1. TELE3119: Trusted Networks Week 2
Course Coordinator:
Prof. Aruna Seneviratne, Room EE 312 –
Course web-page:

2. Basic View of Cryptography
Access &
Availability
Confidentiality
Integrity
Authentication
Cryptography
Symmetric
Asymmetric
Protocols
Stream Ciphers
Block Ciphers
Trusted Networks

3. Key distribution?
In a symmetric crypto system how would you distribute the keys?
A key could be selected by A and physically delivered to B.
A third party could select the key and physically deliver it to A and B.
If A and B have previously and recently used a key, one party could transmit the new key to the other, encrypted using the old key.
If A and B each have an encrypted connection to a third party C (i.e. a key distribution center), C could deliver a key on the encrypted links to A and B.
But for A communicating to B for the first time, the only viable option is (1) which is not practical specially for real time communications

4. Public Key Cryptography
Radically different approach
[Diffie-Hellman76, RSA78]
sender, receiver do not share secret key
public encryption key known to all
private decryption key known only to receiver

5. Public Key Cryptography - Operation+ K
Bob’s public
key B -
Bob’s private
key K B
plaintext
message, m
encryption
algorithm
ciphertext
decryption
algorithm
plaintext
message
K (m) B +
m = K (K (m)) B + -

6. Public Key Encryption Algorithms
requirements: . . + - 1
need K ( ) and K ( ) such that B B
K (K (m)) = m B - + + 2
given public key K , it should be impossible to compute private key K B - B
RSA: Rivest, Shamir, Adelson algorithm

7. Prerequisite: Modular Arithmetic
Let a, r, m ε ℤ and m > 0 then
a ≡ r mod m if m | (a-r) [m divides (a-r)]
x mod n = remainder of x when divided by n
facts:
[(a mod n) + (b mod n)] mod n = (a+b) mod n
[(a mod n) - (b mod n)] mod n = (a-b) mod n
[(a mod n) * (b mod n)] mod n = (a*b) mod n
thus
(a mod n)d mod n = ad mod n
example: x=14, n=10, d=2:
(x mod n)d mod n = 42 mod 10 = 6
xd = 142 = xd mod 10 = 6

8. RSA: Preliminaries (1) Example: Message (m): just a bit pattern
bit pattern can be uniquely represented by an integer number
encrypting a message is equivalent to encrypting a number
Example: m=
Uniquely represented by the decimal number 145.
Encrypt m,
Encrypt the 145, which gives a new number (the ciphertext).

9. RSA: Preliminaries (2)
A prime number is a whole number greater than 1 whose only factors are 1 and itself.
A factor is a whole numbers that can be divided evenly into another number. 
The greatest common factor, or GCF, is the greatest factor that divides two numbers.
List the prime factors of each number.
Multiply those factors both numbers have in common.
If there are no common prime factors, the GCF is 1.
Two integers a and b are said to be relatively prime, mutually prime, or coprime if the only positive integer (factor) that divides both of them is 1.
Trusted Networks

10. RSA: Creating public/private key pair
1. choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. Compute the modulus n = pq and the totient , ϕ (n) or
z = (p-1)(q-1)
3. Choose an integer e (with 1<e<z) that has no common factors
with z (e, z are “relatively prime” or “co-prime”).
4. choose d such that e.d-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 + -

11. RSA: encryption, decryption
0. given (n,e) and (n,d) as computed above
1. to encrypt message m (<n), compute
c = m mod n e
2. to decrypt received bit pattern, c, compute
m = c mod n d
magic
happens!
m = (m mod n) e
mod n d c

12. Bob chooses p=5, q=7. Then n=35, z=24.
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).
encrypting 8-bit messages. e
c = m mod n e
bit pattern m m
encrypt:
0000l000 12
24832 17 c
m = c mod n d 17 12
decrypt:

13. Why does RSA work? Encryption: c = me mod n Decryption: m = cd mod n
Given
pub = <e,n> and priv = <d, n>
n = p x q, z = (p-1)(q-1)
e.d = 1 mod z
x e. d = x mod n
Encryption: c = me mod n
Decryption: m = cd mod n
(me mod n)d mod n = me . d mod n
m mod n = m
since m <n

14. Example Alice and Bob get public numbers
p = 23, g = 9
Alice and Bob compute public values
X = 94 mod 23 = mod 23 = 6
Y = 93 mod 23 = 729 mod = 16
Alice and Bob exchange public numbers

15. Example (cnt’d) Alice and Bob compute symmetric keys
ka = ya mod p = 164 mod 23 = 9
kb = xb mod p = 63 mod 23 = 9

16. RSA: another important property
The following property will be very useful later:
K (K (m)) = m B - +
K (K (m)) =
use public key first, followed by private key
use private key first, followed by public key
result is the same!

