Fundamentals of digital security. Some info I made throughout the years as a refresher for digital security. Basic primer for beginners. If you are an expert, comments and feedbacks welcome.

1. Digital Security 101
Just for Fun: Gary Jan

2. Why Network Security?
• The Internet was not designed to be secured
• No “Security Layer” in the OSI 7 Layer reference model
• Internet was designed in 1970 for academic institutions communicating across
the continent
• Digital information flows in plain text
• Everyone can pick it up and read it
• How does the receiver know the sender is who it claimed to be?
• How does the receiver know the received information is not altered?
• How does the sender make sure only the intended receiver can read it?
• How to enforce network policy and security practice?

3. Trudy the Intruder
Alice BobChannel
Trudy
What are the problems here?
1. Trudy can eavesdrop the communication between A and B
▪ Eavesdropping <-> Confidentiality / Privacy
2. Trudy can intercept the message and alter the content
▪ Man-in-the-Middle Attack <-> Integrity
3. Trudy can pretend to be either A or B
▪ Imposter <-> Authentication
4. Trudy can flood the resource on A and B
▪ Denial of Service <-> Availability / Access Control

4. What is Network Security
4 key elements in secure communication
1. Privacy / Confidentiality
2. Authentication
3. Integrity
4. Nonrepudiation

5. Principles of Cryptography
• Cryptography
• Disguise the data so that Trudy cannot access the intercepted data
• Only the intended receiver can reconstruct the original data
• Disguise the “Plaintext” into “Ciphertext” using Encryption Algorithm
• These algorithms are usually published and standardized
Example
Plaintext: a b c d e f g h i j k l m n o p q r s t u v w x y z
Ciphertext: w x y z s t u v o p q r k l m n g h i j c d e f a b
“How are you” => “vme wgs amc?”

6. Security Key
• Security Key
• A string of numbers or character (cipher-block) as input to the encryption algorithm
to mix with the plaintext
• The key encrypted data is only decrypt-able by the receiver
• Trudy must know the “key” to decrypt
• The encrypted plaintext (ciphertext) is unique and is extremely difficult if not
impossible to be decrypted by Trudy
m X
KA
X
KB
mKA (m)A B
m= Message
x= Encryption Algorithm
KA= A’s Key
KB= B’s Key
KA(m)= Encrypted message using a’s Key

7. Type of Key Systems 1/2
1. Symmetric (Private) Key Systems
• When KA = KB
• The shared secret key is usually express as KAB
2. Asymmetric (Public) Key Systems
• When KA ≠ KB
• Use a pair of key
• A Public Key (K+)
• A Private Key (K-)
• In this example
• KA = KA
+
• KB = KA
-
m X
KA
X
KB
mKA (m)A B

8. Type of Key Systems 2/2
1. Symmetric Key
• Also known as Private Key
• The key is pre-agreed by the two end hosts for all sessions
• A shared secret “Session Key” can also be generated for each session
• Diffie-Hellman Key Exchange
• Internet Key Exchange Protocol
2. Asymmetric Key
• Also known as Public Key
• Each host has a pair of keys: Public and Private Key
• Use Public Key to encrypt; Private key to decrypt
• Requires a Trusted third party to store the public key of a host
• Key Distribution Centre
• CA

9. Symmetric Key- DES and AES
• Data Encryption Standard & Advanced Data Encryption Standard
• A Symmetric (Private) Key based Encryption Algorithm
• DES- 56 bit key size
• AES- 128, 192, and 256 key size
• FYI… it took 4 months to decrypt DES in 1997
• FYI… it took 22 hours to decrypt DES in 1999
• AES is the U.S. government standard replacing 3DES in 2002
m X
KA
X
KB
mKA (m)A B
DES & AES addressed the Confidentiality/Privacy Requirement

10. Symmetric Key- Diffie-Hellman
• Diffie-Hellman Algorithm
• Provide a method to securely generate a private Session Key between two hosts in a public
network
• How it works?
• Tx and Rx agrees on a generator number (g) and a large prime number (p)
• Before exchanging, Tx selects a random number (x) and RX selects a random number (y)
• Tx sends T= (gx mod p); Rx sends R= (gy mod p)
• Tx receives R and generates K; K= gxy mod p
• Rx receives T and generates K; K= gxy mod p
• Trudy can eavesdrop p, g, T, and R but not x and y
• x= logy(T) ; y= logx(R)
Tx Rx
T= (gx mod p)
R= (gy mod p)
K= Rx mod p
K= gxy mod p
K= Ty mod p
K= gxy mod p
K is the Symmetric Session Key
Computation of x and y is extremely time consuming for large numbers of T and R

11. Diffie-Hellman Security Issues
• Diffie-Hellman is subjected to Man-in-the-Middle and Client-
Imposter Attack
• Trudy can intercept T and R and fake it with T’ and R’
• How does Rx know the received message is indeed from Tx?
Tx Rx
T
Trudy
T’
R’ R
K1 K2
In general Key Algorithms provide Privacy/Confidentiality but not
Authentication.
What about Authentication? What do we do?

