IT478 project

College of Computing and Informatics
Network Security

Network Security
Week 2
Network security concepts

Contents
1. Security goals

2. Attacks
3. Security services
4. Security mechanisms
5. Security techniques

Weekly Learning Outcomes
1. Describe the key security goals of confidentiality, integrity,
and availability.
2. Discuss the types of security threats and attacks to different
network assets.
3. Discuss the security services, mechanisms, and techniques.

Computer Security Concepts

• The generic name for the collection of tools designed to
protect data and to prevent hackers.
• The protection afforded to an automated information system
in order to attain the applicable objectives of preserving the
integrity, availability, and confidentiality of information
system resources (includes hardware, software, firmware,
information/data, and telecommunications).

Definitions

• Computer Security – generic name for the collection of tools
designed to protect data and to thwart hackers.
• Network Security – measures to protect data during their
transmission.

• Internet Security – measures to protect data during their
transmission over a collection of interconnected networks.

Computer Security Objectives/ Goals

⚫Confidentiality
⚫Integrity
⚫Availability

Computer Security Objectives/ Goals
• Confidentiality
• Data confidentiality
• Assures that private or confidential information is not made available or disclosed to
unauthorized individuals

• Privacy
• Assures that individuals control or influence what information related to them may be
collected and stored and by whom and to whom that information may be disclosed

• Integrity

• Data integrity
• Assures that information and programs are changed only in a specified and authorized
manner

• System integrity
• Assures that a system performs its intended function in an unimpaired manner, free from
deliberate or inadvertent unauthorized manipulation of the system

• Availability

• Assures that systems work promptly, and service is not denied to authorized
users.

Examples of Security Requirements
• Confidentiality
• student grades

• Integrity
• patient information
• Availability
• authentication service

OSI Security Architecture
• ITU-T X.800 “Security Architecture for OSI”
• Defines a systematic way of defining and providing security
requirements.
• The OSI security architecture focuses in 3 aspects of information
security:
• Security Attack
• Security Mechanism
• Security Service

Security attack
• Any action that compromises the security of information owned by an
organization
• Often threat & attack used to mean same thing.
• The three goals of security – confidentiality, integrity, and availability can
be threatened by security attacks.
• Attacks Threatening Confidentiality
• Attacks Threatening Integrity
• Attacks Threatening Availability
• Passive versus Active Attacks

Taxonomy of Attacks

Taxonomy of Attacks
• Attacks Threading Confidentiality

• Snooping refers to unauthorized access to or interception of data.
• Traffic analysis refers to obtaining some other type of information by monitoring
online traffic.

• Attacks Threatening Integrity

• Modification means that the attacker intercepts the message and changes it.
• Masquerading or spoofing happens when the attacker impersonates somebody
else.
• Replaying means the attacker obtains a copy of a message sent by a user and
later tries to replay it.
• Repudiation means that sender of the message might later deny that she has
sent the message; the receiver of the message might later deny that he has
received the message.

• Attacks Threatening Availability

• Denial of service (DoS) is a very common attack. It may slow down or totally
interrupt the service of a system.

Security Attacks
• A passive attack attempts to learn or make use of information
from the system but does not affect system resources

Security Attacks
• An active attack attempts to alter system resources or affect
their operation

Passive Attacks

• In the nature of eavesdropping or
monitoring of transmissions
• Two types of passive attacks are:
1. Release of message contents
2. Traffic analysis
• Passive attacks are very difficult to detect
• Message transmission apparently normal
• No alteration of the data
• Emphasis on prevention rather than
detection
• By means of encryption

Release of message contents

Traffic analysis

Active Attacks

• Active attacks try to alter system resources or affect their operation
• Modification of data, or creation of false data

• Four types of passive attacks are:
1.
2.
3.
4.

Masquerade
Replay
Modification of messages
Denial of service

• Difficult to prevent because of the wide variety of potential physical,
software, and network vulnerabilities
• The goal is to detect and recover

Active Attacks

• Masquerade
• Takes place when one entity pretends to be a different entity
• Usually includes one of the other forms of active attack

Active Attacks

• Replay

• Involves the passive capture of a data unit and its subsequent retransmission to
produce an unauthorized effect

Active Attacks

• Modification of Messages
• Some portion of a legitimate message is altered, or messages are delayed or
reordered to produce an unauthorized effect

Active Attacks

• Denial of Service
• Prevents or inhibits the normal use or management of communications facilities

Security Service and Mechanism
• ITU-T provides some security services and some mechanisms to
implement those services. Security services and mechanisms are closely
related because …
• Mechanism or combination of mechanisms are used to provide a
service.
• Security Services
• Security Mechanism
• Relation between Services and Mechanisms

Security Services

• Defined by X.800 as:

• A service provided by a protocol layer of communicating open systems and that
ensures adequate security of the systems or of data transfers

• Defined by RFC 4949 as:

• A processing or communication service provided by a system to give a specific kind
of protection to system resources

X.800 Service Categories:
1. Authentication
2. Access control
3. Data confidentiality
4. Data integrity
5. Nonrepudiation

X.800 security service: Authentication

• The assurance that the communicating entity is the one that it claims to be.
• In the case of a single message, assures the recipient that the message is from the
source that it claims to be from.
• In the case of ongoing interaction, assures the two entities are authentic and that
the connection is not interfered with in such a way that a third party can
masquerade as one of the two legitimate parties

• Two specific authentication services are defined in X.800:
• Peer Entity Authentication

• Used in association with a logical connection to provide confidence in the identity of the entities
connected.

• Data-Origin Authentication

• In a connectionless transfer, provides assurance that the source of received data is as claimed.

X.800 security service: Access Control

• The ability to limit and control the access to host systems and applications
via communications links
• To achieve this, each entity trying to gain access must first be indentified, or
authenticated, so that access rights can be tailored to the individual.

X.800 security service: Data Confidentiality
• The protection of transmitted data from passive attacks
• Broadest service protects all user data transmitted between two users over a period
of time
• Narrower forms of service include the protection of a single message or even
specific fields within a message

• The protection of traffic flow from analysis
• This requires that an attacker not be able to observe the source and destination,
frequency, length, or other characteristics of the traffic on a communications facility

X.800 security service: Data Integrity
• Assures that messages are received as sent with no duplication, insertion,
modification, reordering, or replays
• Can apply to a stream of messages, a single message, or selected fields
within a message

X.800 security service: Nonrepudiation
• Prevents either sender or receiver from denying a transmitted message
• When a message is sent, the receiver can prove that the alleged sender in fact sent
the message
• When a message is received, the sender can prove that the alleged receiver in fact
received the message

Security Mechanisms (X.800)
• Specific security mechanisms:
• OSI security services performed on
different protocol layer.
• Encipherment, digital signatures,
access controls, data integrity,
authentication exchange, traffic
padding, routing control, notarization.

• Pervasive security mechanisms:
• The mechanisms that are not specific
to the protocol layer particular.
• Trusted functionality, security labels,
event detection, security audit trails,
security recovery.

Security Mechanisms (X.800)
⚫Encipherment is the use of algorithms mathematics to transform to the
data into a form that cannot be understood.
⚫Digital Signature is a cryptographic transformation of a data unit that is
used to validate the authenticity and integrity of a message. Hashing
algorithm is used.
⚫ Access Control is a mechanism that ensures access to a resource by a
user who have rights.
⚫ Data integrity is a mechanism that used to ensure the integrity of a data
unit or stream of data units.

