1. Security Outline Encryption Algorithms Authentication Protocols
Message Integrity Protocols
Key Distribution
Firewalls

2. Overview Cryptography functions Security services
Secret key (e.g., DES)
Public key (e.g., RSA)
Message digest (e.g., MD5)
Security services
Privacy: preventing unauthorized release of information
Authentication: verifying identity of the remote participant
Integrity: making sure message has not been altered

3. Secret Key DES (data encryption standard)
AES (advanced encryption standard)
Both standardized by NIST (US national inst. of standards & technology), both considered secure, with AES permitting fast implementation and less memory and suitable for small mobile devices.

4. 64-bit key (56-bits + 8-bit parity), 64-bit data block 16 rounds
Each Round L R i ─ 1 i ─ 1 F K i + L R i i
Only known way to break the code is exhaustive search, 2^55*1.5 us = 1.5 kyr, 3K PC bring it down to 6 month
3DES: 3 64-bit keys; AES 128, 192, 256 bits keys

5. Repeat for larger messages (cipher block chaining, CBC)

6. Public Key (RSA) Encryption & Decryption encryption: c = me mod n
decryption: m = cd mod n
e: encryption/public key d: decryption/private key
m: plaintext message, c: cipher text

7. RSA (cont) Choose two large prime numbers p and q (each 256 bits)
Multiply p and q together to get n
Choose the encryption key e, such that e and (p - 1) x (q - 1) are relatively prime.
Two numbers are relatively prime if they have no common factor greater than one
Compute decryption key d such that
d x e= 1 mod ((p - 1) x (q - 1))
Construct public key as (e, n)
Construct private key as (d, n)
Discard (do not disclose) original primes p and q

8. RSA: Example Let p = 7 and q = 11 Choose e = 7
n = 7 x 11 = 77
Choose e = 7
relatively prime to (7 - 1) x (11 - 1) = 60
Select d = 43, which satisfies
d x e= 1 mod ((p - 1) x (q - 1))
Let m = 9, then
c = 9^7 mod 77 = 37
m = 37^43 mod 77 = 9
RSA is based on the premise that factoring large numbers is hard
if n (public) can be factored to p and q, d is easy to compute
512-bit number factorable in the next few years, now 1024-bit or higher considered secure.

9. Message Digest Cryptographic checksum One-way function Relevance
just as a regular checksum protects the receiver from accidental changes to the message, a cryptographic checksum protects the receiver from malicious changes to the message.
One-way function
given a cryptographic checksum for a message, it is virtually impossible to figure out what message produced that checksum; it is not computationally feasible to find two messages that hash to the same cryptographic checksum.
Relevance
if you are given a checksum for a message and you are able to compute exactly the same checksum for that message, then it is highly likely this message produced the checksum you were given.

10. Message Digest MD4, MD5, SHA (secure hash algorithm)
Transform
Initial “
digest
(constant)
Message (padded)
Message digest
512 bits
■ ■ ■
MD5: 128-bit digest
SHA-1: 160-bit digest

11. Implementaion and Performance
Software implementation
numbers below represent those on state of art processors circa 2003
DES: 100 Mbps
MD5: 600 Mbps
RSA: 100 Kbps
Hardware implementation
DES, MD5 ~ Gbps
RSA ~ Mbps
RSA used for encrypt a key, DES for data encryption

12. Security Services
A security system is composed of a collection of security services, which use cryptographic algorithms.
Key Distribution
Message Integrity Protocols
Ascertain the message has not be altered by a third party
Authentication Protocols
Ascertain you (the corresponding partner) are who you claim to be

13. Key Distribution Issues: Types of keys
In secret key cipher, how does the two communicating parties obtain the shared key?
In public key cipher, how does one know what public key belongs to a certain participant?
Types of keys
Session key: valid for one session only, using secret key cipher
Pre-distribution key: used to generate session key, typically using public key cipher.

14. Public Key Distribution
Certificate (standardized in X.509)
special type of digitally signed document:
“I certify that the public key in this document belongs to the entity named in this document, signed X.”
the name of the entity being certified can be:
John Smith
components:
the public key of the entity
the name of the certified authority
a digital signature

15. Public Key Distribution (cont)
Certified Authority (CA)
administrative entity that issues certificates
useful only to someone that already holds the CA’s public key.
Chain of Trust
if X certifies that a certain public key belongs to Y, and Y certifies that another public key belongs to Z, then there exists a chain of certificates from X to Z
someone that wants to verify Z’s public key has to know X’s public key and follow the chain
Certificate Revocation List (CRL)
A user of a certificate will first check with CRL
CRL do not keep certificates which have expired

16. Public Key Distribution (cont)

17. Secret Key Distribution
Harder than public key distribution
For n parties, need n(n-1) secret keys, one for each pair, rather than n public keys, one for each party.
Keys must be kept secret in contrast to public keys.
A common solution: key distribution center (KDC, used in Kerberos).
KDC shares a secret key with each party, needs only n-1 keys, can be distributed using out-of-band method.
When A talks with B, both contact KDC using their keys, which generates a session key.

