1. i-4 security

2. Security taxonomy Physical security Resource exhaustion
Key-based security
cryptography

3. Security dichotomy Computer (system) Security
automated tools and mechanisms to protect data in a computer, even if the computers are connected to a network
against hackers (intrusion)
against viruses
against Denial of Service attacks
Access control, authorization, …
Internet (network) Security
measures to prevent, detect, and correct security violations that involve the transmission of information in a network or interconnected network
Everything on the network can be a target
Every transmitted bit can be tapped

4. Friends and enemies: Alice, Bob, Trudy
well-known in network security world
Bob, Alice want to communicate “securely”
Trudy (intruder) may tap, delete, add, modify messages
Alice
Bob
channel
data, control messages
secure
sender
secure
receiver
data
data
Trudy
Source: Kurose at UMass

5. There are bad guys (and girls) out there!
Q: What can a “bad guy” do?
A: A lot!
eavesdrop: intercept messages
Insert/modify/delete messages into connection
impersonation: can fake (spoof) source address in packet (or any field in packet)
hijacking: “take over” ongoing connection by removing sender or receiver, inserting himself in place
denial of service: prevent service from being used by others (e.g., by overloading resources)
Source: Kurose at UMass

6. Thwart the attacks! Basic Security services authentication
Access control
confidentiality
Data (or message) integrity
Non-repudiation

7. More Security services
Anonymity
Availability
Accountability
Privacy
forensics

8. Security mechanisms Encipherment Message digest Digital Signatures
Encryption and decryption
Keys
Message digest
Hash function characteristics
it is easy to compute the hashed value for any given message,
it is infeasible to find a message that has a given hash,
it is infeasible to find two different messages with the same hash
Can have a key (Cryptographic)
Digital Signatures
demonstrating the authenticity of a digital message or document

9. Meaning of Cryptography
from Greek
Cryptos: secret, hidden
graphos: writing
cryptography: study of secret writing

10. Basics Encryption key Decryption key Encryption (Encipherment)
(Decipherment)
Message
(plaintext,
cleartext)
Ciphertext
(cryptogram)
plaintext
cipher - algorithm for transforming plaintext to ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - the study of principles/methods of deciphering ciphertext without knowing key

11. Classification of Cryptosystems
The way in which keys are used
Symmetric cryptography
Single key
Public key cryptography
Two keys
the way in which plaintext is processed
Block cipher
Stream cipher

12. Symmetric cryptography

13. Symmetric Encryption also known as
Classical, conventional
private-key
single-key
Secret key
sender and recipient share a common key
was only type prior to invention of public-key cryptography
until second half of 1970’s

14. Symmetric Cipher Model
there must be a secure mechanism for the distribution of this key a priori

15. Requirements two requirements for secure use of symmetric encryption:
a strong encryption algorithm
a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
assume encryption algorithm is known
imply a secure channel to distribute the key

16. X-or() in cryptography
Sender wants to send M to receiver
M (Original plaintext): 1010
K (Key): 0011
M  K = 1001 (Encrypted ciphertext)
1001 transmitted
Receiver already knows K
(M  K)  K= 1001  0011 = 1010 = M
-> original message is restored!

17. Some primitives
Substitution
Permutation

18. Two types of symmetric ciphers
Stream cipher
Encrypts one bit at a time
RC4
Block cipher
Encrypts a block of bits at a time
DES, AES

19. Asymmetric cryptography Or Public key cryptography (PKC)

20. PKC – General Characteristics
public-key/two-key/asymmetric cryptography
uses 2 keys
public-key
may be known by anybody, and can be used to encrypt messages, and verify signatures
private-key
known only to the recipient, used to decrypt messages, and sign (create) signatures
keys are related to each other but it is not feasible to find out private key from the public one
Modular arithmetic

21. PKC – General Characteristics
It is computationally easy to en/decrypt messages when the relevant keys are known
RSA
Trap-door one-way function
ku: public-key, kr: private key
Y=fku(X) easy, if ku and X are known
X=fkr-1(Y)easy, if kr and Y are known, but infeasible if Y is known but kr is not known

22. Public-Key Cryptography: Encryption
Bob
Alice

23. Another notation Alice has a public key, kp, and a secret key, ks
Alice’s public key is known to Bob
Asymmetric Cipher: F-1(F(m,kp),ks) = m
Bob
Alice
1. Construct m
2. Compute c= F(m,kp)
3. Send c to Bob c
4. Receive c from Alice
5. Compute d=F-1(c,ks)
6. m = d

24. Public-Key Cryptography - Authentication
Commutative!
Alice
Bob

25. Why PKC? Initially developed to address two challenging issues:
key distribution
symmetric crypto requires how to securely share the key
in PKI you do not need to distribute/know secret keys, but you need trusted third parties
digital signatures (non-repudiation)
not possible with symmetric crypto

26. Diffie-Hellman (D-H) Algorithm
D-H model’s primary contribution:
Take a prime p and a primitive element g
Cyclic group in finite field
Publicize both g and p
Alice chooses some x  Zp* and sends (gx mod p) to Bob
Bob chooses some y  Zp* and sends (gy mod p) to Alice
Eve can see both (gx mod p) and (gy mod p) but she cannot calculate x or y
Discrete logarithm problem

27. D-H Algorithm gx mod p gy mod p Alice Bob
Alice calculates the key; k = (gy)x mod p
Bob calculates the same key; k = (gx)y mod p
Since Eve does not know x or y, she cannot calculate the key k
Diffie and Hellman developed this method to share a key using some publicly available information

28. PKC Applications 3 categories encryption/decryption digital signatures
to provide secrecy
digital signatures
to provide authentication and non-repudiation
key exchange
to agree on a session key (symmetric cipher) to encrypt data packets
Why not use public/private keys?

29. MESSAGE INTEGRITY

30. Source: Kurose at UMass
Message Digest
large
message m
H: Hash
Function
H(m)
Function H( ) that takes as input an arbitrary length message and outputs a fixed-length string: “message signature”
Note that H( ) is a many-to-1 function
H( ) is often called a “hash function”
MD5, SHA-1
Desirable properties:
Easy to calculate
Irreversibility: Can’t determine m from H(m)
Collision resistance: Computationally difficult to produce m and m’ such that H(m) = H(m’)
Seemingly random output
Source: Kurose at UMass

31. Message Authentication Code (MAC)
compare
s = shared secret
Authenticates sender
Verifies message integrity
No encryption !
Also called “keyed hash”
Notation: MDm = H(s||m) ; send m||MDm
HMAC (Hash-based Message Authentication Code)
Source: Kurose at UMass

32. Digital Signatures data integrity, non-repudiation, authentication
Basic idea
use private key on the message to generate a piece of information that can be generated only by yourself
because you are the only person who knows your private key
public key can be used to verify the signature
so everybody can verify
Generally signatures are created and verified over the hash of the message
Not over the original message. Why?

33. Digital Signature – RSA approach
Sender a
Receiver
M: message to be signed H: Hash function
E: RSA Private Key Operation KRa: Sender’s Private Key
D: RSA Public Key Operation KUa: Sender’s Public Key
EKRa[H(M)] Signature of A over hash of M