程序代写代做代考 algorithm 8-1

8-1

Network Security

All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved

George Parisis
School of Engineering and Informatics

University of Sussex

8-2 Network Security

Network Security

v  What is network security?
v  Principles of cryptography
v  Message integrity, authentication
v  Securing e-mail
v  Securing TCP connections: SSL
v  Network layer security: IPsec
v  Operational security: firewalls and IDS

8-3 Network Security

Message digests

computationally
expensive to public-
key-encrypt long
messages

goal: fixed-length, easy-
to-compute digital
“fingerprint”

v  apply hash function H to
m, get fixed size
message digest, H(m).

Hash function properties:
v  produces fixed-size msg

digest (fingerprint)
v  given message digest x,

computationally infeasible
to find m such that x =
H(m)

large
message

m

H: Hash
Function

H(m)

8-4 Network Security

Internet checksum: poor crypto hash
function
Internet checksum has some properties of hash function:
➼  produces fixed length digest (16-bit sum) of message
➼  is many-to-one

But given message with given hash value, it is easy to find
another message with same hash value:

I O U 1
0 0 . 9

9 B O B

49 4F 55 31
30 30 2E 39
39 42 D2 42

message ASCII format

B2 C1 D2 AC

I O U 9
0 0 . 1

9 B O B

49 4F 55 39
30 30 2E 31
39 42 D2 42

message ASCII format

B2 C1 D2 AC different messages
but identical checksums!

8-5 Network Security

Hash function algorithms

v  MD5 hash function widely used (RFC 1321)
§  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
v  SHA-1 is also used

§  US standard [NIST, FIPS PUB 180-1]
§  160-bit message digest

8-6

Secrecy and Integrity (and
authenticity)

•  Alice and Bob share a secret key KAB
•  Alice uses KAB and an agreed encryption function E(KAB, M) to encrypt and

send any number of messages {Mi}KAB to Bob

•  Bob reads the encrypted messages using the decryption function D(KAB, M)
•  Alice and Bob can go on using KAB as long as it is safe to assume that the

key has not been compromised

If the decrypted message makes sense (what does that mean?) then Bob
knows that the message is from Alice and that it hasn’t been tampered with

•  Each message can contain an agreed value
•  It makes sense if this value is in the message – what value to use?

8-7

Secrecy and Integrity (and
authenticity)

•  Alice can include a Message Authentication Code (MAC) in the plaintext
•  A MAC can be the result of applying a secure hash function to the message
•  Computationally infeasible to construct another message which, when

decrypted, will contain the same MAC

•  Bob verifies that the MAC is correct by applying the same hash function to

the decrypted message

Bob can infer that the sender of the message possessed the corresponding

encryption key and hence deduce the identity of the sender if the key is known
only to two parties

•  Message integrity is also preserved

8-8 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”

8-9 Network Security

in a network,
Bob can not “see” Alice,
so Trudy simply declares

herself to be Alice “I am Alice”

Authentication
Goal: Bob wants Alice to “prove” her identity to

him
Protocol ap1.0: Alice says “I am Alice”

8-10 Network Security

Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet

containing her source IP address

Failure scenario??

“I am Alice”
Alice’s

IP address

8-11 Network Security

Trudy can create
a packet
“spoofing”

Alice’s address “I am Alice”
Alice’s

IP address

Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet

containing her source IP address

8-12 Network Security

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

OK Alice’s
IP addr

Authentication: another try

8-13 Network Security

playback attack: Trudy
records Alice’s packet

and later
plays it back to Bob

“I’m Alice” Alice’s
IP addr

Alice’s
password

OK Alice’s
IP addr

“I’m Alice” Alice’s
IP addr

Alice’s
password

Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.

Authentication: another try

8-14 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

OK Alice’s
IP addr

8-15 Network Security

record
and

playback
still works!

“I’m Alice” Alice’s
IP addr

encrypted
password

OK Alice’s
IP addr

“I’m Alice” Alice’s
IP addr

encrypted
password

Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her

encrypted secret password to “prove” it.

8-16 Network Security

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!