18. Message Integrity Protocols
MD is not enough, anyone can produce a valid MD with an altered message
Needs to ascertain the message is generated by true sender
Has to involve a secret privy to the sender (private key, secret key).
Such message digest is called authenticator.

19. Message Integrity Protocols Using public key
Digital signature standard (DSS)
Compute signature (encrypted MD) with private key and verify with public key
Standardized by NIST

20. Message Integrity Protocols Using symmetric key
Message authentication code (MAC) (a)
Hashed message authentication code (HMAC) (b).
The secret value can be the secret key.

21. Authentication Protocols
Objectives are to ascertain:
Originality: you are who you claim to be.
Timeliness: the message is truly sent at this moment
Relay attack: delayed playback of an original message (e.g. replay a bank transaction).
Also establishing a session key in the process

22. Methods to provide timeliness
Timestamp (tamper-proof).
Deficiency: clock synchronization accuracy, protecting against clock tampering
Nonce: a random number used only once
Can check if a nonce has been used before
Nonce + timestamp to reduce the amount of previous nonces to be remembered.

23. A challenge-response protocol
Challenge: a timestamp or nonce
Response: the same encrypted by the shared secret

24. Public-key authentication protocols
Assuming clocks are synchronized, timestamp can be augmented with nonces

25. Public-key authentication protocols: assuming clocks are not synchronized. Alice can check Ta, Bob can check Tb, respectively.

26. Symmetric-key Authentication Protocols: Needham-Schroeder Protocol (using KDC)
Assuming A, B share their master keys with KDC.
KDC doesn’t authenticate the 1st message since its response is useful to Alice only.
N1 assures freshness for Alice. N2 does that for Bob (ex.4)

27. Kerberos An IETF standard based on the NS protocol.
In the NS protocol, the 2nd message has two functions: authenticate Alice (only she can decrypt) and provide a certificate/ticket to present to Bob
In Kerberos, the first function is performed by AS, the second by TGS. The AS can provide multiple tickets to allow Alice to communicate with multiple servers
(In client-server applications, reasonable clock synchronization can be assumed. Kerberos use timestamp to make the problem in ex. 4 go away)

28. Diffie-Hellman Key Management
Features:
Establishes a session key without using any pre-distributed keys.
Messages are open to the public.
But does not provide authentication (thus needs to be augmented to be useful)

29. Mechanism of the Diffie-Hellman Key Management
Two public parameters p and g, satisfying:
p is a prime number
g is primitive root of p (called generator), i.e.,
g^k mod p with varying k covers all the numbers in [1, p-1].

30. Mechanism of the Diffie-Hellman Key Management
Alice/Bob independently selects a random number a/b in [1, p-1], and send values in public:
AB: g^a mod p
BA: g^b mod p
The common session key is: g^ab mod p
which only A, B can compute, e.g. for A: g^ab mod p = received value^a mod p
Revealed to public are: p,g, g^a mod p, g^b mod p, from which it is very difficult to determine a, b (discrete logarithm problem).

31. Man-in-the-middle attack
A and B now end up with both sharing a key with Mallory instead of each other.
Can fix the problem by attaching a public key certificate when sending values

32. Example Security Systems
Can be grouped by the main protocol layer at which they operate
L3: IP Security (IPSEC)
L4: Transport Layer Security (TLS), Secure Socket Layer (SSL)
Application: Pretty Good Privacy (PGP, ), Secure Shell (SSH, telnet)

33. Pretty Good Privacy (PGP)
Provide for encryption and authentication of s
Authentication
Mesh structured trust relationship
Anyone can assign anyone else’s certificate, the more certificate signatures the more trust
Encryption
Sender selects a random secret key k, sends E[m, k] + E[k, receiver’s public key]
Allow different cryptographic algorithms (specified in the header)

34. Secure Shell (SSH) Problem with telnet SSH v.2
password and messages are sent in the clear
SSH v.2
Client connects to the server, the server provides its public key
doubtful value for the key, alternative: out-of-band mechanism
Client authenticates the server using public key algorithm, sets up a session key
a encrypted channel is established
Transmission of password and messages can start now
The secure channel provided by SSH is often used by other applications, e.g. X-windows, IMAP mail readers, etc.

