ECE/CS 372 – introduction to computer networks
Lecture 16
Announcements:
 Lab 4 due now
 Assign 5 is due this Thursday
 Final exam in LPSC 125 (not here)
Credit for lecture slides to Professor Bechir Hamdaoui
Adapted from Jim Kurose & Keith Ross (original copyright)
Chapter 8, slide: 1
Chapter 8: Network Security
Goals:
 understand principles of network security:
cryptography and its many uses beyond
“confidentiality”
 authentication
 message integrity

 Example: securing email
Chapter 8, slide: 2
Bob, Alice want to communicate “securely”
 Trudy is an enemy (intruder): “bad” guy
Q: what should Bob & Alice be concerned about? confidentiality
 eavesdrop: messages are intercepted
integrity
 change: messages are modified
 impersonation: entire communication is hijacked by
replacing sender or receiver by himself
 denial of service: prevent services (e.g., by
overloading resources)
data
sender
receiver
Trudy
availability
Bob
Messages
Alice
authentication
data
Chapter 8, slide: 3
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
Chapter 8, slide: 4
What is network security?
Goals of 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
Integrity: sender, receiver want to ensure message not
altered (in transit, or afterwards) without detection
Availability: services must be accessible and available to
users
Chapter 8, slide: 5
Chapter 8 roadmap
 Principles of cryptography
 Message integrity
 Securing email
Chapter 8, slide: 6
Cryptography
 Cryptography allows a sender to disguise a message so
that an intruder can’t gain information from it
“confidentiality”
Alice’s
K encryption
A
key
plaintext
encryption
algorithm
All terms marked in red
are crypto terminology
ciphertext
Bob’s
K decryption
B key
decryption plaintext
algorithm
Chapter 8, slide: 7
Types of cryptography
symmetric key
- both sender and receiver use identical key
e.g., sender A encrypts with the key
receiver B decrypts with same key
public/private keys
- two keys (public and private) are to be used
e.g., sender A encrypts with B’s public key
receiver B decrypts with its Private key
Chapter 8, slide: 8
Symmetric key cryptography
KA-B
KA-B
plaintext
message, m
encryption ciphertext
algorithm
K (m)
A-B
decryption plaintext
algorithm
m = K ( KA-B(m) )
A-B
symmetric key crypto: Bob and Alice
share/know same (symmetric) key: KA-B
 Q: how do Bob and Alice agree on key value?
Chapter 8, slide: 9
Symmetric key cryptography
monoalphabetic cipher: substituting one letter for another
plaintext:
abcdefghijklmnopqrstuvwxyz
ciphertext:
mnbvcxzasdfghjklpoiuytrewq
E.g.:
Plaintext: bob. i love you. alice
ciphertext: nkn. s gktc wky. mgsbc
Q: what is the encryption & decryption key?:
A: the mapping
Q: How hard to break this simple cipher?:
A: brute force (how hard?) how many possibilities? 26!
Chapter 8, slide: 10
Symmetric key cryptography
polyalphabetic cipher: multiple monoalphabetic ciphers
Eg.: C1, C2, C2, C3 (with 3 monoalphabetic cipher keys)
First letter, apply C1
Second letter, apply C2
Third letter, apply C2
Fourth letter, apply C3
Then, repeat
 Harder to break! By avoiding patterns, same letter may
appear in different positions
 Key is “C1, C2, C2, C3”
Chapter 8, slide: 11
Symmetric key cryptography
block cipher: msg is encrypted in blocks of k bits (independently)
 Each k-bit block is encrypted/mapped
 Possible mappings: 2k!
 Hard to break
Problem
 Hard to implement, with k=64, sender and receiver need to
store a mapping table of 264 entries !! Huge!!
Solution
 Use of functions: break blocks into smaller chunks
Chapter 8, slide: 12
Block Cipher
loop for
n rounds
64-bit input
8bits
8bits
8bits
8bits
8bits
8bits
8bits
8bits
T1
T2
T3
T4
T5
T6
T7
T8
8 bits
 one pass
through: one
input bit
affects eight
output bits
8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits
64-bit scrambler
64-bit output
 multiple passes: each input bit affects all output bits
Chapter 8, slide: 13
Cipher Block Chaining
Problem w/ Cipher Block
 if input block is
repeated, it produces
same cipher text:
 cipher block chaining:
XOR ith input block, m(i),
with previous block of
cipher text, c(i-1)
 c(0) transmitted to
receiver in clear
 what happens in
“HTTP/1.1” scenario
from above?
t=1
…
t=17
m(1) = “HTTP/1.1”
block
cipher
c(1)
m(17) = “HTTP/1.1”
block
cipher
c(17)
= “k329aM02”
= “k329aM02”
m(i)
c(i-1)
+
block
cipher
c(i)
Chapter 8, slide: 14
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
 Two keys
Public key: encryp. key
known to all
 Private key: decryp. key
known only to receiver
 Sender uses public key only
to encryp
 Reciever uses both keys to
decryp.

