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בזמןמי שיגיש את הפרויקט( PA3:) תרגילי בית:6ממוצע של ( הפרויקטPA3)חמשת הטובים )מתוך ששת( התרגילים האחרים( יהיה זהה )(1/6משקל כל תרגיל )מתוך השישה
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Lecture 13: Network Security
Chapter 7: Network security
Foundations: what is security? cryptography authentication message integrity key distribution and certification
Security in practice: application layer: secure e-mail transport layer: Internet commerce, SSL, SET network layer: IP security Firewalls
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Lecture 13: Network Security
What is network security?
Confidentiality: only sender, intended receiver should “understand” message contents sender encrypts message receiver decrypts message
Authentication: sender, receiver want to confirm identity of each other
Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection
Access and Availability: services must be accessible and available to users
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Lecture 13: Network Security
Internet security threatsPacket sniffing:
broadcast media promiscuous NIC reads all packets passing by can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets
A
B
C
src:B dest:A payload
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Lecture 13: Network Security
Internet security threatsIP Spoofing:
can generate “raw” IP packets directly from application, putting any value into IP source address field
receiver can’t tell if source is spoofed e.g.: C pretends to be B
A
B
C
src:B dest:A payload
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Lecture 13: Network Security
Internet security threatsDenial of service (DOS):
flood of maliciously generated packets “swamp” receiver Distributed DOS (DDOS): multiple coordinated sources swamp
receiver e.g., C and remote host SYN-attack A
A
B
C
SYN
SYNSYNSYN
SYN
SYN
SYN
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Cryptography Principles
Lecture 13: Network Security
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Lecture 13: Network Security
Friends and enemies: Alice, Bob, Trudy well-known in network security world Bob, Alice (lovers!) want to communicate “securely” Trudy (intruder) may intercept, delete, add messages
securesender
securereceiver
channel data, control messages
data data
Alice Bob
Trudy
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Lecture 13: Network Security
Who might Bob, Alice be?
… well, real-life Bobs and Alices! Web browser/server for electronic
transactions (e.g., on-line purchases) on-line banking client/server DNS servers routers exchanging routing table updates other examples?
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Lecture 13: Network Security
The language of cryptography
symmetric key crypto: sender, receiver keys identicalpublic-key crypto: encryption key public, decryption
key secret (private)
plaintext plaintextciphertext
KA
encryptionalgorithm
decryption algorithm
Alice’s encryptionkey
Bob’s decryptionkey
KB
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Symmetric Key
CryptographyLecture 13: Network Security
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Lecture 13: Network Security
Symmetric key cryptography
substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another
plaintext: abcdefghijklmnopqrstuvwxyz
ciphertext: mnbvcxzasdfghjklpoiuytrewq
Plaintext: bob. i love you. aliceciphertext: nkn. s gktc wky. mgsbc
E.g.:
Q: How hard to break this simple cipher?:•brute force (how hard?)•other?
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Lecture 13: Network Security
Perfect cipher [Shannon 1948]
Definition: Let C = E[M] Pr[C=c] = Pr[C=c | M]
Example: one time pad Generate random bits b1 ... bn
E[M1 ... Mn] = (M1 b1 ... Mn bn ) Cons: size Pseudo Random Generator
G(R) = b1 ... bn
Indistinguishable from random (efficiently)
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Lecture 13: Network Security
Symmetric key crypto: DES
DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64 bit plaintext input How secure is DES?
DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months
no known “backdoor” decryption approach making DES more secure
use three keys sequentially (3-DES) on each datum use cipher-block chaining
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Lecture 13: Network Security
Symmetric key crypto: DES
initial permutation 16 identical “rounds” of
function application, each using different 48 bits of key
final permutation
DES operation
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Lecture 13: Network Security
Block Cipher chaining
How do we encode a large message Would like to guarantee integrity
Encoding: Ci = E[Mi Ci-1]
Decoding: Mi = D[Ci] Ci-1
Malfunctions: Loss Reorder/ integrity
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7: Network Security 17
Cipher Block Chaining Mode
Cipher block chaining. (a) Encryption. (b) Decryption.