35. Transport Layer Security (TLS, SSL, HTTPS)
TLS (SSL: older version)
Motivated by the need to conduct secure transactions over the web
Provided secure transport layer to the application layer
HTTPS = TLS/(SSL, older) + HTTP
assigned #port, 443.

36. TLS Components
A handshake protocol that authenticate and establishes a secure channel
Through exchange of certificates, keys
A “record” protocol for data transfer
Handles encryption, message integrity, even compression
Maintains persistent session state through session ID
Avoid a security handshake for downloading each item (e.g. picture)

37. Transport Layer Security
The master secret is computed from the session key, client and server nonces.

38. IP Security (IPSEC)
An ambitious framework to provide all security services at the IP level
Modular, allow users to select security algorithms, services, granularity (per connection btw. hosts, per trunk btw. routers)
Two components
Security protocol
Authentication header (AH) (rarely used)
Encapsulation security payload (ESP)
implement authentication, encryption, etc.
Key management protocol (ISAKMP: internet security and association and key management protocol)

39. IP Security (IPSEC)
A secure connection consists of two simplex security associations (SA)
SA is identifiable by the destination IP address and a security parameters index (SPI: id of the SA).
SA is established, modified, and deleted by ISAKMP.
SA defines packet format for exchanging key generation and authentication data.

40. IP Security (IPSEC)
Encapsulating security payload (ESP) provides a secure L3 channel
ESP head
With IPv4, it follows IP header, plus a trailer
With IPv6, it is an IP extension header
Fields
SPI identifies the SA
SeqNum protects against replay attack
PayloadData is the data sent, can be encrypted.
Padlength described how much padding was added (for aligning along block boundaries)
NextHdr describes data
AuthenticationData carries authenticator

41. IPsec support two modes:transport mode and tunnel mode.
ESP format
IPsec support two modes:transport mode and tunnel mode.
Tunnel mode is frequently used in virtual private networks (VPN), where tunnels are between the gateways of remote sites of a company.

42. Wireless Security (802.11i)
Wireless links are particularly exposed to security threats because of lack of physical security.
IEEE i/Wi-Fi Protected Access 2 (WPA2)
Provides authentication, message integrity and confidentiality to /WiFi networks
Backward compatible to WEP (wired equivalent privacy, the first-generation WiFi security protocol which has serious flaws).

43. Wireless Security (802.11i) Two authentication modes
Personal/pre-shared key (PSK) mode:
Provides weaker security, more convenient, suitable for home networks.
The secret key between a wireless device and AP is derived from a preconfigured passphrase

44. Wireless Security (802.11i) Strong mode
Suitable for enterprise networks.
Involves authentication server (AS)
Uses EAP (extensible authentication protocol) which supports multiple authentication methods (Kerberos, public-key authentication, smart cards, one-time passwords, etc.)
Mutual authentication between wireless device and AP
Establishes a pair-wise master key, which generates session keys

45. Firewalls
Firewalls separate the network into zones with different trust levels.
Can be implemented by a router or a host machine
Specifically allow/disallow packets from certain hosts/applications
Typically 3 trust levels:
Internal network
Outside internet
Demilitarized zone (DMZ):
between internal and outside networks,
provides services accessible to the public and internal network, e.g., DNS, , web servers.
Hosts in DMZ do not have access to the internal network

46. Firewalls Using filter to allow/disallow application traffic
Lists of 4-tuples to allow/disallow
Example (prevent web traffic to )
( , 1234, , 80 )
(*,*, , 80 )
Can Filter based on application header contents
Example: disallow packets containing certain URL in the HTTP header.
Default: allow or disallow?
Disallow is more secure, but involves tradeoff between security and convenience.
Dynamic port selection
In some application, the port number is determined at run time. Need stateful firewall to remember session state.

47. Firewalls What firewalls do not protect
Internal adversaries (disgruntled employee, visitors, etc)
Services allowed by firewalls can later turn out to be unsafe.
Limited capability to filter out Malware (virusse, worms, spyware)

48. Denial of Service Attacks that exhaust system resources
bandwidth, processing power, memory space, etc.
Attacks on end hosts
SYN attack
more effective than data packets due to more processing/states incurred
Attacks on routers
Christmas tree packets
Send packets with all IP options turned on, exhausting processing capability
Pollute route cache
Send serial sequence of IP packets, blowing away 1st level route cache and making router spend all its time build new forwarding table

49. Denial of Service Counter attack Distributed DoS attacks
Detect resource usage anomaly
Account for resource usage
Discover if some users/connections are consuming disproportionate amount of resource
Restrict resource usage by misbehaving users/connecitons
Distributed DoS attacks
Makes distinguishing legitimate heavy load and DDOS hard