Security Mechanisms (X.800)-Cont
⚫Authentication Exchange is a mechanism which aims to ensure the
identity of entity for purposes of the exchange of information.
⚫Traffic padding is added to the data bits stream analysis attempts to
confuse traffic.
⚫Routing Control receives the selection of a safe route to certain data and
allow changes routing especially when security breaches made it known.
⚫Notarization is the use of third party reliably during the process of data
exchange.

Relationship between Services &
Mechanisms

Security Techniques
⚫Mechanisms discussed in the previous sections are only theoretical
recipes to implement security. The actual implementation of security
goals needs some techniques.
⚫Two techniques are widespread today:
1. Cryptography
⚫ Cryptography, a word with Greek origins, means “secret writing.”
⚫ However, we use the term to refer to the science and art of transforming messages
to make them secure and immune to attacks.

Security Techniques
2. Steganography

⚫ The word steganography, with origin in Greek, means “covered writing,” in contrast
with cryptography, which means “secret writing.”
⚫ Example: Covering data with image.

Model for Network Security

Model for Network Security
⚫Using this model requires us to:
⚫Design a suitable algorithm for the security transformation.
⚫Generate the secret information (keys) used by the algorithm.
⚫Develop methods to distribute and share the secret information.
⚫Specify a protocol enabling the principals to use the
transformation and secret information for a security service.

Model for Network Access Security

Model for Network Security
⚫Using this model requires us to:
⚫Select appropriate gatekeeper functions to identify users.
⚫Implement security controls to ensure only authorized users
access designated information or resources.
⚫Trusted computer systems may be useful to help implement this
model.

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 3
Symmetric encryption & message confidentiality

Contents
1. Symmetric and asymmetric encryption principles

2. Symmetric block encryption algorithms
3. Random and pseudorandom numbers
4. Stream and block ciphers

Weekly Learning Outcomes
1. Present the main concepts of symmetric and asymmetric
cryptography.
2. Present an overview of DES and AES algorithms.
3. Explain the concepts of randomness and random number
generators.
4. Present an overview of stream and block ciphers.

Model of symmetric Encryption
• The original message from Alice to Bob is called plaintext; the message
that is sent through the channel is called the ciphertext. To create the
ciphertext from the plaintext, Alice uses an encryption algorithm and a
shared secret key. To create the plaintext from ciphertext, Bob uses a
decryption algorithm and the same secret key.

Requirements
• There are two requirements for secure use of symmetric
Encryption/Cipher:
• A strong encryption algorithm
• Sender and receiver must have obtained copies of the secret key in a
secure fashion and must keep the key secure
• The security of symmetric encryption depends on the secrecy of the
key, not the secrecy of the algorithm
• This makes it feasible for widespread use
• Manufacturers can and have developed low-cost chip
implementations of data encryption algorithms
• These chips are widely available and incorporated into a number of
products

Model of symmetric Encryption
• If P is the plaintext, C is the ciphertext, and K is the key,

• We assume that Bob creates P1; we prove that P1 = P:

Model of symmetric Encryption
• If P is the plaintext, C is the ciphertext, and K is the key,

• We assume that Bob creates P1; we prove that P1 = P:

Cryptography and Cryptanalysis
• Cryptography is the process of creating secret codes.
• Cryptanalysis is the process of breaking/discovering those codes.
• Types Cryptanalysis Attacks on Encrypted Messages:

Ciphertext-Only Attack
• A type of attack in which the intruder has only the intercepted
ciphertext to analyze.
• Example:
• Brute-Force Attack.
• Statistical Attack
• Pattern Attack

Known-Plaintext Attack
• An attack in which the attacker uses a set of known plaintexts and their
corresponding ciphertexts to find the cipher key.

Chosen-Plaintext Attack
• A type of attack in which the attacker chooses a set of plaintexts and
somehow finds the corresponding ciphertexts. She then analyzes the
plaintext/ciphertext pairs to find the secret key.

Chosen-Ciphertext Attack
• The chosen-ciphertext attack is similar to the chosen-plaintext attack,
except that Eve chooses some ciphertext and decrypts it to form a
ciphertext/plaintext pair.
• This can happen if Eve has access to Bob’s computer.

Cryptography
• Cryptographic systems are generically classified along three
independent dimensions.
1. The type of operations used for transforming plaintext to ciphertext
• Substitution
• Each element in the plaintext is mapped into another element
• Transposition
• Elements in the plaintext are rearranged
• Fundamental requirement is that no information be lost
• Product systems
• Involve multiple stages of substitutions and transpositions

Cryptography (Cont…)
• Cryptographic systems are generically classified along three
independent dimensions.
2. The number of keys used
• Referred to as symmetric, single-key, secret-key, or conventional
encryption if both sender and receiver use the same key
• Referred to as asymmetric, two-key, or public-key encryption if the
sender and receiver each use a different key

Cryptography (Cont…)
• Cryptographic systems are generically classified along three
independent dimensions.
3. The way in which the plaintext is processed
• Block cipher processes the input one block of elements at a time,
producing an output block for each input block
• Stream cipher processes the input elements continuously, producing
output one element at a time, as it goes along

Substitution Ciphers
• A substitution cipher replaces one symbol with another.
• Substitution ciphers can be categorized as either monoalphabetic
ciphers or polyalphabetic ciphers.
• The following shows a plaintext and its corresponding ciphertext. The
cipher is probably monoalphabetic because both l’s (els) are encrypted
as O’s.
• The following shows a plaintext and its corresponding ciphertext. The
cipher is polyalphabetic because each l (el) is encrypted by a different
character.

Transposition Ciphers
• A transposition cipher does not substitute one symbol for another,
instead it changes the location of the symbols.
• A good example of a keyless cipher using the first method is the rail
fence cipher. The ciphertext is created reading the pattern row by row.
For example, to send the message “Meet me at the park” to Bob, Alice
writes:

• She then creates the ciphertext “memateaketethpr”.

Transposition Ciphers
• Alice and Bob can agree on the number of columns and use the second
method. Alice writes the same plaintext, row by row, in a table of four
columns.
• For example, to send the message “Meet me at the park” to Bob, Alice
writes:
• Key = 4 (no of columns)

• She then creates the ciphertext “mmtaeehreaekttp”.

Stream and Block Ciphers
• The literature divides the symmetric ciphers into two broad categories:

• Stream ciphers and
• Block ciphers.

Stream Cipher
• Call the plaintext stream P, the ciphertext stream C, and the key stream
K.

Block cipher
• In a block cipher, a group of plaintext symbols of size m (m > 1) are
encrypted together creating a group of ciphertext of the same size. A
single key is used to encrypt the whole block even if the key is made of
multiple values. Figure 3.27 shows the concept of a block cipher.

