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Wireless Network Security
Prof. Nasir Memon
Department of Computer Science
11/28/2001 Wireless Security 2
Overview Introduction Data Encryption
Private Key Cryptography Public Key Cryptography
Digital Signatures Cryptographic Hash Functions Wireless Security Issues WEP
11/28/2001 Wireless Security 3
Network Security – Issues. Confidentiality– Can you keep a secret? Integrity – Did you get the message I sent? Availability – Are you there when needed? Identification – Who are you? Authentication – Can you prove who you
are? Access Control – What are you allowed to
do? Non-repudiability – Yes you did! Audit Trails – What have you been up to? Privacy – Can you treat my like a human?
11/28/2001 Wireless Security 4
Network Security - Why is it difficult? Complexity. Resource sharing. Unknown Perimeter. Many points of attack. Anonymity. Unknown Paths.
11/28/2001 Wireless Security 5
Type of Attacks in Computer Systems
11/28/2001 Wireless Security 6
Security Mechanisms Three basic building blocks are used:
Encryption is used to provide confidentiality, can provide authentication and integrity protection
Digital signatures are used to provide authentication, integrity protection, and non-repudiation
Checksums/hash algorithms are used to provide integrity protection, can provide authentication
One or more security mechanisms are combined to provide a security service
11/28/2001 Wireless Security 7
Services, Mechanisms, Algorithms A typical security protocol provides one
or more services Services are built from mechanisms Mechanisms are implemented using
algorithms
11/28/2001 Wireless Security 8
Protocol Stack
Socket
TCP/UDP
IP/IPSec
Mobile IP
Physical
Session mobility & WTLS
Data link
Application
Session Layer
Transport Layer
Network Layer
11/28/2001 Wireless Security 9
Network Layer: IPSec Advantages of IPSec
• Can protect a mix of application protocols running over a complex combination of media• Provides security services in the background• Secure all data communication• Multiple secure sessions can share the exchanged secret
Limitations of IPSec• Does not provide end-to-end security• Does not cover all security features• Does not prevent denial of service attacks• Does not stop traffic analysis
11/28/2001 Wireless Security 10
WTLS
Advantages of WTLS• Manages end-to-end as well as end-to-gatewaysecure wireless connections• Supports key refresh and transaction recovery
Limitations of WTLS• Does not cover all security features• Limited prevention against denial of service attacks• Does not stop traffic analysis• Secures data specific to the session only• Secure sessions cannot share exchanged secrets
11/28/2001 Wireless Security 11
Data Encryption Encryption is the process of encoding a message
such that its meaning is not obvious. Decryption is the reverse process.
We denote plaintext by P and ciphertext by C. C = E(P), P = D(C) and P = D(E(P)), where E() is
the encryption function (algorithm) and D() the decryption function.
Encryption DecryptionPlaintext PlaintextCiphertext
Secret Key
11/28/2001 Wireless Security 12
Symmetric and Asymmetric Cryptosystems
If the encryption key and the decryption key are the same then we have a symmetric encryption scheme (also private key, one-key).
If the encryption key and the decryption key are different then we have an asymmetric encryption scheme (also public key, two-key).
11/28/2001 Wireless Security 13
Example – Caesar Cipher Let messages be all lower case from a through
z (no spaces or punctuation).itsnotthathardtoread
Represent letters by numbers from 0 to 25. Encryption function
Ci = E(Pi ) = Pi + K.
where K is secret key and addition done modulo 26.
Decryption isPi = D(Ci ) = Ci - K.
UNIX ROT13 uses K as 13.
11/28/2001 Wireless Security 14
Cryptanalysis A cryptosystem should be secure
against the following kinds of attacks: Ciphertext only attack. Known plaintext attack. Chosen plaintext attack. Adaptive chosen plaintext attack. Chosen ciphertext attack. Chosen key attack.
11/28/2001 Wireless Security 15
Brute Force Attacks. If key space is finite, given a ciphertext a
cryptanalyst can try and check all possible keys. For above to be not feasible, key space should
be large!! How large? How about 256?
Large enough to make it impractical for an adversary. But what is impractical today, may not be so tomorrow.
In practice, for a “good” cryptosystem, the only possible attack should be the brute force attack, which should be impractical into the foreseeable future, as long as message may have value.
11/28/2001 Wireless Security 16
DES – Data Encryption Standard Private key. Encrypts by series of
substitution and transpositions. Worldwide standard for more than 20
years. Has a history of controversy. Designed by IBM (Lucipher) with later
help (interference?) from NSA. No longer considered secure for highly
sensitive applications. Replacement standard (AES - Rijndael)
has been selected.