12. Authentication Protocol 1/x
• Authentication
• To verify the validity of the person is who it claimed to be
• Passport Photo
• Driver’s License Number
• Social Insurance Number and etc…
• In digital communication world
• ???

13. 1. A indicates it wants to talk
to B
3. A encrypts r using the
shared key KAB and sends m=
KAB(r)
5. A then sends a Nonce (r’) to
B
7. A decrypts m’ using shared
secret key KAB.
If the result is r, B knows it is
indeed talking to A
Authentication Protocol 2/x
• Nonce- Number used Once
• Also Known as Challenge-Response approach
• Use Nonce to confirm the receiver is indeed talking to the intended transmitter
• A random pseudonumber
• Symmetric Key + Nonce
• A and B can share a secret key, or
• A and B can share a session key
• Used to Authenticate the host and to verify the sender is “live”
A B
“I am A”
r
m= KAB(r)
r’
m’= KAB(r’)
2. B sends a Nonce (r) to A
4. B decrypts m using shared
secret key KAB.
If the result is r, B knows it is
indeed talking to A
6. B encrypts r’ using the
shared key KAB and sends m’=
KAB(r’)
Using Nonce+ Key Cryptography, A knows it is indeed talking to B; B knows
it is indeed talking to A

14. Authentication Protocol 3
• Asymmetric Key + Nonce
• A and B knows each others Public and Private Key
• Used to Authenticate the host and to verify the sender is “live”
A B
“I am A”
r
m= KA
- (r)
“What’s your public key?”
My Public Key= KA
+
2. B sends a Nonce (r) to A
4. B asks for A’s public key
6. B decrypts m using A’s
public key KA
+
If the result is r, B knows it is
indeed talking to A
1. A indicates it wants to talk
to B
3. A encrypts r using A’s
secret key KA
- and sends m=
KA
-(r)
5. A sends it’s Public Key, KA
+,
to B
How does B know KA
+ is indeed A’s public key?
A sends data using KA
- to Encrypt and
receive data using KA
- to Decrypt
B sends data using KA
+ to Encrypt and
receive data using KA
+ to Decrypt
Encrypted Data

15. Authentication Protocol 4
• Authentication using Asymmetric Key still subjects to Man-in-the-Middle Attack
A BTrudy“I am A”
r
m= KA
- (r)
“What’s your public key?”
My Public Key= KA
+
“I am A”
r
m= KA
- (r)
“What’s your public key?”
My Public Key= KT
+
B sends data using KT
+
to Encrypt and receive
data using KT
+ to
Decrypt
Encrypted Data
B sends data using KA
-
to Encrypt and receive
data using KA
- to
Decrypt
Encrypted Data
Trudy decrypts
received
message from B
using KT
-
Trudy sends
modified data to
A
How does A and B ensure each other is who it claimed to be?- Authentication
How does A and B ensure the received data is not altered?- Integrity

16. CA- Certificate Authority
• Certificate Authority
• Validate identities and issue certificates
• A certificate is the binding of an Identity with a Public Key
• One must trust CA in identifying the validity of a Identity/Public Key pair
• Some popular CA- VeriSign, Comodo, and GoDaddy
• How is a Certificate issued?
X
A’s
Certificate
KCA
-
[A, KA
+] CA= KCA
-[A, KA
+]
CA
A
A sends its identity and public key in
Certificate Authority
CA encrypts A’s identity and public
key using CA’s private key KCA
- and
produce a Certificate CA of A.
The certificate is now registered and
available only in CA’s server

17. Authentication Protocol 5
• Authentication using Asymmetric Key + Certificate
A B
“I am A”
r
m= KA
- (r)
“What’s your public key?”
My Public Key= KA
+
2. B sends a Nonce (r) to A
4. B asks for A’s public key
6. B go to CA to verify A’s Certificate
7. B decrypts A’s Certificate using CA’s public key
If the identity and the public key matches B knows
it’s A’s public key
B decrypts m using A’s public key KA
+
If the result is r, B knows it is indeed talking to A
1. A indicates it
wants to talk to
B
3. A encrypts r
using A’s secret
key KA
- and
sends m= KA
-(r)
5. A sends it’s
Public Key, KA
+,
to B
A sends data using KA
- to
Encrypt and receive data
using KA
- to Decrypt
Encrypted Data
CA
What’s A’s Certificate?
A’s Certificate= CA
= KCA
-[A, KA
+]
B sends data using KA
+
to Encrypt and receive
data using KA
+ to
Decrypt
How does A and B ensure each other is who it claimed to be?- Authentication
How does A and B ensure the received data is not altered?- Integrity

18. Security Elements
• 5 key elements in secure communication
1. Privacy / Confidentiality
• Symmetric, Asymmetric, and Session Key
2. Authentication
• Symmetric Key, Asymmetric Key + Certificate Authority
3. Integrity
4. Nonrepudiation
5. Availability and Access Control