Lecture 13: Network Security
Diffie-Hellman key exchange protocol Goal: Allow strangers establish a shared secret
key for later communication Assume two parties (Alice and Bob) want to
establish a secret key. Alice and Bob agree on two large numbers, n and
g usually, these are publicly known, and have some
additional conditions applied (e.g., n must be prime)
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Lecture 13: Network Security
Diffie-Hellman Key Exchange Alice picks large x, Bob picks large y (e.g., 512 bits)
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Lecture 13: Network Security
Man in the middle attack
Eavesdropper can’t determine secret key (gxy mod n) from (gx mod n) or (gy mod n)
However, how does Alice and Bob know if there is a third party adversary in between?
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Lecture 13: Network Security
Exponentiation
Compute gx mod nExpg,n (x) Assume x = 2y + b Let z = Expg,n (y) R=z2
If (b=1) R = g R mod n Return R
Complexity: logarithmic in x
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Public Key Cryptograph
yLecture 13: Network Security
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Lecture 13: Network Security
Public Key Cryptography
symmetric key crypto
requires sender, receiver know shared secret key
Q: how to agree on key in first place (particularly if never “met”)?
public key cryptography
radically different approach [Diffie-Hellman76, RSA78]
sender, receiver do not share secret key
encryption key public (known to all)
decryption key private (known only to receiver)
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Lecture 13: Network Security
Public key cryptography
plaintextmessage, m
ciphertextencryptionalgorithm
decryption algorithm
Bob’s public key
plaintextmessageK (m)
B+
K B+
Bob’s privatekey
K B-
m = K (K (m))B+
B-
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Lecture 13: Network Security
Public key encryption algorithms
need d ( ) and e ( ) such that
d (e (m)) = m BB
B B. .
need public and private keysfor d ( ) and e ( ). .
BB
Two inter-related requirements:
1
2
RSA: Rivest, Shamir, Adelson algorithm
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Lecture 13: Network Security
RSA: Choosing keys
1. Choose two large prime numbers p, q. (e.g., 1024 bits each)
2. Compute n = pq, z = (p-1)(q-1)
3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”).
4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ).
5. Public key is (n,e). Private key is (n,d).
K B+ K B
-
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Lecture 13: Network Security
RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern, m, compute
c = m mod n
e (i.e., remainder when m is divided by n)e
2. To decrypt received bit pattern, c, compute
m = c mod n
d (i.e., remainder when c is divided by n)d
m = (m mod n)
e mod n
dMagichappens!
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Lecture 13: Network Security
RSA example:
Bob chooses p=5, q=7. Then n=35, z=24.e=5 (so e, z relatively prime).d=29 (so ed-1 exactly divisible by z).
letter m me c = m mod ne
l 12 1524832 17
c m = c mod nd
17 481968572106750915091411825223072000 12
cdletter
l
encrypt:
decrypt:
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Lecture 13: Network Security
RSA: Why m = (m mod n)
e mod n
d
Number theory result:
• IF pq = n, p and q primes then:
x y mod n = x (y mod (p-1)(q-1) ) mod n
• (m e)d mod n = m (ed mod (p-1)(q-1)) mod n
•But ed – 1 divisible by (p-1)(q-1) i.e., ed mod (p-1)(q-1) = 1
• = m 1 mod n = m
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Lecture 13: Network Security
modified Diffie-Hellman Key Exchange
Encrypt 1 with Bob’s public key, 2 with Alice’s public key Prevents man-in-the-middle attack Actually, nonces and a third message are needed to
fully complete this exchange (in a few slides)
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Authentication
Lecture 13: Network Security
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Lecture 13: Network Security
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
Failure scenario??“I am Alice”
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Lecture 13: Network Security
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
in a network,Bob can not “see”
Alice, so Trudy simply declares
herself to be Alice“I am Alice”
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Lecture 13: Network Security
Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packetcontaining her source IP address
Failure scenario??