Symmetric Block encryption algorithms
• Block cipher is the most commonly used symmetric encryption
algorithms
• Processes the plaintext input in fixed-sized blocks and
• Produces a block of ciphertext of equal size for each plaintext block
Data
Encryption
Standard
(DES)

Advanced
Encryption
Standard
(AES)

The three most
important
symmetric
block ciphers
Triple DES
(3DES)

Symmetric Block encryption algorithms

Key Length (bits)
Key Strength
Processing
Requirements
RAM Requirements
Remarks

RC4
40 bits or more

DES
56

3DES
112 or 168

AES
128, 192, or 256

Very weak at 40 bits
Low

Weak
Moderate

Strong
High

Strong
Low

Low

Moderate

Moderate

Low

Can use keys of
variable lengths

Created in the
1970s

Applies DES three
times with two or
three different DES
keys

Today’s gold
standard for
symmetric key
encryption

Random and pseudorandom Numbers
• A number of network security algorithms based on cryptography make
use of random numbers
• Examples:
• Generation of keys for the RSA public-key encryption algorithm and other
public-key algorithms
• Generation of a symmetric key for use as a temporary session key; used in a
number of networking applications such as Transport Layer Security, Wi-Fi, email security, and IP security
• In a number of key distribution scenarios, such as Kerberos, random numbers
are used for handshaking to prevent replay attacks

• Two distinct and not necessarily compatible requirements for a
sequence of random numbers are:
• Randomness
• Unpredictability

Randomness
• The following criteria are used to validate that a sequence of numbers is
random:

Uniform
distribution

Independence

• The distribution of bits in the sequence should be
uniform
• Frequency of occurrence of ones and zeros should
be approximately the same

• No one subsequence in the sequence can be
inferred from the others
• There is no test to “prove” independence
• The general strategy is to apply a number of tests
until the confidence that independence exists is
sufficiently strong

Unpredictability
• In applications such as reciprocal authentication and session key
generation, the requirement is not so much that the sequence of
numbers be statistically random but that the successive members of the
sequence are unpredictable
• With “true” random sequences, each number is statistically
independent of other numbers in the sequence and therefore
unpredictable
• Care must be taken that an opponent not be able to predict future
elements of the sequence on the basis of earlier elements

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 4
Public-key cryptography & message authentication

Contents
1. Message integrity and authentication

2. Secure hash functions
3. Public-key cryptography principles
4. Public-key cryptography algorithms
5. Digital signatures

Weekly Learning Outcomes
1. Explain the message integrity and authentication concept.
2. Explain the needs for a hash function in message
authentication.
3. Present the basic principles of asymmetric cryptography.
4. Present an overview of the RSA algorithm.
5. Understand the digital signature concept.

Message Integrity
• The cryptography systems that we have studied so far provide secrecy, or
confidentiality, but not integrity. However, there are occasions where we may not even
need secrecy but instead must have integrity.
• Integrity means that attackers cannot change or destroy information.
• Document and Fingerprint
• One way to preserve the integrity of a document is through the use of a fingerprint. If Alice needs to
be sure that the contents of her document will not be changed, she can put her fingerprint at the
bottom of the document.

• Message and Message Digest
• The electronic equivalent of the document and fingerprint pair is the message and digest pair.

Message Integrity/ Hash function
• Hash function is a mathematical algorithm that maps data of arbitrary size (often called
the “message”) to a fixed size (the “hash value”, “hash”, or “message digest”).
• Accepts a variable-size message M as input and produces a fixed-size message digest
H(M) as output
• Hash function is a one-way function, that is, a function which is practically infeasible to
invert or reverse the computation.
• the only way to find a message that produces a given hash is to attempt a brute-force search of
possible inputs to see if they produce a match, or use a rainbow table of matched hashes.

Message

Message digest

Secure Hash Functions
• To be useful for message authentication, a hash function H must have the following
properties:
1. H can be applied to a block of data of any size.
2. H produces a fixed-length output.
3. H(x) is relatively easy to compute for any given x, making both hardware and software
implementations practical.
4. For any given code h, it is computationally infeasible to find x such that H(x) = h. A hash function
with this property is referred to as one-way or preimage resistant.
5. For any given block x, it is computationally infeasible to find y x with H(y) = H(x). A hash function
with this property is referred to as second preimage resistant. This is sometimes referred to as
weak collision resistant.
6. It is computationally infeasible to find any pair (x, y) such that H(x) = H(y). A hash function with this
property is referred to as collision resistant. This is sometimes referred to as strong collision
resistant.

Secure Hash Functions
• There are two approaches to attacking a secure hash function:
1. Cryptanalysis
• Involves exploiting logical weaknesses in the algorithm
2. Brute-force attack
• The strength of a hash function against this attack depends solely on
the length of the hash code produced by the algorithm

Checking Integrity
• To check the integrity of a message, or document, we run the cryptographic hash
function again and compare the new message digest with the previous one. If both
are the same, we are sure that the original message has not been changed.

Message Authentication
• A message digest does not authenticate the sender of the message. To provide
message authentication, Alice needs to provide proof that it is Alice sending the
message and not an attacker.
• The digest created by a cryptographic hash function is normally called a modification
detection code (MDC). What we need for message authentication is a message
authentication code (MAC).

Modification Detection Code (MDC)
• A modification detection code (MDC) is a message digest that can prove the integrity
of the message: that message has not been changed. If Alice needs to send a
message to Bob and be sure that the message will not change during transmission,
Alice can create a message digest, MDC, and send both the message and the MDC to
Bob. Bob can create a new MDC from the message and compare the received MDC
and the new MDC. If they are the same, the message has not been changed.

Message Authentication Code (MAC)
• To ensure the integrity of the message and the data origin authentication that Alice
is the originator of the message, not somebody else we need to change a
modification detection code (MDC) to a message authentication code (MAC). The
difference between a MDC and a MAC is that the second includes a secret between
Alice and Bob for example, a secret key that Eve does not own.
• The security of a MAC depends on the security of the underlying hash algorithm.

Approaches to Message Authentication
Using conventional
encryption
• Symmetric encryption alone is not a
suitable tool for data authentication
• We assume that only the sender and
receiver share a key, so only the genuine
sender would be able to encrypt a
message successfully
• The receiver assumes that no alterations
have been made and that sequencing is
proper if the message includes an error
detection code and a sequence number
• If the message includes a timestamp, the
receiver assumes that the message has
not been delayed beyond that normally
expected for network transit

Without message
encryption
• An authentication tag is generated and
appended to each message for
transmission
• The message itself is not encrypted and
can be read at the destination
independent of the authentication
function at the destination
• Because the message is not encrypted,
message confidentiality is not provided

Cryptographic hash algorithms
• SHA was developed by NIST and published as a federal information processing
standard (FIPS 180) in 1993
• Was revised in 1995 as SHA-1 and published as FIPS 180-1
• The actual standards document is entitled “Secure Hash Standard”
• Based on the hash function MD4 and its design closely models MD4
• Produces 160-bit hash values
• In 2005 NIST announced the intention to phase out approval of SHA-1 and move to
a reliance on SHA-2 by 2010

Public-key (asymmetric-key) cryptography
• Symmetric and asymmetric-key cryptography will exist in parallel and continue to
serve the community. We actually believe that they are complements of each
other; the advantages of one can compensate for the disadvantages of the other.
• Asymmetric key cryptography uses two separate keys: one private and one public.