19. Integrity and Nonrepudiation
• Nonrepudiation
• The sender must be able to prove it is the creator of the content
• The sender must be able to prove it is the approver of the content
• The receiver must be able to verify the creator and approver of the received
content
• Exactly like Human Signature
• Sign checks, credit card receipts, and etc…
• Signature indicates the signer has verified and is responsible for the content
• Digital Signature
• A cryptographic technique used to achieve the same goals of Human
Signature

20. Digital Signature
• To prove that a document signed by an individual was indeed signed by that individual
(verifiable)
• To prove that only that individual could have signed the document (Nonrepudiation)
• How does it work in Digital World?
• Use Asymmetric Key to produce Digital Signature
• Whoever signed the document must have used KA
-
• Verifiable
• “A” must be the only person who possesses the key KA
• Nonrepudiate
KA
-
[m] X
Signed Message
= KA
-[m]
X
KA
+
KA
+[KA
-[m]]
[m]
A B

21. Security Elements
• 5 key elements in secure communication
1. Privacy / Confidentiality
• Symmetric, Asymmetric, and Session Key
2. Authentication
• Symmetric Key, Asymmetric Key + Certificate Authority
3. Integrity
4. Nonrepudiation
• Digital Signature
5. Availability and Access Control

22. Integrity- Message Digest
• To verify the received data is not altered and is what is being sent by the sender
• Message Digest
• A mathematical function which takes an input message (m) and produces an mathematically
calculated output (H(m)).
• The Hash Function must have extremely low probability of producing same output with different
inputs.
• i.e. H(x) != H(y)
• Popular Hash Algorithms
• MD5, MD6
• SHA-1/2/3
[m]
Hash Function
H( )
H(m)

23. Integrity + Authentication 1/2
• HMAC- Hashed Message Authentication Code
• Provides Integrity and Authentication
• Two techniques: HMAC + Key Cryptography
[m]
Hash Function
H( )
H(m)
A
KA
-
X
{m, K A
- [H(m)]}+
K A
- [H(m)]
Message is Hashed and Signed.
Signed Hash message is sent along original message.

24. Integrity + Authentication 2/2
• HMAC- Hashed Message Authentication Code
• Provides Integrity and Authentication
• Two techniques: HMAC + Key Cryptography
B
{m, K A
- [H(m)]} -
K A
- [H(m)]
m
Hash Function
H( )
X KA
+
=
?
H(m)
H(m) Y Message is
not altered
Received signed hashed message is unlocked to reveal H(m)
Received original message is hashed to compare results

25. IPSec
▪ IPSec- Internet Protocol Security
▪ Suite of protocols to ensure Confidentiality, Authentication, and Integrity
across IP network
▪ Works on Layer 3 Packets
▪ Three Components
▪ AH- Authentication Header
▪ Authentication & Integrity
▪ ESP
▪ Authentication, Integrity, and Confidentiality
▪ IKE
▪ Key Management and Security Association Management
▪ Two Modes
▪ Tunnel Mode
▪ Transport Mode

26. IPSec- ESP
• ESP- Encapsulating Security Payload
• Confidentiality- Encrypted over payload, only intended receiver can decrypt
• Authentication & Integrity- HMAC + MD5
IP HDR ESP HDR TCP Data ESP Trailer
ESP
Authenticat
ion
IP HDR TCP Data
Encrypted
Authenticated

27. IP HDR TCP Data
TCP Data + X
ESP Trailer
K
K[TCP, Data, ESP Trailer]
+
ESP HDR
ESP HDR
Encrypted
Data
HMAC
MD5
H[ESP HDR, Encrypted Data]
KA
-
X
+
K A
- [H(m)]
i.e. ESP Auth HDR
ESP HDR, Encrypted Data
IP HDR ESP HDR Encrypted Data
ESP
Authenticati
on
TCP Data ESP Trailer

28. ESP- Confidentiality
IP HDR TCP Data
TCP Data + X
ESP Trailer
KDES
KDES[TCP, Data, ESP Trailer]
TCP Data ESP Trailer
Encrypted Payload
▪ ESP Confidentiality
▪ Achieved by encrypting [TCP, Data] payload
▪ AES, DES, 3DES or etc
DES encrypted payload
DES key arranged during IKE Session

29. ESP- Authentication & Integrity
Encrypted Payload +
ESP HDR
Hash
H(ESP HDR, Encrypted Data)
KHMAC
+
KHMAC[H(m)]
i.e. ESP Auth HDR
IP HDR,
ESP HDR,
Encrypted Data
IP HDR ESP HDR Encrypted Data
ESP
Authenticat
ion
ESP HDR
Encrypted
Data
▪ ESP Authentication & Integrity
▪ Achieved by using HMAC-MD5/SHA over DES encrypted payload
X