17. Why ? K (K (m)) = m K (K (m)) - + = B
follows directly from modular arithmetic:
(me mod n)d mod n = med mod n
= mde mod n
= (md mod n)e mod n

18. Why is RSA secure? Fact: factoring a big number is hard
Suppose you know Bob’s public key (n,e). How hard is it to determine d?
Essentially need to find factors of n without knowing the two factors p and q
Fact: factoring a big number is hard

19. RSA in practice: session keys
Exponentiation in RSA is computationally intensive
DES is at least 100 times faster than RSA
Use public key crypto to establish secure connection, then establish second key – symmetric session key – for encrypting data
session key, KS
Bob and Alice use RSA to exchange a symmetric key KS
Once both have KS, they use symmetric key cryptography

20. Summary Asymmetric Encryption RSA Algorithm Why RSA works
Trusted Networks

21. Basic View of Cryptography
Access &
Availability
Confidentiality
Integrity
Authentication
Cryptography
Symmetric
Asymmetric
Protocols
Stream Ciphers
Block Ciphers
Trusted Networks

22. Protocol ap1.0: Alice says “I am Alice”
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
“I am Alice”
Failure scenario??
Security
8-22

23. 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”
Security
8-23

24. Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
“I am Alice”
Alice’s
IP address
Failure scenario??
Security
8-24

25. Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
Trudy can create
a packet “spoofing”
Alice’s address
“I am Alice”
Alice’s
IP address
Security
8-25

26. Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
“I’m Alice”
Alice’s
IP addr
password
Failure scenario?? OK
Alice’s
IP addr
Security
8-26

27. Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Alice’s
IP addr
Alice’s
password
“I’m Alice”
playback attack: Trudy records Alice’s packet
and later
plays it back to Bob OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
password
Security
8-27

28. Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
“I’m Alice”
Alice’s
IP addr
encrypted
password
Failure scenario?? OK
Alice’s
IP addr
Security
8-28

29. Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
Alice’s
IP addr
encrypted
password
“I’m Alice”
record
and
playback
still works! OK
Alice’s
IP addr
“I’m Alice”
Alice’s
IP addr
encrypted
password
Security
8-29

30. Authentication: yet another try
Goal: avoid playback attack
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!
Failures, drawbacks?
Security
8-30

31. “send me your public key”
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”
Bob computes R
(K (R)) = R A - K +
K (R) A -
and knows only Alice could have the private key, that encrypted R such that
“send me your public key” K A +
(K (R)) = R A - K +
Security
8-31

32. sends m to Alice encrypted with Alice’s public key
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) - R
Send me your public key A
K (R) - T K +
Send me your public key A K + T
K (m) +
Trudy gets T
m = K (K (m)) + - A
K (m) + A
m = K (K (m)) + -
sends m to Alice encrypted with Alice’s public key
Security
8-32

33. ap5.0: security hole difficult to detect:
man (or 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!
Security
8-33

34. Summary
Understand the challenges of authentication
Trusted Networks

35. Basic View of Cryptography
Access &
Availability
Confidentiality
Integrity
Authentication
Cryptography
Symmetric
Asymmetric
Protocols
Stream Ciphers
Block Ciphers
Trusted Networks

36. Digital signatures (1)
cryptographic technique analogous to hand-written signatures:
sender (Bob) digitally signs document, establishing he is document owner/creator.
verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document

37. Bob’s message, m, signed (encrypted) with his private key
Digital signatures (2)
simple digital signature for message m:
Bob signs m by encrypting with his private key KB, creating “signed” message, KB(m) - - K B -
Bob’s message, m
Bob’s private
key -
m,K
(m) B
Dear Alice
Oh, how I have missed you. I think of you all the time! …(blah blah blah)
Bob
Bob’s message, m, signed (encrypted) with his private key
Public key
encryption
algorithm

38. Digital signatures (3) -
suppose Alice receives msg m, with signature: m, KB(m)
Alice verifies m signed by Bob by applying Bob’s public key KB to KB(m) then checks KB(KB(m) ) = m.
If KB(KB(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 KB(m) to court and prove that Bob signed m -

39. Message Digests
large
Message m
H: Hash
Function
computationally expensive to public-key-encrypt long messages
goal: fixed-length, easy- to-compute digital “fingerprint”
apply hash function H to m, get fixed size message digest, H(m).
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)

40. 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:
message
ASCII format
message
ASCII format
I O U 1
9 B O B
49 4F 55 31
E 39
39 42 D2 42
I O U 9
9 B O B
49 4F 55 39
E 31
39 42 D2 42
B2 C1 D2 AC
B2 C1 D2 AC
different messages
but identical checksums!