Public-key (asymmetric-key) cryptography

General idea of asymmetric-key cryptosystem

Applications for public-key cryptosystems
• Public-key systems are characterized by the use of a cryptographic type of
algorithm with two keys, one held private and one available publicly
• Depending on the application, the sender uses either the sender’s private key, the
receiver’s public key, or both to perform some type of cryptographic function

The use of public-key
cryptosystems can be
classified into three
categories:

Encryption/decryption

The sender encrypts a
message with the
recipient’s public key

Digital signature

The sender “signs” a
message with its
private key

Key exchange

Two sides cooperate to
exchange a session key

Applications for public-key cryptosystems

RSA Cryptosystem
• The most common public-key algorithm is the RSA cryptosystem, named for its
inventors (Rivest, Shamir, and Adleman).

RSA Cryptosystem
• The security of RSA depends on it being used in such a way as to counter
potential attacks
• Possible attack approaches are:
1. Mathematical attacks
2. Timing attacks
3. Chosen ciphertext attacks
• To counter sophisticated chosen ciphertext attacks, RSA Security Inc
recommends modifying the plaintext using a procedure known as
optimal asymmetric encryption padding (OAEP)

Diffie-Hellman Key Exchange
• First published public-key algorithm
• A number of commercial products employ this key exchange technique
• Purpose of the algorithm is to enable two users to exchange a secret key
securely that then can be used for subsequent encryption of messages
• The algorithm itself is limited to the exchange of the keys
• Depends for its effectiveness on the difficulty of computing discrete
logarithms

Digital Signature
• A conventional signature is included in the document; it is part of the
document. But when we sign a document digitally, we send the
signature as a separate document.
• For a conventional signature, when the recipient receives a document,
she compares the signature on the document with the signature on file.
For a digital signature, the recipient receives the message and the
signature. The recipient needs to apply a verification technique to the
combination of the message and the signature to verify the authenticity.
• For a conventional signature, there is normally a one-to-many
relationship between a signature and documents. For a digital signature,
there is a one-to-one relationship between a signature and a message

Digitized Written Signature!
• Simply taking a digital picture of a written signature does not provide
adequate security.
• Such a digitized written signature could easily be copied from one
electronic document to another with no way to determine whether it is
legitimate.
• Electronic signatures, on the other hand, are unique to the message
being signed and will not verify if they are copied to another document.

Digital signatures are used just like
handwritten signatures.
• Digital signatures are used just like handwritten signatures.
• When you add them to a document, you are “signing” that document as
a way of endorsing or agreeing with what the document says.
• Unlike handwritten signatures, digital signatures are used only with
computers. They are electronic signatures that can be used to sign
electronic documents, like word processing files or spreadsheets.

What is a digital signature?
• A digital signature is a kind of ID. You can use it on the Internet to
identify yourself in a secure manner.
• This is extremely useful in areas such as electronic commerce. For
instance, when making a credit card purchase on the Internet, you can
use your digital signature to “sign” that purchase.
• This helps to ensure that only you can make purchases with your credit
card number.

Requirements for a Digital Signature
• The signature must be a bit pattern that depends on the message being
signed
• The signature must use some information unique to the sender, to
prevent both forgery and denial.
• It must be relatively easy to produce digital signature.

• It must be relatively easy to recognize and verify the digital signature.
• It must be computationally infeasible to forge a digital signature, either
by constructing a new message for an existing digital signature or by
constructing a fraudulent digital signature for a given message.
• It must be practical to hold a copy of the digital signature in storage.

How is a Digital Signature Produced?
• Very briefly, a typical digital signature works like this:
• A signature in the form of a code is generated by applying an
algorithm, such as RSA, and the sender’s private key to some or
all of the message contents.
• The recipient verifies the signature by decrypting it using the
sender’s public key.

Steps in making a digital signature
• Figure below shows the digital signature process.
• The sender uses a signing algorithm to sign the message. The message and the
signature are sent to the receiver.
• The receiver receives the message and the signature and applies the verifying
algorithm to the combination. If the result is true, the message is accepted;
otherwise, it is rejected.
• A digital signature needs a public-key system.
• The signer signs with her private key; the verifier verifies with the signer’s public key.
Sender

Receiver

!#1&4

!#1&4

!#1&4

If the hashes are equal,
the signature is valid
and message accepted

Steps in making a digital signature
• Sender runs a one-way hash function to create a fixed length message digest (hash)
from the message to be sent
• Sender encrypts the message digest with his private key to create a digital signature.
• Joe sends the signature and the message to Alice
• Receiver decrypts the signature with sender’s public key to reveal the message digest
• Receiver then applies the same one-way function to the message he/she received
from Sender to produce a message digest
• Receiver compares the message digest he/she created with the message digest sent
by sender. If they compare the integrity of the messages is verified.

Steps in making a digital signature
• Public key cryptography verifies integrity by using of public key signatures and secure
hashes.
• A secure hash algorithm is used to create a message digest. The message digest,
called a hash, is a short form of the message that changes if the message is modified.
• The hash is then signed with a private key. Anyone can recalculate the hash and use
the corresponding public key to verify the integrity of the message.

Importance of Digital Signatures
• Digital Signatures are a central component of modern cryptographic systems.
• In analogy to handwritten signatures on paper documents digital signatures are used
to guarantee the authenticity of electronic documents.
• Thus they play an important role for example in secure and reliable systems for
electronic commerce.

What makes a digital signature break?
• Digital signatures contain a special number. This number is generated by a complex
mathematical formula when you sign a document. When the digital signature is
added to a document, the document is passed to the formula. The formula examines
the document and generates a number. This number is then saved as part of the
digital signature.
• When somebody uses your public key to decode your signature, the same process
occurs. The document is again passed to the formula, and the formula returns a
number. The returned number is then compared to the number stored in the
signature. If the numbers are the same, then the document hasn’t been tampered
with, and the signature is good. If the numbers are different, then something in the
document has changed, and the signature will break.
• This means that once a document is signed, it can’t be changed without breaking the
signature.

How do I know the signature isn’t a fake?
• Even if the signature isn’t broken, you might be concerned that somebody has
falsified a signature. For example, if your friend Bob managed to create his own
digital certificate with your name on it, he could send documents with your signature
on them. In effect, Bob would be forging your signature.
• To make sure that a signature is authentic, you can check who issued or created the
certificate. Each certificate is issued by what is called a certificate authority (CA).
Certificate authorities can be anyone, from the government to your next door
neighbor. Whenever you view a digital signature, you can see who the certificate
authority was that issued the original certificate. You then have to decide for yourself
whether you can trust that certificate authority.

How do I know the signature isn’t a fake?
• For example, if you looked at a signature and saw that the certificate authority was
the State of California, you would probably want to trust that signature. The State of
California would have rigorous guidelines for issuing digital certificates. However, if
the certificate authority was “Wild Bill”, you might have second thoughts — who
knows what criteria Wild Bill might use?
• Since digital certificates are stored on your desktop computer, the only other way for
somebody to “forge” your signature is for them to get access to your computer.
However, digital certificates can also be password protected, in order to prevent this
from happening.

Are digital signatures being used today?
• Electronic commerce is also turning to digital signatures. “Smart cards,” which are
much like credit cards, can be used to store your digital certificate. You can then
“swipe” these cards on your computer to sign things on the Internet, such as credit
card purchases or bank deposits.
• Over the next year, the number of applications using digital signatures will continue
to grow. It will likely become the standard for identifying yourself on the Internet.

