i-4 security
Security taxonomy
• Physical security
• Resource exhaustion
• Key-based security
• cryptography
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
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
secure
sender
data, control
messages
secure
receiver
data
Trudy
Source: Kurose at UMass
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
Thwart the attacks!
• Basic Security services
– authentication
• Access control
– confidentiality
– Data (or message) integrity
– Non-repudiation
More Security services
•
•
•
•
•
Anonymity
Availability
Accountability
Privacy
forensics
Security mechanisms
• Encipherment
– 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
Meaning of Cryptography
• from Greek
– Cryptos: secret, hidden
– graphos: writing
– cryptography: study of secret writing
Basics
Encryption key
Message
(plaintext,
cleartext)
Encryption
(Encipherment)
Ciphertext
(cryptogram)
Decryption key
Decryption
(Decipherment)
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
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
Symmetric cryptography
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
Symmetric Cipher Model
there must be a
secure mechanism
for the distribution of
this key a priori
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
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!
Some primitives
• Substitution
• Permutation
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
Asymmetric cryptography
Or Public key cryptography (PKC)
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
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
Public-Key Cryptography: Encryption
Bob
Alice
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
Public-Key Cryptography - Authentication
Commutative!
Bob
Alice
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
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
26
D-H Algorithm
gx mod p
Alice
Bob
gy mod p
• 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
27
PKC Applications
• 3 categories
– encryption/decryption
• 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?
MESSAGE INTEGRITY
Message Digest
• Function H( ) that takes as
input an arbitrary length
message and outputs a fixedlength string: “message
signature”
• Note that H( ) is a many-to-1
function
• H( ) is often called a “hash
function”
– MD5, SHA-1
large
message
m
H: Hash
Function
H(m)
• 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
Message Authentication Code (MAC)
s = shared secret
message
s
message
message
s
H( )
compare
H( )
•
•
•
•
•
•
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
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?
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