11/28/2001 Wireless Security 17
DES - Overview
11/28/2001 Wireless Security 18
DES – Each iteration.
11/28/2001 Wireless Security 19
Triple DES
Triple DES (2 keys) requires 2112 search. Is reasonably secure.
3 keys requires 2168 .
11/28/2001 Wireless Security 20
Other Private Key Cryptosystems IDEA Twofish Blowfish RC4, RC5, RC6 Rijndael (AES Winner) Serpent MARS Feal
11/28/2001 Wireless Security 21
Private key cryptography revisited.
Key distribution and management is a serious problem! N users – O(N2) keys!
11/28/2001 Wireless Security 22
Public key cryptography
Key management problem not really that simple as we will see later!!! (trust).
11/28/2001 Wireless Security 23
One-way functions and trapdoors. A function f() is said to be one-way if given
x it is “easy” to compute y = f (x), but given y it is “hard” to compute x = f -1(y).
A trap-door one-way function fK() is such that to compute y = fK(x) is easy if K and x are known. x = f -1
K(y) is easy if K and y are known. x = f -1
K(y) is hard if y is known but K is unknown.
Given a trap-door one-way function one can design a public key cryptosystem.
11/28/2001 Wireless Security 24
Encryption and 1-way trap doors Two keys:
public encryption key e private decryption key d
encryption easy when e is known decryption hard when d is not known d provides “trap door”: decryption easy
when d is known We’ll study the RSA public key
encryption scheme.
11/28/2001 Wireless Security 25
RSA overview - setup Alice wants people to be able to send her
encrypted messages. She chooses two (large) prime numbers, p and
q and computes n=pq and . [“large” = 100 digits +]
She chooses a number e such that e is relatively prime to and computes d, the inverse of e in
She publicizes the pair (e,n) as her public key. She keeps d secret and destroys p, q, and
Plaintext and ciphertext messages are elements of Zn and e is the encryption key.
)(n
( ) ( 1) ( 1)n p q )(nZ
)(n
11/28/2001 Wireless Security 26
RSA overview - encryption Bob wants to send a message x (an
element of Zn) to Alice. He looks up her encryption key, (e,n), in
a directory. The encrypted message is
Bob sends y to Alice.
nxxEy e mod)(
11/28/2001 Wireless Security 27
RSA overview - decryption
To decrypt the message
she’s received from Bob, Alice
computes
Claim: D(y) = x
nyyD d mod)(
nxxEy e mod)(
11/28/2001 Wireless Security 28
Tiny RSA example. Let p = 7, q = 11. Then n = 77 and
Choose e = 13. Then d = 13-1 mod 60 = 37.
Let message = 2. E(2) = 213 mod 77 = 30. D(30) = 3037 mod 77=2
60)( n
11/28/2001 Wireless Security 29
Authentication and Authorization Authentication is a service that
allows receivers of a messages to identify its origin. makes is difficult for third parties to masquerade as
someone else. e.g., your driver’s license and photo authenticates
your image to a name, address, and birth date.
Authorization is a service that Allows only entities that have been authenticated
and who appear on an access list to utilize a service. E.g., your date of birth on your driver’s license
authorizes you to drink as someone who is over 21.
11/28/2001 Wireless Security 30
Authentication Authentication codes provide assurance that
message has not been tampered with and has indeed originated from a specific source.
Independent of encryption. In fact, encryption may even be undesirable.
Alice(Transmitter)
OscarBob
(Receiver)X Y Y’ X’
Au
then
tic?
Authentication Key Verification Key
11/28/2001 Wireless Security 31
Substitution and Impersonation Two types of attacks on authentication
schemes: Substitution attack
Impersonation attack
Hello Bob, I love you- Alice
Hello Bob, I hate you
- Alice
Hello Bob, I love you- Olivia
11/28/2001 Wireless Security 32
Digital Signatures Desirable properties of handwritten signatures:
Signed document is authentic. Signature is unforgeable. Signature is not reusable. Signed document is unalterable. Signature cannot be repudiated. (Above not strictly true but mostly so)
Same properties and more can be achieved by digital signatures.
Digital Signatures use public key cryptography.
11/28/2001 Wireless Security 33
RSA based signature
Alice signs message by encrypting with private key. Bob decrypts message with Alice’s public key. If meaningful message then it must have been
encrypted with Alice’s private key!
Hello, I love you
EncryptWith
Privatekey
HjkhrkHj837**ji8hj]
DecryptWith
Publickey
Hello, I love you
Message Alice signs Signed messageBob verifies Message
11/28/2001 Wireless Security 34
Signing With Message Digests A fixed length “fingerprint” of a
message. Instead of signing message, sign the
message digest.