Digital Signature Services
• A secure digital signature scheme, like a secure conventional signature
can provide message authentication.
• A digital signature provides message integrity.
• Nonrepudiation can be provided using a trusted party.
• A digital signature does not provide privacy.
• If there is a need for privacy, another layer of encryption/decryption must be
applied.

Digital Signature Algorithms
1. Digital signature algorithm (DSA)
2. RSA digital signature algorithm
3. Elliptic curve digital signature algorithm (ECDSA)

Digital Signature Variations
• Time Stamped Signatures
• Sometimes a signed document needs to be time stamped to prevent
it from being replayed by an adversary. This is called time-stamped
digital signature scheme.
• Blind Signatures
• Sometimes we have a document that we want to get signed without
revealing the contents of the document to the signer.

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 5
User Authentication and Key Distribution

Contents
1. Remote user authentication principles

2. Symmetric key distribution using symmetric encryption
3. Kerberos
4. Key distribution using asymmetric encryption
5. Public-key infrastructure

Weekly Learning Outcomes
1. Understand the issues involved in the use of symmetric
encryption to distribute symmetric keys.
2. Explain the Kerberos network authentication protocol.
3. Understand the issues involved in the use of asymmetric
encryption to distribute symmetric keys.
4. Present an overview of public-key infrastructure concepts.

Remote user authentication principles
• In most computer security contexts, user authentication is the
fundamental building block and the primary line of defense
• User authentication is the basis for most types of access control and for
user accountability.
• RFC 4949 (Internet Security Glossary) defines user authentication as the
process of verifying an identity claimed by or for a system entity
• Identification step
• Presenting an identifier to the security system
• Verification step
• Presenting or generating authentication information that
corroborates the binding between the entity and the identifier
• An entity can be a person, a process, a client, or a server.

Remote user authentication principles
• An entity can be a person, a process, a client, or a server.
• There are two differences between message authentication (data-origin
authentication), discussed in Chapter 13, and entity authentication,
discussed in this chapter.
1. Message authentication might not happen in real time; entity
authentication does.
2. Message authentication simply authenticates one message; the process
needs to be repeated for each new message. Entity authentication
authenticates the claimant for the entire duration of a session.

Means of authentication
• There are four general means of authenticating a user’s identity, which can
be used alone or in combination
1. Something the individual knows
• Examples include a password, a personal identification number (PIN), or
answers to a prearranged set of questions
2. Something the individual possesses
• Examples include cryptographic keys, electronic keycards, smart cards, and
physical keys
• This type of authenticator is referred to as a token
3. Something the individual is (static biometrics)
• Examples include recognition by fingerprint, retina, and face
4. Something the individual does (dynamic biometrics)
• Examples include recognition by voice pattern, handwriting characteristics, and
typing rhythm

PASSWORDS
• The simplest and oldest method of entity authentication is the passwordbased authentication, where the password is something that the claimant
knows
• Fixed Password
• One-Time Password

Fixed Password
User ID and password file

•

Attacks on the first approach
• Eavesdropping
• Stealing a password
• Accessing a password file
• Guessing

Fixed Password
Hashing the password

•

Dictionary attack
• Create a list of password, calculate the hash value, and
search the second-column entries to find a match.

One-Time Password
A one-time password is a password that is used only once.
• In the first approach, the user and the system agree upon a list of
passwords.
• In the second approach, the user and the system agree to sequentially
update the password.
• The user and the system agree on an original pwd, P1, which is valid
only for the first access.
• During the first access, the user generates a new pwd P2, and encrypt
this pwd with P1 as the key, P2 is the pwd for the second access.
• If Eve can guess the first pwd P1, she can find all of the subsequent
ones.
•

Key Management

SYMMETRIC-KEY DISTRIBUTION
• Symmetric-key cryptography is more efficient than asymmetric-key
cryptography for enciphering large messages. Symmetric-key cryptography,
however, needs a shared secret key between two parties. The distribution
of keys is another problem.

Key-Distribution Center: KDC

Flat Multiple KDCs.

Hierarchical Multiple KDCs

Session Keys
• A KDC creates a secret key for each member. This secret key can be used
only between the member and the KDC, not between two members.
• A session symmetric key between two parties is used only once.
• A Simple Protocol Using a KDC

Key Distribution
• For two parties A and B, there are the following options:
1

2

• A key can be selected by A and physically delivered to B

• A third party can select the key and physically deliver it to A and B

3

• If A and B have previously and recently used a key, one party could transmit
the new key to the other, using the old key to encrypt the new key

4

• If A and B each have an encrypted connection to a third party C, C could
deliver a key on the encrypted links to A and B

Kerberos
• KDC and user authentication service developed at MIT
• Provides a centralized authentication server whose function is to
authenticate users to servers and servers to users
• Several systems, including Windows 2000, use Kerberos.
• Relies exclusively on symmetric encryption, making no use of public-key
encryption
Two versions are in use
• Version 4 implementations still exist, although this version
is being phased out
• Version 5 corrects some of the security deficiencies of
version 4 and has been issued as a proposed Internet
Standard (RFC 4120)

Kerberos Servers
• Three servers are involved in the Kerberos protocol:
• Authentication Server (AS)
• The authentication server (AS) is the KDC in the Kerberos protocol.
• Ticket-Granting Server (TGS)
• The ticket-granting server (TGS) issues a ticket for the real server (Bob).
• Real Server
• The real server (Bob) provides services for the user (Alice).

Kerberos servers

Kerberos Operation
• A client process (Alice) can access a process running on the real server (Bob) in six steps, as shown in Figure 15.8.
1. Alice sends her request to the AS in plain text using her registered identity.
2. The AS sends a message encrypted with Alice’s permanent symmetric key, KA-AS. The message contains two
items: a session key, KA-TGS, that is used by Alice to contact the TGS, and a ticket for the TGS that is encrypted
with the TGS symmetric key, KAS-TGS. Alice does not know KA-AS, but when the message arrives, she types her
symmetric password. The password and the appropriate algorithm together create KA-AS if the password is
correct. The password is then immediately destroyed; it is not sent to the network and it does not stay in the
terminal. It is used only for a moment to create KA-AS. The process now uses KA-AS to decrypt the message
sent. KA-TGS and the ticket are extracted.
3. Alice now sends three items to the TGS. The first is the ticket received from the AS. The second is the name of
the real server (Bob), the third is a timestamp that is encrypted by KA-TGS. The timestamp prevents a replay by
Eve.
4. Now, the TGS sends two tickets, each containing the session key between Alice and Bob, KA-B. The ticket for
Alice is encrypted with KA-TGS; the ticket for Bob is encrypted with Bob’s key, KTGS-B. Note that Eve cannot
extract KAB because Eve does not know KA-TGS or KTGS-B. She cannot replay step 3 because she cannot replace
the timestamp with a new one (she does not know KA-TGS). Even if she is very quick and sends the step 3
message before the timestamp has expired, she still receives the same two tickets that she cannot decipher.
5. Alice sends Bob’s ticket with the timestamp encrypted by KA-B.
6. Bob confirms the receipt by adding 1 to the timestamp. The message is encrypted with KA-B and sent to Alice.

