Advanced Computer Networks: Lecture 40 - Dr. Amir Qayyum
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CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1
1
Lecture No. 40
2
Security
Outline
–
–
–
–
–
Encryption Algorithms
Authentication Protocols
Message Integrity Protocols
Key Distribution
Firewalls
3
Overview
• Cryptography functions
– Secret key (e.g. DES)
– Public key (e.g. RSA)
– Message digest (e.g. MD5)
• Security services
– Privacy: preventing unauthorized release of
information
– Authentication: verifying identity of the remote
participant
– Integrity: making sure message has not been
altered
4
Taxonomy of Network Security
Security
Cryptography
algorithms
Security
services
Secret
Public
Message
key
key
digest
(e.g. DES) (e.g. RSA) (e.g. MD5)
Privacy Authentication Message
integrity
5
Secret Key Encryption
6
Secret Key Encryption (DES)
Plaintext
Plaintext
Encrypt with
secret key
Decrypt with
secret key
Ciphertext
7
DES Algorithm
Initial permutation
• 64bit key (56bits + 8bit parity)
• 16 rounds
Round 1
• Each Round
Round 2
56bit
key
L ─1
i
R ─1
i
F
+
Round 16
Ri
Li
Final permutation
8
Ki
Expansion Phase of DES
4-bit chunk
■■■
■■■
■■■
■■■
Expanded to 6 bits by stealing
a bit from left and right chunks
9
Secret Key Encryption
Plaintext Block 3
Plaintext Block 2
Plaintext Block 1
Plaintext Block 0
Encryption
Function
Blocks of Ciphertext
Initialization Vector
(For block 0 only)
10
Cipher Block Chaining (CBC)
• Repeat for larger messages
Block
IV
1
Block
2
Block
Block
3
4
+
+
+
+
DES
DES
DES
DES
Cipher
1
Cipher
2
Cipher
Cipher
3
11
4
Public Key Encryption
12
Public Key Authentication
13
Public Key Encryption (RSA)
Plaintext
Plaintext
Encrypt with
public key
Decrypt with
private key
Ciphertext
•
Encryption & Decryption
c = memod n
m = cdmod n
14
RSA(cont)
ã Choosetwolargeprimenumberspand
q(each256bits)
ã Multiplypandqtogethertogetn
ã Choosetheencryptionkeye,suchthate
and(pư1)ì(qư1)arerelativelyprime.
Twonumbersarerelativelyprimeifthey
havenocommonfactorgreaterthanone
15
RSA(cont)
ã Computedecryptionkeydsuchthat
d=eư1mod((pư1)ì(qư1))
ã Constructpublickeyas(e,n)
ã Constructprivatekeyas(d,n)
ã Discard(donotdisclose)original
primespandq
16
Message Digest
• Cryptographic checksum
– Just as a regular checksum protects the
receiver from accidental changes to the
message, a cryptographic checksum
protects the receiver from malicious
changes to the message.
17
Message Digest
• Oneway function
– Given a cryptographic checksum for a
message, it is virtually impossible to
figure out what message produced that
checksum; it is not computationally
feasible to find two messages that hash to
the same cryptographic checksum.
18
Message Digest
• Relevance
– If you are given a checksum for a
message and you are able to compute
exactly the same checksum for that
message, then it is highly likely this
message produced the checksum you
were given.
19
Overview of Message Digest Operation
Initial “digest”
(constant)
Message (padded)
512 bits 512 bits
512 bits
■■■
Transform
Transform
Transform
Message digest
20
Authentication Protocols
• Threeway handshake
Client
Server
ClientI
d, E
,
E(x + 1
S
(x , C H
K)
HK)
S
,
Y
(
HK), E
E(y + 1
, CHK)
HK)
S
,
K
E(S
21
Third Party Authentication
• Trusted third party (Kerberos)
S
A
B
A, B
E((T,
L, K,
K
B) ,
E(A()T,
, L,
K, A
), K
B
)
E( ( A
, T),
E((TK),
, L,
K, A
),
KB )
, K)
1
+
E(T
22
Public Key Authentication
A
B
E(x,
Pu
blic
B )
x
23
Message Integrity
24
Message Integrity Protocols
• Keyed MD5
– Sender: m + MD5 (m + k) + E(E(k, rcv – pub),
private)
– Receiver
• Recovers random key using the sender’s
public key
• Applies MD5 to the concatenation of this
random key message
25