“I am Alice”Alice’s
IP address
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Lecture 13: Network Security
Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packetcontaining her source IP address
Trudy can createa packet
“spoofing”Alice’s address“I am Alice”
Alice’s IP address
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Lecture 13: Network Security
Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.
Failure scenario??
“I’m Alice”Alice’s IP addr
Alice’s password
OKAlice’s IP addr
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Lecture 13: Network Security
Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it.
playback attack: Trudy records Alice’s
packetand later
plays it back to Bob
“I’m Alice”Alice’s IP addr
Alice’s password
OKAlice’s IP addr
“I’m Alice”Alice’s IP addr
Alice’s password
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Lecture 13: Network Security
Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.
Failure scenario??
“I’m Alice”Alice’s IP addr
encrypted password
OKAlice’s IP addr
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Lecture 13: Network Security
Authentication: another try
Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it.
recordand
playbackstill works!
“I’m Alice”Alice’s IP addr
encryptedpassword
OKAlice’s IP addr
“I’m Alice”Alice’s IP addr
encryptedpassword
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Lecture 13: Network Security
Authentication: yet another try
Goal: avoid playback attack
Failures, drawbacks?
Nonce: number (R) used only once –in-a-lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice
must return R, encrypted with shared secret key“I am Alice”
R
K (R)A-B
Alice is live, and only Alice knows key to encrypt
nonce, so it must be Alice!
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Lecture 13: Network Security
Authentication: ap5.0
ap4.0 requires shared symmetric key can we authenticate using public key techniques?ap5.0: use nonce, public key cryptography
“I am Alice”
RBob computes
K (R)A-
“send me your public key”
K A+
(K (R)) = RA
-K A
+
and knows only Alice could have the
private key, that encrypted R such that
(K (R)) = RA-
K A+
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Lecture 13: Network Security
ap5.0: security holeMan (woman) in the middle attack: Trudy poses
as Alice (to Bob) and as Bob (to Alice)
I am Alice I am Alice
R
TK (R)
-
Send me your public key
TK
+A
K (R)-
Send me your public key
AK
+
TK (m)+
Tm = K (K (m))+
T-
Trudy gets
sends m to Alice encrypted
with Alice’s public key
AK (m)+
Am = K (K (m))+
A-
R
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Lecture 13: Network Security
ap5.0: security holeMan (woman) in the middle attack: Trudy poses
as Alice (to Bob) and as Bob (to Alice)
Difficult to detect: Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation) problem is that Trudy receives all messages as well!
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Message Integrity
(Signatures etc)
Lecture 13: Network Security
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Lecture 13: Network Security
Digital Signatures
Cryptographic technique analogous to hand-written signatures.
Sender (Bob) digitally signs document, establishing he is document owner/creator.
Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document.
Assumption: eB(dB(m)) = dB(eB(m)) RSA
Simple digital signature for message m:
Bob decrypts m with his private key dB, creating signed message, dB(m).
Bob sends m and dB(m) to Alice.
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Lecture 13: Network Security
Digital Signatures (more)
Suppose Alice receives msg m, and digital signature dB(m)
Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m.
If eB(dB(m) ) = m, whoever signed m must have used Bob’s private key.
Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not
m’.Non-repudiation:
Alice can take m, and signature dB(m) to court and prove that Bob signed m.
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Lecture 13: Network Security
Message Digests
Computationally expensive to public-key-encrypt long messages
Goal: fixed-length,easy to compute digital signature, “fingerprint”
apply hash function H to m, get fixed size message digest, H(m).
Hash function properties: Many-to-1 Produces fixed-size msg
digest (fingerprint) Given message digest x,
computationally infeasible to find m such that x = H(m)
computationally infeasible to find any two messages m and m’ such that H(m) = H(m’).
