

Public Key Cryptography•How can Alice and Bob start secure

communication, if they cannot come together in the physical world?- Send shared secret in plaintext?- Send encrypted shared secret?- Hide the secret somewhere in plaintext?- Any other crazy ideas?- Or shall we simply use public key cryptography?


Plaintextmessage

m = KB-(KB

+(m))

Public Key Cryptography


encryptioencryptionn

algorithalgorithmm

decryptiodecryptionn

algorithalgorithmm

Plaintextmessage, m

CiphertextKB

+(m)

Public key: KB+(m)

Private key: KB-(m)

Plaintextmessage

m = KB-(KB

+(m))

Public Key Cryptography


encryptioencryptionn

algorithalgorithmm

decryptiodecryptionn

algorithalgorithmm

Plaintextmessage, m

CiphertextKB

+(m)

Public key: KB+(m)

Private key: KB-(m)

What could go wrong here?•Hint 1: Who can use the public key?•Hint 2: What happens when same text, algorithm and key are used?

Public Key Cryptography•Prerequisite: Modular Arithmetic•x mod n = remainder of x when divided by

n• facts:

- [(a mod n) + (b mod n)] mod n = (a+b) mod n- [(a mod n) - (b mod n)] mod n = (a-b) mod n- [(a mod n) * (b mod n)] mod n = (a*b) mod n

• thus: - (a mod n)d mod n = ad mod n

•example:- a=14, n=10, d =2- (14 mod 10)2 mod 10 = 42 mod 10 = 6- 142 mod 10 = 196 mod 10 = 6


RSA: Encryption/Decryption•Encryption

- c = me mod n- c is ciphertext - m is plaintext- e is encryption key- (n, e) is the public key

•Decryption- m = cd mod n = (me mod n)d mod n = me*d mod n- d is decryption key- (n, d) is the private key

• Do you notice something when m = me*d mod n?Adapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

RSA: Creating public/private key pair•Choose two large prime numbers p and q (1024

bits each)

•Compute (n = p * q) and (z = (p-1) * (q-1))

•Choose e < n that has no common factors with z (relatively prime) - e.g. (3 and 7) and (5 and 12) are relatively prime.

•Choose d that fulfills (e * d mod z = 1)

•Public key (n,e)•Private key (n,d)Adapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

RSA Encryption

Source: Kurose Ross

p=5q=7

n=35z=24e=5

d=29Adapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

RSA Decryption

Source: Kurose Ross

p=5q=7

n=35z=24e=5

d=29Adapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

Why does RSA work?•m = cd mod n •m = (me mod n)d mod n •m = me*d mod n

• fact: - cd mod n = c(d mod z) mod n- where n = p*q and z=(p-1)*(q-1)

• thus:- m = m((e*d) mod z) mod n- m = m1 mod n


Why is RSA secure?• We know the public key (n,e). Can we compute d

using n and e?• We need to find the factors of n= p*q• p and q are two very large prime numbers (at

least 1024 bits)

• 136064817260489928484113640026944941480975382962539945337862848254226224034275820538310008858403955437239102681465761388249980135083342434428721426840110617593953169835450968550730769430412845048185659381370857105323219453521491277894773367539216680431287506338710965204349119030528157752992551375455100484051 (309 digits)

• Factoring a big number is hard!Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

RSA in practice: Session keys•Exponentiation in RSA is computationally

intensive

•Use public key crypto to establish secure connection

•Establish symmetric session key for encrypting data- Shared secret


Message Integrity•Apply hash function H to m and get fixed

size message digest H(m).

•Good to rely on- MD5 (128 bit message digest)- SHA-1 (160 bit message digest) (US standard)

•Bad to rely on- Internet checksum (16 bit digest)- “IOU100.99BOB” and - “IOU900.19BOB” have identical checksum (B2 C1 D2 AC)


Message Integrity• If Alice sends (m, H(m)) to Bob, can Bob trust

the message m comes from Alice?•No; because Trudy can prevent Bob from

receiving (m, H(m)) and instead send (m’, H(m’)). Bob will check that H(m’) is indeed digest/hash of m’.

•There is a solution to this problem:- Message Authentication Code (e.g. HMAC)- Used together with a cryptographically secure hash

function such as MD5 or SHA-1- There is a shared authentication key between Alice

and Bob.

- So, Alice will send (m, H(m+s)) instead of (m, H(m)).Adapted from: Computer Networking, Kurose/Ross and lecture notes, Rohner

Authentication•Bob wants Alice to “prove” her identity to

him

•Bob wants to know that if he receives a message from Alice, the message actually comes from her.