Kerberos version 4 vs 5
• The minor differences between version 4 and version 5 are briefly listed
below:
1. Version 5 has a longer ticket lifetime.
2. Version 5 allows tickets to be renewed.
3. Version 5 can accept any symmetric-key algorithm.
4. Version 5 uses a different protocol for describing data types.
5. Version 5 has more overhead than version 4.

Overview of Kerberos

Kerberos Realms
• Kerberos realm
• A set of managed nodes that share the same
Kerberos database
• Kerberos allows the global distribution of ASs and
TGSs, with each system called a realm. A user may
get a ticket for a local server or a remote server.
• The Kerberos database resides on the Kerberos
master computer system, which should be kept in a
physically secure room
• A read-only copy of the Kerberos database might
also reside on other Kerberos computer systems
• All changes to the database must be made on the
master computer system
• Changing or accessing the contents of a Kerberos
database requires the Kerberos master password

A Kerberos environment
consists of:
A Kerberos server

A number of clients

A number of application
servers

Key distribution using asymmetric
encryption
• One of the major roles of public-key encryption is to address the problem
of key distribution
• There are two distinct aspects to the use of public-key encryption in this
regard:
• The distribution of public keys
• The use of public-key encryption to distribute secret keys

Public-key certificate
• Public-key certificate
• Consists of a public key plus a user ID of the key owner, with the whole block signed
by a trusted third party
• Typically, the third party is a certificate authority (CA) that is trusted by the user
community, such as a government agency or a financial institution
• A user can present his or her public key to the authority in a secure manner and
obtain a certificate
• The user can then publish the certificate
• Anyone needing this user’s public key can obtain the certificate and verify that it is
valid by way of the attached trusted signature

Generation of a public-key certificate

X.509 Certificates
• ITU-T recommendation X.509 is part of the X.500 series of
recommendations that define a directory service
• Defines a framework for the provision of authentication services by the
X.500 directory to its users
• The directory may serve as a repository of public-key certificates
• Defines alternative authentication protocols based on the use of public-key
certificates
• Was initially issued in 1988
• Based on the use of public-key cryptography and digital signatures
• The standard does not dictate the use of a specific algorithm but
recommends RSA

X.509 Formats

Obtaining a user’s certificate
• User certificates generated by a CA have the following characteristics:
• Any user with access to the public key of the CA can verify the user public key that
was certified
• No party other than the certification authority can modify the certificate without
this being detected

• Because certificates are unforgeable, they can be placed in a directory
without the need for the directory to make special efforts to protect them

Revocation of certificates
• Each certificate includes a period of validity
• Typically, a new certificate is issued just before the expiration of the old
one
• It may be desirable on occasion to revoke a certificate before it expires for
one of the following reasons:
• The user’s private key is assumed to be compromised
• The user is no longer certified by this CA; reasons for this include subject’s name has
changed, the certificate is superseded, or the certificate was not issued in
conformance with the CA’s policies
• The CA’s certificate is assumed to be compromised

X.509 Version 3

Includes a number of optional extensions that may be
added to the version 2 format

Each extension consists of:
• A n extension identifier
• A criticality indicator
• An extension value

The certificate extensions fall into three main
categories:
• Key and policy information
• Subject and issuer attributes
• Certification path constraints

Key and policy information
• These extensions convey additional information about the subject and
issuer keys, plus indicators of certificate policy
• A certificate policy is a named set of rules that indicates the applicability of
a certificate to a particular community and/or class of application with
common security requirements

Includes:
• Authority key identifier
• Subject key identifier
• Key usage
• Private-key usage period
• Certificate policies
• Policy mappings

Certificate subject and issuer attributes
• These extensions support alternative names, in alternative formats, for a
certificate subject or certificate issuer and can convey additional
information about the certificate subject to increase a certificate user’s
confidence that the certificate subject is a particular person or entity

Includes:
• Subject alternative name
• Issuer alternative name
• Subject directory attributes

Certification path constraints
• These extensions allow constraint specifications to be included in
certificates issued for CAs by other CAs
• The constraints may restrict the types of certificates that can be issued by
the subject CA or that may occur subsequently in a certification chain

Includes:
• Basic constraints
• Name constraints
• Policy constraints

Public-Key Infrastructures (PKI)
• Public-Key Infrastructure (PKI) is a model for creating, distributing, and
revoking certificates based on the X.509.

Some duties of a PKI

Standards
The Extensible Markup Language (XML)
• Appear similar to HTML documents that are visible as Web pages, but provide greater functionality
• Includes strict definitions of the data type of each field
• Provides encoding rules for commands that are used to transfer and update data objects
The Simple Object Access Protocol (SOAP)
• Minimal set of conventions for invoking code using XML over HTTP
• Enables applications to request services from one another with XML-based requests and receive
responses as data formatted with XML

WS-Security
• A set of SOAP extensions for implementing message integrity and confidentiality in Web services
• Assigns security tokens to each message for use in authentication
Security Assertion Markup Language (SAML)
• An XML-based language for the exchange of security information between online business partners
• Conveys authentication information in the form of assertions about subjects

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 6
Network Access Control

Contents
1. Network access control
2. Extensible authentication protocol
3. IEEE 802.1X port-based network access control

Weekly Learning Outcomes
1. Discuss the principal elements of a network access control
system.
2. Discuss the principal network access enforcement methods.
3. Present an overview of the Extensible Authentication
Protocol.
4. Understand the operation and role of the IEEE 802.1X portbased network access control mechanism.

Network Access Control (NAC)
• An umbrella term for managing access to a network
• Authenticates users logging into the network and determines what data
they can access and actions they can perform
• Also examines the health of the user’s computer or mobile device

NAC systems deal with three categories
of components:

Access requester (AR)

Policy server

Network access server (NAS)

• Node that is attempting to
access the network and may be
any device that is managed by
the NAC system, including
workstations, servers, printers,
cameras, and other IP-enabled
devices
• Also referred to as supplicants,
or clients

• Determines what access
should be granted
• Often relies on backend
systems

• Functions as an access control
point for users in remote locations
connecting to an enterprise’s
internal network
• Also called a media gateway,
remote access server (RAS), or
policy server
• May include its own
authentication services or rely on
a separate authentication service
from the policy server

Network Access Control Context

Network Access Enforcement Methods
• The actions that are applied to ARs to regulate access to the enterprise
network
• Many vendors support multiple enforcement methods simultaneously,
allowing the customer to tailor the configuration by using one or a
combination of methods

Common NAC enforcement methods:

• IEEE 802.1X
• Virtual local area networks (VLANs)
• Firewall
• DHCP management

Network Access Enforcement Methods
• The Extensible Authentication Protocol
(EAP), defined in RFC 3748, acts as a
framework for network access and
authentication protocols.
• EAP provides a set of protocol
messages that can encapsulate various
authentication methods to be used
between a client and an
authentication server.
• EAP can operate over a variety of
network and link level facilities,
including point-to-point links, LANs,
and other networks, and can
accommodate the authentication
needs of the various links and
networks.