41. Digital signature = signed message digest
Bob sends digitally signed message:
Alice verifies signature, integrity of digitally signed message:
large
message m
H: Hash
function
H(m)
KB(H(m)) -
encrypted
msg digest
Bob’s
private
key
digital
signature
(encrypt)
large
message m K B -
Bob’s
public
key
digital
signature
(decrypt) K B +
H: Hash
function
KB(H(m)) -
encrypted
msg digest +
H(m)
H(m)
Equal ?

42. Hash Function Algorithms
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
SHA-1 is also used
US standard [NIST, FIPS PUB 180-1]
160-bit message digest

43. Public-key Certification
motivation: Trudy plays pizza prank on Bob
Trudy creates 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
Security
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44. Certification Authorities
certification authority (CA): binds public key to particular entity, E.
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”
digital
signature
(encrypt) K B +
Bob’s
public
key K B + CA
private
key
certificate for Bob’s public key, signed by CA -
Bob’s
identifying information K CA
Security
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45. Certification Authorities
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 K B +
digital
signature
(decrypt)
Bob’s
public
key K B + CA
public
key + K CA
Security
8-45

46. Secure e-mail . - Alice wants to send confidential e-mail, m, to Bob.
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
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
Security
8-46

47. Secure e-mail . - Alice wants to send confidential e-mail, m, to Bob.
KS( ) .
KB( ) + -
KS(m )
KB(KS ) m KS KB
Internet
Bob:
uses his private key to decrypt and recover KS
uses KS to decrypt KS(m) to recover m
Security
8-47

48. Secure e-mail (continued)
Alice wants to provide sender authentication message integrity
H( ) .
KA( ) - +
H(m )
KA(H(m)) m KA
Internet
compare
Alice digitally signs message
sends both message (in the clear) and digital signature
Security
8-48

49. Secure e-mail (continued)
Alice wants to provide secrecy, sender authentication, message integrity.
H( ) .
KA( ) - +
KA(H(m)) m KA
KS( )
KB( )
KB(KS ) KS KB
Internet
Alice uses three keys: her private key, Bob’s public key, newly created symmetric key (Pretty Good Privacy – PGP)
Security
8-49

50. Summary How integrity can be maintained Digital Signatures
Message Digests
Need for Certification Authorities
Trusted Networks

51. Basic View of Cryptography
Access &
Availability
Confidentiality
Integrity
Authentication
Cryptography
Symmetric
Asymmetric
Protocols
Transport Layer
Stream Ciphers
Block Ciphers
Trusted Networks

52. SSL: Secure Sockets Layer
widely deployed security protocol
supported by almost all browsers, web servers
https
billions $/year over SSL
mechanisms: [Woo 1994], implementation: Netscape
variation -TLS: transport layer security, RFC 2246
provides
confidentiality
integrity
authentication
original goals:
Web e-commerce transactions
encryption (especially credit-card numbers)
Web-server authentication
optional client authentication
minimum hassle in doing business with new merchant
available to all TCP applications
secure socket interface
Security
8-52

53. SSL and TCP/IP
Application
TCP IP
normal application
Application
SSL
TCP IP
application with SSL
SSL provides application programming interface (API) to applications
C and Java SSL libraries/classes readily available
Security
8-53

54. Could do something like PGP:KA -
H( ) .
KA( ) - +
KA(H(m)) m
KS( )
KB( ) KS KB
Internet
KB(KS ) +
But want
To send byte streams & interactive data
Set of secret keys for entire connection
Certificate exchange as part of protocol: handshake phase
Security
8-54

55. Toy SSL: a simple secure channel
handshake: Alice and Bob use their certificates, private keys to authenticate each other and exchange shared secret
key derivation: Alice and Bob use shared secret to derive set of keys
data transfer: Data to be transferred is broken up into series of records
connection closure: Special messages to securely close connection
Security
8-55