Authentication: yet another try

8-17 Network Security

Authentication: ap5.0
ap4.0 requires shared symmetric key
v  can we authenticate using public key techniques?
ap5.0: use nonce, public key cryptography

“I am Alice”

R
Bob computes

K (R) A

“send me your public key”

K A
+

(K (R)) = R A

K A
+

and knows only Alice
could have the private
key, that encrypted R

such that
(K (R)) = R A


K A

+

8-18 Network Security

ap5.0: security
hole
man (or woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

I am Alice I am Alice
R

T
K (R)

Send me your public key

T
K

+
A

K (R)

Send me your public key

A
K +

T K (m)
+

T
m = K (K (m)) +

T

Trudy gets

sends m to Alice
encrypted with

Alice’s public key

A K (m)
+

A
m = K (K (m)) +

A

R

8-19 Network Security

difficult to detect:
v  Bob receives everything that Alice sends, and vice

versa. (e.g., so Bob, Alice can meet one week later
and recall conversation!)

v  problem is that Trudy receives all messages as well!

ap5.0: security
hole
man (or woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

8-20 Network Security

Digital signatures

cryptographic technique analogous to hand-
written signatures:

v  sender (Bob) digitally signs document,
establishing he is document owner/creator.

v  verifiable, non-forgeable: recipient (Alice) can
prove to someone that Bob, and no one else
(including Alice), must have signed document

8-21 Network Security

simple digital signature for message m:
v  Bob signs m by encrypting with his private key KB,

creating “signed” message, KB(m)

Dear Alice
Oh, how I have missed
you. I think of you all the
time! …(blah blah blah)

Bob

Bob’s message, m

Public key
encryption
algorithm

Bob’s private
key

K B

Bob’s message,
m, signed

(encrypted) with
his private key

m,K B

(m)

Digital signatures

8-22 Network Security

Alice thus verifies that:
➼ Bob signed m
➼ no one else signed m
➼ Bob signed m and not m‘

non-repudiation:
ü  Alice can take m, and signature KB(m) to court

and prove that Bob signed m

Digital signatures
v  suppose Alice receives msg m, with signature: m, KB(m)
v  Alice verifies m signed by Bob by applying Bob’s public

key KB to KB(m) then checks KB(KB(m) ) = m.
v  If KB(KB(m) ) = m, whoever signed m must have used

Bob’s private key.

– –

+

+ +

8-23 Network Security

large
message

m
H: Hash
function H(m)

digital
signature
(encrypt)

Bob’s
private

key K B

+

Bob sends digitally signed
message:

Alice verifies signature,
integrity of digitally signed
message:

KB(H(m))

encrypted
msg digest

KB(H(m))

encrypted
msg digest

large
message

m

H: Hash
function

H(m)

digital
signature
(decrypt)

H(m)

Bob’s
public

key K B
+

equal
?

Digital signature = signed message
digest

8-24 Network Security

Recall: ap5.0 security
hole
man (or woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

I am Alice I am Alice
R

T
K (R)

Send me your public key

T
K

+
A

K (R)

Send me your public key

A
K +

T K (m)
+

T
m = K (K (m)) +

T

Trudy gets

sends m to Alice
encrypted with

Alice’s public key

A K (m)
+

A
m = K (K (m)) +

A

R

8-25 Network Security

Public-key certification
v  motivation: Trudy plays pizza prank on Bob

§  Trudy creates e-mail order:
Dear Pizza Store, Please deliver to me four
pepperoni pizzas. Thank you, Bob

§  Trudy signs order with her private key
§  Trudy sends order to Pizza Store
§  Trudy sends to Pizza Store her public key, but

says it’s Bob’s public key
§  Pizza Store verifies signature; then delivers four

pepperoni pizzas to Bob
§  Bob doesn’t even like pepperoni

8-26 Network Security

Certification authorities
v  certification authority (CA): binds public key to

particular entity, E.
v  E (person, router) registers its public key with CA.

§  E provides “proof of identity” to CA.
§  CA creates certificate binding E to its public key.
§  certificate containing E’s public key digitally signed by CA –

CA says “this is E’s public key”

Bob’s
public

key K B
+

Bob’s
identifying

information

digital
signature
(encrypt)

CA
private

key K CA

K B
+

certificate for
Bob’s public key,

signed by CA

8-27 Network Security

v  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

Bob’s
public

key K B
+

digital
signature
(decrypt)

CA
public

key
K CA

+

K B
+

Certification authorities

8-28 Network Security

Network Security (summary)

basic techniques……
§  message integrity
§  end-point authentication