Authentication Methods
• EAP provides a generic transport service (framework) for the exchange of
authentication information between a client system and an authentication
server
• The basic EAP transport service is extended by using a specific
authentication protocol that is installed in both the EAP client and the
authentication server

Commonly supported EAP methods:
• EAP Transport Layer Security
• EAP Tunneled TLS
• EAP Generalized Pre-Shared Key
• EAP-IKEv2

EAP protocol exchanges

EAP message flow

Authentication Methods
• EAP provides a generic transport service for the exchange of authentication
information between a client system and an authentication server
• The basic EAP transport service is extended by using a specific
authentication protocol that is installed in both the EAP client and the
authentication server

Commonly supported EAP methods:
• EAP Transport Layer Security
• EAP Tunneled TLS
• EAP Generalized Pre-Shared Key
• EAP-IKEv2

IEEE 802.1X and EAP
• Extensible Authentication
Protocol is a universal
authentication framework
frequently used in wireless
networks and Point-to-Point
connections.
• IEEE 802.1X is simply the
standard Extensible
Authentication Protocol (EAP),
by which WiFi authentication is
able to transmit.

Terminology Related to IEEE 802.1X

IEEE 802.1X Access control
• As indicated in Figure 5.5, 802.1X uses the
concepts of controlled and uncontrolled ports.
• Ports are logical entities defined within the
authenticator and refer to physical network
connections.
• Each logical port is mapped to one of these two
types of physical ports.
• An uncontrolled port allows the exchange of
protocol data units (PDUs) between the supplicant
and the AS, regardless of the authentication state
of the supplicant.
• A controlled port allows the exchange of PDUs
between a supplicant and other systems on the
network only if the current state of the supplicant
authorizes such an exchange.

EAPOL (EAP over LAN)
• The essential element defined in 802.1X is a protocol known as EAPOL (EAP over LAN).
• EAPOL operates at the network layers and makes use of an IEEE 802 LAN, such as Ethernet
or Wi-Fi, at the link level.
• EAPOL enables a supplicant to communicate with an authenticator and supports the
exchange of EAP packets for authentication.

Common EAPOL Frame Types
• The essential element defined in 802.1X is a protocol known as EAPOL (EAP over LAN).
• EAPOL operates at the network layers and makes use of an IEEE 802 LAN, such as Ethernet
or Wi-Fi, at the link level.
• EAPOL enables a supplicant to communicate with an authenticator and supports the
exchange of EAP packets for authentication.

Timing diagram for IEEE 802.1X

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 7
Cloud Security

Contents
1. Cloud computing
2. Cloud security risks and countermeasures
3. Data protection in the cloud

4. Addressing cloud computing security concerns

Weekly Learning Outcomes
1. Present an overview of cloud computing concepts.
2. Understand the unique security issues related to cloud
computing.

Cloud Computing
• Cloud Computing provides us a means by
which we can access the applications as
utilities, over the Internet. It allows us to
create, configure, and customize
applications online.
• With Cloud Computing users can access
database resources via the internet from
anywhere for as long as they need without
worrying about any maintenance or
management of actual resources.
• It offers online data storage, infrastructure
and application.
• Cloud Computing is both a combination of
software and hardware
• Eg: Email, Search engines, Social network

Data storage

Cloud Computing Models
• There are certain services and models working behind the scene
making the cloud computing feasible and accessible to end users.
1. Deployment Models
• Deployment models define the type of access to the cloud,
i.e., how the cloud is located? Cloud can have any of the
four types of access: Public, Private, Hybrid and
Community.
2. Service Models
• Service Models are the reference models on which the
Cloud Computing is based.

Deployment Models
• PUBLIC CLOUD : The Public Cloud allows systems and services to be easily accessible to
the general public. Public cloud may be less secure because of its openness, e.g., e-mail.
• PRIVATE CLOUD : The Private Cloud allows systems and services to be accessible within an
organization. It offers increased security because of its private nature.
• COMMUNITY CLOUD : The Community Cloud allows systems and Services to be
accessible by group of organizations.
• HYBRID CLOUD : The Hybrid Cloud is mixture of public and private cloud. However, the
critical activities are performed using private cloud while the non-critical activities are
performed using public cloud.

Service Models
1. Infrastructure as a Service (IaaS)
2. Platform as a Service (PaaS)
3. Software as a Service (SaaS)

Infrastructure as a Service (IaaS)
• IaaS is the delivery of technology infrastructure as an on demand scalable
service.
• IaaS provides access to fundamental resources such as physical machines,
virtual machines, virtual storage, etc.
• Usually billed based on usage
• Usually, multi-tenant virtualized environment
• Can be coupled with Managed Services for OS and application support)
• Examples: DigitalOcean, Linode, Rackspace, Amazon Web Services (AWS),
Cisco Metacloud, Microsoft Azure, Google Compute Engine (GCE)

Platform as a Service (PaaS)
• PaaS provides the runtime environment for applications, development &
deployment tools, etc.
• PaaS provides all of the facilities required to support the complete life cycle
of building and delivering web applications and services entirely from the
Internet.
• Typically applications must be developed with a particular platform in mind
• Multi-tenant environments
• Highly scalable multi-tier architecture
• Examples: Microsoft Azure, Google App Engine, IBM Cloud Foundry, Red
Hat OpenShift, Oracle Cloud Platform

Cloud Security Risks and
Countermeasures
• The Cloud Security Alliance [CSA10] lists the following as the top cloud
specific security threats, together with suggested countermeasures:
Abuse and nefarious use of cloud computing

• Countermeasures: stricter initial registration and validation processes;
enhanced credit card fraud monitoring and coordination; comprehensive
introspection of customer network traffic; monitoring public blacklists for
one’s own network blocks
Malicious insiders
• Countermeasures: enforce strict supply chain management and conduct a
comprehensive supplier assessment; specify human resource
requirements as part of legal contract; require transparency into overall
information security and management practices, as well as compliance
reporting; determine security breach notification processes

Cloud Security Risks and
Countermeasures (continued)

Insecure interfaces
and APIs

Shared technology
issues

Data loss or
leakage

Countermeasures:
analyzing the security
model of CP interfaces;
ensuring that strong
authentication and access
controls are implemented
in concert with encryption
machines; understanding
the dependency chain
associated with the API

Countermeasures: implement
security best practices for
installation/configuration;
monitor environment for
unauthorized changes/activity;
promote strong authentication
and access control for
administrative access and
operations; enforce SLAs for
patching and vulnerability
remediation; conduct
vulnerability scanning and
configuration audits

Countermeasures: implement
strong API access control;
encrypt and protect integrity of
data in transit; analyze data
protection at both design and
run time; implement strong key
generation, storage and
management, and destruction
practices

Software as a Service (SaaS)
• SaaS model allows to use software applications as a service to end users.
• SaaS is a software delivery methodology that provides licensed multitenant access to software and its functions remotely as a Web-based
service.
• Usually billed based on usage
• Usually, multi-tenant environment
• Highly scalable architecture
• Examples: Dropbox, Google Applications (Gmail, Google Docs, Google
Sheets, Google Drive…etc)

Cloud Security Risks and
Countermeasures (continued)
• Account or service hijacking
• Countermeasures: prohibit the sharing of account credentials between
users and services; leverage strong two-factor authentication
techniques where possible; employ proactive monitoring to detect
unauthorized activity; understand CP security policies and SLAs
• Unknown risk profile
• Countermeasures: disclosure of applicable logs and data; partial/full
disclosure of infrastructure details; monitoring and alerting on
necessary information

NIST Guidelines on Security and Privacy
Issues and Recommendations

Advantages of Cloud Computing
• Back-up and restore data

• Once the data is stored in the cloud, it is easier to get back-up and restore that data using the cloud.