56. Toy: a simple handshakeCA
hello
Cryto Algorithms agreement
Session ID
Servers Digital Certificate
Servers public key
public key certificate
KB+(MS) = EMS
finished
Encyrpted with EMS
MS: master secret
EMS: encrypted master secret
Security
8-56

57. Toy: key derivation
Considered bad to use same key for more than one cryptographic operation
Use different keys for message authentication code (MAC) and encryption
Four keys:
Kc = encryption key for data sent from client to server
Mc = MAC key for data sent from client to server
Ks = encryption key for data sent from server to client
Ms = MAC key for data sent from server to client
Keys derived from key derivation function (KDF)
Takes master secret and (possibly) some additional random data and creates the keys
Security
8-57

58. Toy: data records
Why not encrypt data in constant stream as we write it to TCP?
where would we put the MAC? If at end, no message integrity until all data processed.
e.g., with instant messaging, how can we do integrity check over all bytes sent before displaying?
Solution: Break stream in series of records
each record carries a MAC
receiver can act on each record as it arrives
Issue: in record, receiver needs to distinguish MAC from data
Solution: include a length
want to use variable-length records
length
data
MAC
Security
8-58

59. Toy: sequence numbers
Issue: attacker can capture and replay record or re-order records
Solution: put sequence number into MAC:
MAC = MAC(Mx, sequence||data)
note: no sequence number field
Issue: attacker could replay all records
Solution: use nonce
Security
8-59

60. Toy: control information
Issue: truncation attack:
attacker forges TCP connection close segment
one or both sides thinks there is less data than there actually is.
Solution: record types, with one type for data and another for closure
type 0 for data; type 1 for closure
MAC = MAC(Mx, sequence || type || data)
length
type
data
MAC
Security
8-60

61. Toy SSL: summary hello certificate, nonce KB+(MS) = EMS
type 0, seq 1, data
type 0, seq 2, data
type 0, seq 3, data
type 1, seq 4, close
type 1, seq 2, close
bob.com
encrypted
Security
8-61

62. Toy SSL isn’t complete Issues: Solution: negotiation - Allow
How long are fields?
Which encryption protocols?
Solution: negotiation - Allow
Client and server to support different encryption algorithms
Client and server to choose together specific algorithm before data transfer
Security
8-62

63. SSL cipher suite Cipher Suite SSL supports several cipher suites
public-key algorithm
symmetric encryption algorithm
MAC algorithm
SSL supports several cipher suites
Negotiation: client, server agree on cipher suite
client offers choice
server picks one
common SSL symmetric ciphers
DES – Data Encryption Standard: block
3DES – Triple strength: block
RC2 – Rivest Cipher 2: block
RC4 – Rivest Cipher 4: stream
SSL Public key encryption
RSA
Security
8-63

64. Real SSL: handshake (1) Purpose server authentication
negotiation: agree on crypto algorithms
establish keys
client authentication (optional)
Security
8-64

65. Real SSL: handshake (2)
client sends list of algorithms it supports, along with client nonce
server chooses algorithms from list; sends back: choice + certificate + server nonce
client verifies certificate, extracts server’s public key, generates pre_master_secret, encrypts with server’s public key, sends to server
client and server independently compute encryption and MAC keys from pre_master_secret and nonces
client sends a MAC of all the handshake messages
server sends a MAC of all the handshake messages
Security
8-65

66. Real SSL: handshaking (3)
last 2 steps protect handshake from tampering
client typically offers range of algorithms, some strong, some weak
man-in-the middle could delete stronger algorithms from list
last 2 steps prevent this
last two messages are encrypted
Security
8-66

67. Real SSL: handshaking (4)
why two random nonces?
suppose Trudy sniffs all messages between Alice & Bob
next day, Trudy sets up TCP connection with Bob, sends exact same sequence of records
Bob (Amazon) thinks Alice made two separate orders for the same thing
solution: Bob sends different random nonce for each connection. This causes encryption keys to be different on the two days
Trudy’s messages will fail Bob’s integrity check
Security
8-67