Chapter 8, slide: 15
Public key cryptography
+ Bob’s public
B key
K
K
plaintext
message, m
encryption ciphertext
algorithm
+
K (m)
B
- Bob’s private
B key
decryption plaintext
algorithm message
+
m = K B(K (m))
B
 Note: only Bob is able to understand (decrypt)
message m. Because only Bob has Bob’s private key
 This assures “confidentiality”
Chapter 8, slide: 16
Public key encryption algorithms
Requirements:
1
2
+
need K ( ) and K - ( ) such that
B
B
- +
K (K (m)) = m
B B
.
.
+
given public key KB , it should be
impossible to compute
private key KB
RSA: Rivest, Shamir, Adleman algorithm
Chapter 8, slide: 17
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).
+
KB
-
KB
Chapter 8, slide: 18
RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above
1. To encrypt bit pattern, m, compute
e
e
c = m mod n (i.e., remainder when m is divided by n)
2. To decrypt received bit pattern, c, compute
d
m = c d mod n (i.e., remainder when c is divided by n)
Magic
d
m = (m e mod n) mod n
happens!
c
Chapter 8, slide: 19
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).
encrypt:
decrypt:
letter
m
me
l
12
1524832
c
17
d
c
481968572106750915091411825223071697
c = me mod n
17
m = cd mod n letter
12
l
Chapter 8, slide: 20
Another RSA example:
Bob chooses p=5, q=11.
Question: Find (n,e) and (n,d)
One answer: 1) n=55; z = 40; 2) e=27; 3) d=3 (ed – 1 = 80 is divisible by 40)
4) public key: (n,e) = (55,27) ; private key: (n,d) = (55,3)
Step 1: Compute n = pq, z = (p-1)(q-1)
Step 2: Choose e (with e<n) that has no common factors
with z. (e, z are “relatively prime”).
Step 3: Choose d such that ed-1 is exactly divisible by z.
(in other words: ed mod z = 1 ).
Step 4: Public key is (n,e). Private key is (n,d).
K
+
B
K
B
Chapter 8, slide: 21
RSA: another important property
The following property will be very useful later:
-
+
B
B
K (K (m))
+ = m = K (K (m))
B B
use public key
first, followed
by private key
use private key
first, followed
by public key
Result is the same!
Chapter 8, slide: 23
Chapter 8 roadmap
 Principles of cryptography
 Message integrity
 Securing email
Chapter 8, slide: 24
Message Integrity/Authentication
Bob receives msg from Alice, wants to ensure:


Authentication: message originally came from Alice
Integrity: message not changed since sent by Alice
Cryptographic Hashing:
 What:
 take input m, produce fixed length value, H(m)
 e.g., as in Internet checksum
 Properties of H:
 given m = H(x), (x unknown), it is computationally infeasible to
determine x.
 difficult to find x  y such that H(x) = H(y)
 note: Internet checksum fails this requirement!
 Examples
 widely used hash functions: MD5, SHA
Chapter 8, slide: 25
MAC: Message Authentication Code
(shared secret)
s
H(.)
(message)
m
append
H(.)
m
H(m+s)
public
Internet
H(m+s)
m
compare
H(m+s)
H(m+s)
s
(shared secret)
 Does MAC solve
 Integrity ?? How ??
via Hashing