11/28/2001 Wireless Security 35
Cryptographic Hash Functions Requirements of cryptographic hash
functions: Can be applied to data of any length. Output is fixed length. Relatively easy to compute h(x), given x. Infeasible to get x, given h(x). Given x, infeasible to find y such that h(x) =
h(y). Weak collision property. Infeasible to find any pair x and y such that
h(x) = h(y). Strong collision property.
11/28/2001 Wireless Security 36
Wireless Security
How is wireless different? 802.11 Security
11/28/2001 Wireless Security 37
Wireless Dimension
Access to Medium:Unlike wired medium
(cables) wireless medium (air) is
ubiquitous hence access restrictions to the medium must be handled explicitly, where as in wired environments it is
implicit.
War Dialing:Attacker gains access to wired
medium by exhaustive dialing of
phone numbers
War Driving:Attacker gains
access to wireless medium by just driving by the
network coverage area.
11/28/2001 Wireless Security 38
How is wireless different? The Medium
Wireless medium has no explicit packet boundary This property weaken privacy and authentication
mechanisms adopted from wired environment Portability
Wireless devices are smaller in size and portable Data in those devices require more protection than
data on non-portable devices Mechanisms to recover stolen or lost devices are
important Mechanisms for self-destruction of data is also
important
11/28/2001 Wireless Security 39
How is wireless different? Mobility
Mobility brings even bigger challenges Trust in infrastructure
Wired networks assume certain level of trust in local infrastructure (we trust our routers)
In wireless networks this is a weak assumption Would you put same level of trust on an Access Point in JFK as
you put on your home AP? Security mechanisms should anticipate these variances in trust Or, security mechanisms should be independent of location or
infrastructure Trust in location
Wired networks implicitly assume network address is equivalent to physical location (128.238.x.x is Poly’s resources)
In wireless networks physical location is not tied to network address. Physical location may change transparent to end nodes.
11/28/2001 Wireless Security 40
How is wireless different? Mobility
Privacy of location On wired network privacy of location is not a
concern In wireless networks location privacy of the user is
a serious issue because users can be tracked, their travel behaviors can be used for marketing purposes etc.
Similar scenario exists on the Web: A user’s web surfing pattern can be tracked and this raised several privacy issues in 1999 (Double Click’s Cookie Tracking)
11/28/2001 Wireless Security 41
How is wireless different? Processing power, memory & energy
requirements Handheld devices have stringent processing
power, memory, and energy requirements Current security solutions require expensive
processing power & memory Handheld devices mandate inexpensive
substitutes for Crypto algorithms (AES instead of 3-DES) Authentication schemes
Better one-time password schemes with feasible remote key updates
11/28/2001 Wireless Security 42
Power consumption & crypto algorithms
Piy
ush
Mis
hra
et
al.
11/28/2001 Wireless Security 43
How is wireless different? Network Topologies
Wired networks usually rely on network topology to deploy security solutions
E.g: firewall is installed on a machine where all traffic is visible
Wireless networks (esp. ad-hoc) have dynamic topologies
Wireless networks may not have single point of convergence (hidden host problem!)
Wireless networks put emphasis on host based solutions e.g: distributed firewalls
11/28/2001 Wireless Security 44
Wired Equivalent Privacy (WEP) Wired equivalence privacy?
Wireless medium has no packet boundaries WEP control access to LAN via authentication
Wireless is an open medium Provides link-level security equivalent to a closed medium No end-to-end privacy
Security Goals of WEP Access Control
Provide access control to the underlying medium through authentication
Confidentiality Provide confidentiality to data on the underlying
medium through encryption Data Integrity
Provide means to determine integrity of data between links
11/28/2001 Wireless Security 45
Wired Equivalent Privacy (WEP) An attack on WEP should compromise at least
one of these properties Three levels of security
Open system – WEP is disabled in this mode. No security. Shared Key Authentication – provides access control to
medium Encryption – provides confidentiality to data on network
You can have confidentiality on an open system! That is, you can encrypt all the traffic and not have
access control to the medium! Which also means, a wily hacker can have all his traffic
encrypted on our network so that no one “see” what s/he is doing!
11/28/2001 Wireless Security 46
Properties of WEP It is reasonably strong
Withstand brute force attacks and cryptanalysis It is self-synchronizing
Uses self-synchronizing stream cipher It is efficient
Hardware/software implementation It may be exportable
Rest of the world needs security too! It is optional
WEP layer should be independent of other layers