68. SSL record protocol record header: content type; version; length
data
fragment
MAC
encrypted
data and MAC
record
header
record header: content type; version; length
MAC: includes sequence number, MAC key Mx
fragment: each SSL fragment 214 bytes (~16 Kbytes)
Security
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69. SSL record format data and MAC encrypted (symmetric algorithm) content
type
SSL version
length
MAC
data
1 byte
2 bytes
3 bytes
data and MAC encrypted (symmetric algorithm)
Security
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70. Real SSL connection everything henceforth is encrypted TCP FIN follows
handshake: ClientHello
handshake: ServerHello
handshake: Certificate
handshake: ServerHelloDone
handshake: ClientKeyExchange
ChangeCipherSpec
handshake: Finished
application_data
Alert: warning, close_notify
Real SSL connection
everything
henceforth
is encrypted
TCP FIN follows
Security
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71. Key Derivation
client nonce, server nonce, and pre-master secret input into pseudo random-number generator.
produces master secret
master secret and new nonces input into another random-number generator: “key block”
because of resumption: TBD
key block sliced and diced:
client MAC key
server MAC key
client encryption key
server encryption key
client initialization vector (IV)
server initialization vector (IV)
Security
8-71

72. Summary How security can be provided at the transport layer SSL/HTTPs
Trusted Networks

73. Basic View of Cryptography
Access &
Availability
Confidentiality
Integrity
Authentication
Cryptography
Symmetric
Asymmetric
Protocols
Network Layer
Stream Ciphers
Block Ciphers
Trusted Networks

74. What is network-layer confidentiality ?
Between two network entities:
Sending entity encrypts datagram payload, payload could be:
TCP or UDP segment, ICMP message, OSPF message ….
All data sent from one entity to other would be hidden:
Web pages, , P2P file transfers, TCP SYN packets …
Provides “blanket coverage”
Security
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75. IPsec services Data Integrity Origin Authentication
Replay Attack Prevention
Confidentiality
Two protocols providing different service models:
Authentication Header (AH) Protocol
Encapsulation Security (ES) Protocol
Security
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76. IPsec Transport Mode
IPsec
The transport mode encrypts only the payload and ESP trailer; so the IP header of the original packet is not encrypted.
The transport mode is usually used when another tunnelling protocol (such as GRE, L2TP) is used to first encapsulate the IP data packet, then IPsec is used to protect the GRE/L2TP tunnel packets.
IPsec datagram emitted and received by end-system
Protects upper level protocols
Security
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77. IPsec – Tunneling Mode
IPsec
Edge Routers IPsec-aware
Hosts IPsec-aware
Tunnel mode protects the internal routing information by encrypting the IP header of the original packet.
The original packet is encapsulated by a another set of IP headers.
Security
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78. Two IPsec protocols Authentication Header (AH) protocol
provides source authentication & data integrity but not confidentiality
Encapsulation Security Protocol (ESP)
provides source authentication, data integrity, and confidentiality
more widely used than AH
Security
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79. Four combinations are possible!
Host mode with AH
Host mode with ESP
Tunnel mode with AH
Tunnel mode with ESP
most common and most important
Security
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80. Security associations (SAs)
Before sending data, “security association (SA)” established from sending to receiving entity
SAs are simplex: for only one direction
Sending, receiving entitles maintain state information about SA
recall: TCP endpoints also maintain state info
IP is connectionless; IPsec is connection-oriented!
How many SAs in VPN w/ headquarters, branch office, and n traveling salespeople?
Security
8-80

81. Example SA from R1 to R2 R1 stores for SA:
/24
/24
security association
Internet
headquarters
branch office R1 R2
R1 stores for SA:
32-bit SA identifier: Security Parameter Index (SPI)
Origin SA interface ( )
Destination SA interface ( )
Type of encryption used (e.g., 3DES with CBC)
encryption key
type of integrity check used (e.g., HMAC with MD5)
authentication key
Security
8-81

82. Security Association Database (SAD)
Endpoint holds SA state in security association database (SAD), where it can locate them during processing
With n salespersons, 2 + 2n SAs in R1’s SAD
When sending IPsec datagram, R1 accesses SAD to determine how to process datagram
When IPsec datagram arrives to R2, R2 examines SPI in IPsec datagram, indexes SAD with SPI, and processes datagram accordingly
Security
8-82

83. IPsec Datagram focus for now on tunnel mode with ESP
encrypted
“enchilada” authenticated
new IP header
ESP
hdr
original IP hdr
Original IP
datagram payload
ESP
trl
ESP
auth
SPI
Seq #
padding
pad length
next header
Security
8-83