Authentication ?? How ??
via secret key
 Any problem ??
 Secret key distribution ??
 So we can’t really
authenticate via MAC alone
Chapter 8, slide: 27
Digital Signatures via Public Key Crypto
simple digital signature for message m:
 Bob “signs” m by encrypting with his private key
K -, creating “signed” message, K -(m)
B
Bob’s message, m
Dear Alice
Oh, how I have missed
you. I think of you all the
time! …(blah blah blah)
Bob
B
K B Bob’s private
key
public key
encryption
algorithm
-
K B(m)
Bob’s message,
m, signed
(encrypted) with
his private key
Chapter 8, slide: 28
MAC via private/public keys
Alice’s public key
+
KA
-
K (m)
m
A
(message)
m
m
append
m
-
-
K (m)
A
public
Internet
-
K (m)
A
compare
m
m
K (m)
A
-
KA
Alice’s private key
 Note: only Alice would have had her private key
 This assures “authentication”
Chapter 8, slide: 30
Digital Signatures via Public Key Crypto (more)
Problem
 Signing data by encryption and decryption is
computationally expensive
 Imagine encrypting (signing) huge files of data !!!
Solution
 Sign hashed output of original msg (sign H(m) only)
 Recall hash algorithms turn large msgs into small, fixed
length msg
 … signed MAC is the solution
-
Chapter 8, slide: 31
Digital signature = signed MAC
= authentication + integrity
Alice verifies signature and
integrity of digitally signed
message:
Bob sends digitally signed
message:
large
message
m
H: hash
function
Bob’s
private
key
+
-
KB
encrypted
msg digest
H(m)
digital
signature
(encrypt)
encrypted
msg digest
KB(H(m))
large
message
m
H: hash
function
KB(H(m))
Bob’s
public
key
+
KB
digital
signature
(decrypt)
H(m)
H(m)
equal
?
Chapter 8, slide: 32
Public Key Certification
Problem with public key:
 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)
Chapter 8, slide: 33
Certification Authorities
 Certification Authority (CA): binds public key to
particular entity, E.
 E 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.”
- +
K CA(KB )
Bob’s
public
key
Bob’s
identifying
information
+
KB
digital
signature
(encrypt)
CA
private
key
K-
CA
+
KB
certificate for
Bob’s public key,
signed by CA
Chapter 8, slide: 34
Certification Authorities
 when Alice wants Bob’s public key:
gets Bob’s certificate
 apply CA’s public key to Bob’s certificate, get
Bob’s public key

+
KB
-
+
K CA(KB )
digital
signature
(decrypt)
CA
public
key
Bob’s
public
+
key
KB
+
K CA
Chapter 8, slide: 35
Get Bob’s public key from CA
CA’s public key
+
K CA
(Bob’s public key)
+
KB
append
+
KB
-
K
+
B
K - (K + )
CA
B
public
Internet
+
K CA(KB )
K
+
KB
+
KB
CA
(K + )
B
compare
+
KB
-
K CA
CA’s private key
 Alice just got Bob’s public key (authenticated key)
 Of course, here we assume that you have CA’s public
key !!! Need to get it physically !!!
Chapter 8, slide: 36
Chapter 8: Recap
So far:
Note: Sender applies
receiver’s public key
 Cryptography & confidentiality
 Symmetric key
 Public key: A wants to send msg m to B. What does A send?
A sends KB+(m); hence, ONLY B understands m by applying KB(KB+(m))
=> confidentiality
Note: Sender applies
 Authentication & integrity
its private key
 MAC (Msg Authen. Code):
requires symmetric key
 Signed MAC: A -> B
A sends (m,KA-(m)) to B,
Hence, All get m by applying KA+(KA-(m));
Comparison => authen. + integrity, but NOT confidentiality
Chapter 8, slide: 37
Chapter 8 roadmap
 Principles of cryptography
 Message integrity
 Securing e-mail
Chapter 8, slide: 38
Secure e-mail (confidentiality)

Alice wants to send confidential e-mail, m, to Bob.
KS
m
K (.)
S
+
A
KS
+
.
K B( )
+
KS(m )
KS(m )
-
Internet
+
KB(KS )
KB
.
KS( )
+
KB(KS )
m
KS
-
.
B
K B( )
-
KB
Alice:
 generates random symmetric private key, KS.
Bob:
encrypts
message
with
KS (for efficiency)
 uses
his to
private
key
to
decrypt
and recover
KS encrypts with
Note
that
provide
confidentiality,
sender
also encrypts
KSkey
with
key.
 uses
KS public
to decrypt
K(ONLY
(m) topublic
recover
m should see msg)
SBob’s
receiver’s
receiver
 sends both KS(m) and KB(KS) to Bob.
Chapter 8, slide: 39
Secure e-mail (authen. + integrity)
• Alice wants to provide sender authentication/integrity.
+
-
KA
m
A
.
H( )
-
.
KA( )
-
-
KA(H(m))
KA(H(m))
+
Internet
m
KA
+
.
KA( )
m
H(m )
compare
.
H( )
H(m )
• Alice digitally signs message.
• sends both message (in the clear) and digital signature.
Again note that to provide authenticate/integrity,
sender encrypts with its private (all can understand msg)
Chapter 8, slide: 40
B
Secure e-mail (all: confid. + auth. + integrity)
• Alice wants to provide secrecy, sender authentication,
message integrity.
-
KA
m
.
H( )
-
.
KA( )
-
KA(H(m))
+
A
KS
.
KS( )
+
m
KS
+
.
K B( )
+
Internet
+
KB(KS )
KB
Alice uses three keys: her private key, Bob’s public
key, newly created symmetric key
Chapter 8, slide: 41
The end of new material!
Final Review on Thursday!
Chapter 8, slide: 42