•Bob wants to be sure that the message was not tampered with on its way to him.


RSA: Another important property• KB

-(KB+(m)) = m = KB

+(KB-(m))

• private(public(m)) = m = public(private(m))

• Everyone can encrypt• Only one can decrypt

• Only one can claim it• Everyone can check it


Digital Signatures•Cryptographic technique analogous to hand-

written signatures•Bob (sender) digitally signs document,

establishing he document owner/creator•Bob signs message m by encrypting with his

private key KB-, creating signed message KB

-

(m).• Verifiable, non-forgeable: Alice (recipient) can

prove to someone that Bob and no one else must have signed the document

• Non-repudiation:- Alice can take m and signature KB

-(m) to court and prove that Bob signed m

- Only Bob possesses KB-Adapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

Digital Signature

Source: Kurose Ross


Signed Message Digests•Computationally expensive to encrypt long

messages with public key crypto•Goal:

- Fixed-length- Easy-to-compute- Digital fingerprint

•Apply hash function H to m and get fixed size message digest H(m).

•Sign H(m)•Send (m, KB

-(H(m)))


Digital Signature

Source: Kurose Ross


Impersonation Attack

Source: Kurose Ross


Replay Attack

Source: Kurose Ross


Nonce (timeliness)•Nonce: number R used only once-in-a-

lifetime•KA-B : Shared secret key


Nonce (timeliness)

Source: Kurose Ross


(Wo)Man-in-the-Middle Attack

Source: Kurose RossAdapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

(Wo)Man-in-the-Middle Attack•Difficult to detect

•Alice receives everything Bob sends

•Bob and Alice can meet later and still recall the last conversation

•Trudy receives all messages as well!


Public Key Certification•Certification Authority (CA)

- binds public key to particular entity (Bob)

•Bob provides proof of identity to CA•CA creates certificate binding Bob to his public

key•Certificate containing Bob’s public key digitally

signed by CA - CA says “this is Bob’s public key”

•When Alice wants Bob’s public key- gets Bob’s certificate (from Bob or elsewhere)- apply CA’s public key to Bob’s certificate- gets Bob’s public key


Security at Different Layers

ApplicationMail: MIME/S, PGP

TransportTLS (Secure Socket Layer, SSL)

NetworkIP Security (IPSec)Packet Filter

LinkFrame Filter, WPA

PhysicalWEP


Firewalls• Isolates organization’s internal network

from larger Internet, allowing some packets to pass, blocking others


Firewalls•Prevent denial of service attacks

- SYN flooding: attacker establishes many bogus TCP connections, no resources left for real connections

•Prevent illegal modification/access of internal data- Attacker replaces website’s homepage with something

else

•Allow only authorized access to inside network- Set of authenticated users

•Three types of firewalls- Stateless packet filters- Stateful packet filters- Application gateways


Stateless Packet Filtering• Internal network connected to Internet via

router firewall

•Router filters packet-by-packet, decision to forward/drop packet based on- Source IP address, destination IP address- TCP/UDP source and destination port numbers- ICMP message type- TCP SYN and ACK bits

•Example: - Block incoming and outgoing datagrams with IP

protocol field 17- All incoming and outgoing UDP flows are blockedAdapted from: Computer Networking, Kurose/Ross and lecture

notes, Rohner

Access Control Lists

ActionAction SourceSourceAddressAddress

DestDestAddressAddress

ProtoProtocolcol

SourceSourcePortPort

DestDestPortPort Flag BitFlag Bit

allow222.22/1

6

outside of

222.22/16

TCP >1023 80 any

allow

outside of

222.22/16

222.22/16

TCP 80 >1023 ACK

deny all all all all all all


Stateful Packet Filtering•Stateless packet filtering

- Admits packets that makes no sense- e.g. dest port=80, ACK bit set, even though no

TCP connection established

•Stateful packet filtering tracks- Status of every TCP connection- Connection setup (SYN)- Connection teardown (FIN)- Timeout inactive connections at firewall


Intrusion Detection Systems•Packet filtering

- operates on TCP/IP headers only- no correlation check among sessions

• Intrusion Detection System- Deep packet inspection: Look at packet contents

for viruses, attack patterns, etc.

- Examine correlation among multiple packets for port scanning, network mapping, Denial of Service (DoS) attack, etc.


communications research group network security volkan cambazoglu adapted from: computer networking,...

Documents

rohner slide

lecture notes

computer networking

integrity security

users alice

examples of alice

bob insertion insert

bob deletion