84. What Happens? Internet headquarters branch office R1 R2
/24
/24
security association
Internet
headquarters
branch office R1 R2
encrypted
“enchilada” authenticated
new IP header
ESP
hdr
original IP hdr
Original IP
datagram payload
ESP
trl
ESP
auth
SPI
Seq #
padding
pad length
next header
Security
8-84

85. R1: convert original datagram to IPsec datagram
Appends to back of original datagram (which includes original header fields!) an “ESP trailer” field
Encrypts result using algorithm & key specified by SA
Appends to front of this encrypted quantity the “ESP header, creating “enchilada”
creates authentication MAC over the whole enchilada, using algorithm and key specified in SA
appends MAC to back of enchilada, forming payload
creates brand new IP header, with all the classic IPv4 header fields, which it appends before payload
Security
8-85

86. Inside the enchilada: ESP trailer: Padding for block ciphers
encrypted
“enchilada” authenticated
new IP header
ESP
hdr
original IP hdr
Original IP
datagram payload
ESP
trl
ESP
auth
SPI
Seq #
padding
pad length
next header
ESP trailer: Padding for block ciphers
ESP header:
SPI, so receiving entity knows what to do
Sequence number, to thwart replay attacks
MAC in ESP auth field is created with shared secret key
Security
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87. IPsec sequence numbers
for new SA, sender initializes seq. # to 0
each time datagram is sent on SA:
sender increments seq # counter
places value in seq # field
goal:
prevent attacker from sniffing and replaying a packet
receipt of duplicate, authenticated IP packets may disrupt service
method:
destination checks for duplicates
doesn’t keep track of all received packets; instead uses a window
Security
8-87

88. Security Policy Database (SPD)
policy: For a given datagram, sending entity needs to know if it should use IPsec
needs also to know which SA to use
may use: source and destination IP address; protocol number
info in SPD indicates “what” to do with arriving datagram
info in SAD indicates “how” to do it
Security
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89. Summary: IPsec services
suppose Trudy sits somewhere between R1 and R2. she doesn’t know the keys.
will Trudy be able to see original contents of datagram? How about source, dest IP address, transport protocol, application port?
flip bits without detection?
masquerade as R1 using R1’s IP address?
replay a datagram?
Security
8-89

90. IKE: Internet Key Exchange
previous examples: manual establishment of IPsec SAs in IPsec endpoints:
Example SA
SPI: 12345
Source IP:
Dest IP:
Protocol: ESP
Encryption algorithm: 3DES-cbc
HMAC algorithm: MD5
Encryption key: 0x7aeaca…
HMAC key:0xc0291f…
manual keying is impractical for VPN with 100s of endpoints
instead use IPsec IKE (Internet Key Exchange)
Security
8-90

91. IKE: PSK and PKI authentication (prove who you are) with either
pre-shared secret (PSK) or
with PKI (pubic/private keys and certificates).
PSK: both sides start with secret
run IKE to authenticate each other and to generate IPsec SAs (one in each direction), including encryption, authentication keys
PKI: both sides start with public/private key pair, certificate
run IKE to authenticate each other, obtain IPsec SAs (one in each direction).
similar with handshake in SSL.
Security
8-91

92. IKE phases IKE has two phases
phase 1: establish bi-directional IKE SA
note: IKE SA different from IPsec SA
aka ISAKMP security association
phase 2: ISAKMP is used to securely negotiate IPsec pair of SAs
phase 1 has two modes: aggressive mode and main mode
aggressive mode uses fewer messages
main mode provides identity protection and is more flexible
Security
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93. IPsec summary
IKE message exchange for algorithms, secret keys, SPI numbers
either AH or ESP protocol (or both)
AH provides integrity, source authentication
ESP protocol (with AH) additionally provides encryption
IPsec peers can be two end systems, two routers/firewalls, or a router/firewall and an end system
Security
8-93

94. Virtual Private Networks (VPNs)
motivation:
Institutions often want private networks for security
costly: separate routers, links, DNS infrastructure.
VPN: institution’s inter-office traffic is sent over public Internet instead
encrypted before entering public Internet
logically separate from other traffic
Security
8-94

95. Virtual Private Networks (VPNs)
laptop
w/ IPsec
public Internet IP
header
IPsec
Secure
payload
salesperson in hotel
header IP
IPsec
Secure
payload
header IP
IPsec
payload
Secure
router w/
IPv4 and IPsec
router w/
IPv4 and IPsec IP
header
payload
header IP
payload
headquarters
branch office
Security

