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, last update 1999)
AES (advanced encryption standard, 2000)
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 public 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 ~ Kbps
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
Authentication Protocols
Ascertain you (the corresponding partner) are who you claim to be
Message Integrity Protocols
Ascertain the message has not be altered by a third party

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, 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. 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. Key Distribution (cont)

17. Authentication Protocols
Three-way handshake (secret key based, e.g., DES)
A challenge and response model
E(m, k): encryption of message m with key k
D(m, k): decryption of message m with key k
Assumption: client and server each know the other’s key
x: random number
CHK: client handshake key
SHK: server handshake key
SK: session key
CHK, SHK used only for the authentification, can be generated from password (not suitable for key)
SK used for data encryption after authentication, which is exposed more often, but useful for one secession only
Client
Server
ClientId, E (
, CHK) E( y +
E(SK, SHK) Y

18. Trusted third party (Kerberos)
uses an authentication server (S) to help two parties (A, B) to authenticate each other, also secret key based
Assumption: S and A share a secret key KA, S, B, key KB
T: time stamp, works as a R.N.
L: life time,
K: session key, validation period determined by T, L
A can decrypt the 1st part of message from S, not the 2nd part,
The 2nd part is passed to B, challenging B to respond T+1.

19. Public key authentication( x
, Public )
x: a R.N.
No shared secret key required, only needs to know the other party’s public key

20. Message Integrity Protocols
Digital signature using RSA
special case of a message integrity that also ascertains the message is generated by true sender
compute signature with private key and verify with public key
DSS standardized by NIST, using a variant of RSA

21. Message Integrity Protocols
Keyed MD5
MD only does not provide integrity, any one can produce a valid MD with an altered message; has to involve a secret privy to the sender.
if the sender and receiver shares a secret key k, then sender: m + MD5(m + k), if not:
sender: m + MD5(m + k) + E[E(k, rcv-pub), snd-prv]
k: RN, the third part allows recovery of k by the receiver
receiver
recovers random key using the sender’s public key
applies MD5 to the concatenation of this random key message

22. Message Integrity Protocols
MD5 with RSA signature
sender: m + E(MD5(m), snd-prv)
receiver
decrypts signature with sender’s public key
compares result with MD5 checksum sent with message

23. 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)

24. 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)

25. 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.

26. Transport Layer Security (TLS, SSL, HTTPS)
TLS (SSL, older version) provides
A handshake protocol that authenticate and establishes a secure channel
Through exchange of certificates, keys
Hierarchical key distribution, root keys included in the browser.
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)
HTTPS = TLS/(SSL, older) + HTTP
assigned #port, 443.

27. Transport Layer Security

28. IP Security (IPSEC)
A 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
Authentication header (AH) + Encapsulation Security Payload (ESP)
implement authentication, encryption, etc
Key management protocol (ISAKMP)
A secure connection consists of two simplex security associations (SA), each identifiable by an IP address and a security parameters index (SPI)

29. Wireless Security (802.11i)
Wi-Fi Protected Acess 2 (WPA2, i)

30. Firewalls A firewall is a router that enforces security policies
A centralized approach, in contrast to end-to-end approach such as IPSEC
Filter-Based Solution
Lists of 4-tuples to allow/disallow
Example (prevent web traffic to )
( , 1234, , 80 )
(*,*, , 80 )
Default: forward or not forward? Tradeoff between security and convenience
Dynamic port selection: deals with the case that port number is determined by the application at run time

31. Proxy-Based Firewalls
Problem: complex policy
Example: web server
restrict certain web pages (URLs) to company users
Solution: proxy
determine whether to fetch the page depending on the user category
Works at application layer (HTTP), can also cache web pages, balance load
Limitations: attacks from within
malicious users, visitors, WiFi access

32. Denial of Service Attacks on end hosts Attacks on routers 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 route cache and making router spend all its time build new forwarding table

33. 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