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Lecture 13: Network Security
Hash Function Algorithms
Internet checksum would make a poor message digest. Too easy to find
two messages with same checksum.
MD5 hash function widely used. Computes 128-bit
message digest in 4-step process.
arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x.
SHA-1 is also used. US standard 160-bit message digest
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Lecture 13: Network Security
large message
mH: Hashfunction H(m)
digitalsignature(encrypt)
Bob’s private
key K B-
+
Bob sends digitally signed message:
Alice verifies signature and integrity of digitally signed message:
KB(H(m))-
encrypted msg digest
KB(H(m))-
encrypted msg digest
large message
m
H: Hashfunction
H(m)
digitalsignature(decrypt)
H(m)
Bob’s public
key K B+
equal ?
Digital signature = signed message digest
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Key Distribution
CentersLecture 13: Network Security
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Lecture 13: Network Security
Problems with Public-Key Encryption A way for Trudy to subvert public-key
encryption.
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Lecture 13: Network Security
Trusted Intermediaries
Problem: How do two
entities establish shared secret key over network?
Solution: trusted key
distribution center (KDC) acting as intermediary between entities
Problem: When Alice obtains
Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s?
Solution: trusted certification
authority (CA)
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Lecture 13: Network Security
Key Distribution Center (KDC)
Alice,Bob need shared symmetric key.
KDC: server shares different secret key with each registered user.
Alice, Bob know own symmetric keys, KA-
KDC KB-KDC , for communicating with KDC.
Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1)
Alice sends Bob KB-KDC(A,R1), Bob extracts R1
Alice, Bob now share the symmetric key R1.
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Lecture 13: Network Security
Certification Authorities
Certification authority (CA) binds public key to particular entity.
Entity (person, router, etc.) can register its public key with CA. Entity provides “proof
of identity” to CA. CA creates certificate
binding entity to public key.
Certificate digitally signed by CA.
When Alice wants Bob’s public key:
gets Bob’s certificate (Bob or elsewhere).
Apply CA’s public key to Bob’s certificate, get Bob’s public key
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Lecture 13: Network Security
X.509 is the standard for certificates The basic fields of an X.509 certificate.
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Lecture 13: Network Security
Public-Key Infrastructures
PKIs are a way to structure certificates (a) A hierarchical PKI. (b) A chain of
certificates.
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Lecture 13: Network Security
Example revisited (solved with certificates) Trudy presents Alice a certificate,
purporting to be Bob, but Alice is unable to trace Trudy’s certificate back to a trusted root
Be wary if your browser warns about certs!
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Secure email
Lecture 13: Network Security
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Lecture 13: Network Security
Secure e-mail
Alice: generates random symmetric private key, KS. encrypts message with KS (for efficiency) also encrypts KS with Bob’s public key. sends both KS(m) and KB(KS) to Bob.
Alice wants to send confidential e-mail, m, to Bob.
KS( ).
KB( ).+
+ -
KS(m
)
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m
)
KB(KS )+
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Lecture 13: Network Security
Secure e-mail
Bob: uses his private key to decrypt and recover KS
uses KS to decrypt KS(m) to recover m
Alice wants to send confidential e-mail, m, to Bob.
KS( ).
KB( ).+
+ -
KS(m
)
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m
)
KB(KS )+
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Lecture 13: Network Security
Secure e-mail (continued)
• Alice wants to provide sender authentication message integrity.
• Alice digitally signs message.• sends both message (in the clear) and digital signature.
H( ). KA( ).-
+ -
H(m )KA(H(m))-
m
KA-
Internet
m
KA( ).+
KA+
KA(H(m))-
mH( ). H(m )
compare
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Lecture 13: Network Security
Pretty good privacy (PGP)
Internet e-mail encryption scheme, a de-facto standard.
Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described.
Provides secrecy, sender authentication, integrity.
Inventor, Phil Zimmerman, was target of 3-year federal investigation.