96. Summary How security can be provided at the network layer IPSec
The use of IPSec to develop services
VPN
Trusted Networks

97. Basic View of Cryptography
Access &
Availability
Confidentiality
Integrity
Authentication
Cryptography
Symmetric
Asymmetric
Protocols
Data Link
Stream Ciphers
Block Ciphers
Trusted Networks

98. WEP design goals symmetric key crypto
confidentiality
end host authorization
data integrity
self-synchronizing: each packet separately encrypted
given encrypted packet and key, can decrypt; can continue to decrypt packets when preceding packet was lost (unlike Cipher Block Chaining (CBC) in block ciphers)
Efficient
implementable in hardware or software
Security
8-98

99. Review: symmetric stream ciphers
keystream
generator
key
combine each byte of keystream with byte of plaintext to get ciphertext:
m(i) = ith unit of message
ks(i) = ith unit of keystream
c(i) = ith unit of ciphertext
c(i) = ks(i)  m(i) ( = exclusive or)
m(i) = ks(i)  c(i)
WEP uses RC4
Security
8-99

100. Stream cipher and packet independence
recall design goal: each packet separately encrypted
if for frame n+1, use keystream from where we left off for frame n, then each frame is not separately encrypted
need to know where we left off for packet n
WEP approach: initialize keystream with key + new IV for each packet:
keystream
generator
Key+IVpacket
keystreampacket
Security
8-100

101. WEP encryption (1)
sender calculates Integrity Check Value (ICV, four-byte hash/CRC over data
each side has 104-bit shared key
sender creates 24-bit initialization vector (IV), appends to key: gives 128-bit key
sender also appends keyID (in 8-bit field)
128-bit key inputted into pseudo random number generator to get keystream
data in frame + ICV is encrypted with RC4:
bytes of keystream are XORed with bytes of data & ICV
IV & keyID are appended to encrypted data to create payload
payload inserted into frame
encrypted
data
ICV IV
MAC payload
Key ID
Security
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102. WEP encryption (2)
new IV for each frame
Security
8-102

103. WEP decryption overview
encrypted
data
ICV IV
MAC payload
Key ID
receiver extracts IV
inputs IV, shared secret key into pseudo random generator, gets keystream
XORs keystream with encrypted data to decrypt data + ICV
verifies integrity of data with ICV
note: message integrity approach used here is different from MAC (message authentication code) and signatures (using PKI).
Security
8-103

104. End-point authentication w/ nonce
Nonce: number (R) used only once –in-a-lifetime
How 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!
Security
8-104

105. WEP authentication Notes: not all APs do it, even if WEP is being used
authentication request
nonce (128 bytes)
nonce encrypted shared key
success if decrypted value equals nonce
Notes:
not all APs do it, even if WEP is being used
AP indicates if authentication is necessary in beacon frame
done before association
Security
8-105

106. Breaking 802.11 WEP encryption
security hole:
24-bit IV, one IV per frame, -> IV’s eventually reused
IV transmitted in plaintext -> IV reuse detected
attack:
Trudy causes Alice to encrypt known plaintext d1 d2 d3 d4 …
Trudy sees: ci = di XOR kiIV
Trudy knows ci di, so can compute kiIV
Trudy knows encrypting key sequence k1IV k2IV k3IV …
Next time IV is used, Trudy can decrypt!
Security
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107. 802.11i: improved security
numerous (stronger) forms of encryption possible
provides key distribution
uses authentication server separate from access point
Security
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108. 802.11i: four phases of operation
AP: access point
STA:
client station
AS:
Authentication
server
wired
network
1 Discovery of
security capabilities
STA and AS mutually authenticate, together
generate Master Key (MK). AP serves as “pass through” 2 3
STA derives
Pairwise Master
Key (PMK)
AS derives
same PMK,
sends to AP 4
STA, AP use PMK to derive
Temporal Key (TK) used for message
encryption, integrity
Security
8-108

109. EAP: extensible authentication protocol
EAP: end-end client (mobile) to authentication server protocol
EAP sent over separate “links”
mobile-to-AP (EAP over LAN)
AP to authentication server (RADIUS over UDP)
wired
network
EAP TLS
EAP
EAP over LAN (EAPoL)
RADIUS
IEEE
UDP/IP
Security
8-109

110. Summary How security is provided at the network layer - wireless WEP
Trusted Networks