---BEGIN PGP SIGNED MESSAGE---Hash: SHA1
Bob:My husband is out of town tonight.Passionately yours, Alice
---BEGIN PGP SIGNATURE---Version: PGP 5.0Charset: noconvyhHJRHhGJGhgg/
12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2
---END PGP SIGNATURE---
A PGP signed message:
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Secure Socket layer
(SSl)Lecture 13: Network Security
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Lecture 13: Network Security
Secure sockets layer (SSL)
PGP provides security for a specific network app.
SSL works at transport layer. Provides security to any TCP-based app using SSL services.
SSL: used between WWW browsers, servers for E-commerce (https).
SSL security services: server authentication data encryption client authentication
(optional)
Server authentication: SSL-enabled browser
includes public keys for trusted CAs.
Browser requests server certificate, issued by trusted CA.
Browser uses CA’s public key to extract server’s public key from certificate.
Visit your browser’s security menu to see its trusted CAs.
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Lecture 13: Network Security
Internet Explorer:Tools Internet options Content Certificates
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Lecture 13: Network Security
Internet Explorer: Error Message
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Lecture 13: Network Security
SSL (continued)
Encrypted SSL session: Browser generates
symmetric session key, encrypts it with server’s public key, sends encrypted key to server.
Using its private key, server decrypts session key.
Browser, server agree that future msgs will be encrypted.
All data sent into TCP socket (by client or server) is encrypted with session key.
SSL: basis of IETF Transport Layer Security (TLS).
SSL can be used for non-Web applications, e.g., IMAP.
Client authentication can be done with client certificates.
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SSL basics
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Lecture 13: Network Security 68
Access Control in
the NetworkLecture 13: Network Security
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Lecture 13: Network Security
Firewalls
Two firewall types: packet filter application
gateways
To prevent denial of service attacks: SYN flooding: attacker
establishes many bogus TCP connections. Attacked host alloc’s TCP buffers for bogus connections, none left for “real” connections.
To prevent illegal modification of internal data. e.g., attacker replaces
CIA’s homepage with something else
To prevent intruders from obtaining secret info.
isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.
firewall
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Lecture 13: Network Security
Packet Filtering
Internal network is connected to Internet through a router.
Router manufacturer provides options for filtering packets, based on: source IP address destination IP address TCP/UDP source and
destination port numbers
ICMP message type TCP SYN and ACK bits
Example 1: block incoming and outgoing datagrams with IP protocol field = 17 and with either source or dest port = 23. All incoming and outgoing
UDP flows and telnet connections are blocked.
Example 2: Block inbound TCP segments with ACK=0. Prevents external clients
from making TCP connections with internal clients, but allows internal clients to connect to outside.
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Lecture 13: Network Security
Virtual Private Networks
Using firewalls and IPsec encryption to provide a “leased-line” like connection over the Internet
(a) A leased-line private network. (b) A virtual private network.
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Lecture 13: Network Security
Application gateways
Filters packets on application data as well as on IP/TCP/UDP fields.
Example: allow select internal users to telnet outside.
host-to-gatewaytelnet session
gateway-to-remote host telnet session
applicationgateway
router and filter
1. Require all telnet users to telnet through gateway.2. For authorized users, gateway sets up telnet
connection to dest host. Gateway relays data between 2 connections
3. Router filter blocks all telnet connections not originating from gateway.
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Lecture 13: Network Security
Limitations of firewalls and gateways
IP spoofing: router can’t know if data “really” comes from claimed source
If multiple app’s. need special treatment, each has own app. gateway.
Client software must know how to contact gateway. e.g., must set IP
address of proxy in Web browser
Filters often use all or nothing policy for UDP.
Tradeoff: degree of communication with outside world, level of security
Many highly protected sites still suffer from attacks.
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Lecture 13: Network Security
Network Security (summary)
Basic techniques…... cryptography (symmetric and public) authentication message integrity…. used in many different security scenarios secure email secure transport (SSL) IP sec Firewalls
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