• Improved collaboration

• Cloud applications improve collaboration by allowing groups of people to quickly and easily share information in the
cloud via shared storage.

• Excellent accessibility

• Cloud allows us to quickly and easily access store information anywhere, anytime in the whole world, using an
internet connection. An internet cloud infrastructure increases organization productivity and efficiency by ensuring
that our data is always accessible.

• Low maintenance cost

• Cloud computing reduces both hardware and software maintenance costs for organizations.

• Mobility

• Cloud computing allows us to easily access all cloud data via mobile.

• Services in the pay-per-use model

• Cloud computing offers Application Programming Interfaces (APIs) to the users for access services on the cloud and
pays the charges as per the usage of service.

• Unlimited storage capacity

• Cloud offers us a huge amount of storing capacity for storing our important data such as documents, images, audio,
video, etc. in one place.

• Data security

• Data security is one of the biggest advantages of cloud computing. Cloud offers many advanced features related to
security and ensures that data is securely stored and handled.

Advantages of Cloud Computing
• Back-up and restore data

• Once the data is stored in the cloud, it is easier to get back-up and restore that data using the cloud.

• Improved collaboration

• Cloud applications improve collaboration by allowing groups of people to quickly and easily share information in the
cloud via shared storage.

• Excellent accessibility

• Cloud allows us to quickly and easily access store information anywhere, anytime in the whole world, using an
internet connection. An internet cloud infrastructure increases organization productivity and efficiency by ensuring
that our data is always accessible.

• Low maintenance cost

• Cloud computing reduces both hardware and software maintenance costs for organizations.

• Mobility

• Cloud computing allows us to easily access all cloud data via mobile.

• Services in the pay-per-use model

• Cloud computing offers Application Programming Interfaces (APIs) to the users for access services on the cloud and
pays the charges as per the usage of service.

• Unlimited storage capacity

• Cloud offers us a huge amount of storing capacity for storing our important data such as documents, images, audio,
video, etc. in one place.

• Data security

• Data security is one of the biggest advantages of cloud computing. Cloud offers many advanced features related to
security and ensures that data is securely stored and handled.

Data Protection in the Cloud
• The threat of data compromise increases in the cloud
• Database environments used in cloud computing can vary significantly
Multi-instance model
• Provides a unique DBMS running on a virtual machine instance for each cloud
subscriber
• This gives the subscriber complete control over role definition, user
authorization, and other administrative tasks related to security
Multi-tenant model
• Provides a predefined environment for the cloud subscriber that is shared with
other tenants, typically through tagging data with a subscriber identifier
• Tagging gives the appearance of exclusive use of the instance, but relies on the
CP to establish and maintain a sound secure database environment

Data Protection in the Cloud
• Data must be secured while at rest, in transit, and in use, and access to the data must be
controlled
• The client can employ encryption to protect data in transit, though this involves key
management responsibilities for the CP
• For data at rest the ideal security measure is for the client to encrypt the database and
only store encrypted data in the cloud, with the CP having no access to the encryption
key
• A straightforward solution to the security problem in this context is to encrypt the entire
database and not provide the encryption/decryption keys to the service provider
• The user has little ability to access individual data items based on searches or
indexing on key parameters
• The user would have to download entire tables from the database, decrypt the
tables, and work with the results
• To provide more flexibility it must be possible to work with the database in its
encrypted form

Data Protection in the Cloud

Cloud Security as a Service (SecaaS)
• The Cloud Security Alliance defines SecaaS as the provision of security applications and
services via the cloud either to cloud-based infrastructure and software or from the
cloud to the customers’ on-premise systems
• The Cloud Security Alliance has identified the following SecaaS categories of service:
• Identity and access management
• Data loss prevention
• Web security
• E-mail security
• Security assessments
• Intrusion management
• Security information and event management
• Encryption
• Business continuity and disaster recovery
• Network security

Cloud Security as a Service (SecaaS)

Control Functions and Classes

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 7
Transport-Level Security

Contents
1. Web Security Considerations
2. Transport Layer Security
3. HTTPS
4. Secure Shell (SSH)

Weekly Learning Outcomes
1. Summarize Web security threats and Web traffic security
approaches.
2. Present an overview of Transport Layer Security (TLS).
3. Understand the differences between Secure Sockets Layer
and Transport Layer Security.
4. Present an overview of HTTPS (HTTP over SSL).
5. Present an overview of Secure Shell (SSH).

Web Security Considerations
• The World Wide Web is fundamentally a client/server application running
over the Internet and TCP/IP intranets
• The following characteristics of Web usage suggest the need for tailored
security tools:
• Web servers are relatively easy to configure and manage
• Web content is increasingly easy to develop
• The underlying software is extraordinarily complex
• May hide many potential security flaws

• A Web server can be exploited as a launching pad into the corporation’s or
agency’s entire computer complex
• Casual and untrained (in security matters) users are common clients for
Web-based services
• Such users are not necessarily aware of the security risks that exist and do not have
the tools or knowledge to take effective countermeasures

A Comparison of Threats on the Web

SSL and TLS protocols
• Two protocols are dominant today
for providing security at the
transport layer: the Secure Sockets
Layer (SSL) Protocol and the
Transport Layer Security (TLS)
Protocol.
• One of the goals of these protocols
is to provide server and client
• authentication, data confidentiality,
and data integrity

SSL vs TLS

SSL Services
• Fragmentation
• First, SSL divides the data into blocks of 214 bytes or less.

• Compression
• Each fragment of data is compressed using one of the lossless compression
methods negotiated between the client and server. This service is optional.

• Message Integrity
• To preserve the integrity of data, SSL uses a keyed-hash function to create a MAC.

• Confidentiality
• To provide confidentiality, the original data and the MAC are encrypted using
symmetric key cryptography.

• Framing
• A header is added to the encrypted payload. The payload is then passed to a
reliable transport layer protocol.

SSL Sessions and Connections
• In a session, one party has the role of a client and the other the role of a
server; in a connection, both parties have equal roles, they are peers.
• A session can consist of many connections. A connection between two
parties can be terminated and reestablished within the same session

SSL Protocols
• SSL defines four protocols in two layers.

SSL Handshake Protocol
• The Handshake Protocol uses messages to negotiate the cipher suite, to
authenticate the server to the client and the client to the server if needed,
and to exchange information for building the cryptographic secrets.

SSL ChangeCipherSpec Protocol
• The change cipher spec protocol is used to change the encryption being
used by the client and server.
• It is normally used as part of the handshake process to switch to symmetric
key encryption.
• The CCS protocol is a single message that tells the peer that the sender
wants to change to a new set of keys, which are then created from
information exchanged by the handshake protocol.

SSL Alert Protocol
• SSL uses the Alert Protocol for reporting errors and abnormal conditions. It
has only one message type, the Alert message, that describes the problem
and its level (warning or fatal).
• Table 17.4 shows the types of Alert messages defined for SSL.

SSL Record Protocol
• The Record Protocol carries messages from the upper layer (Handshake
Protocol, ChangeCipherSpec Protocol, Alert Protocol, or application layer).
• The SSL record protocol provides two services for SSL connections:
• Confidentiality, by encrypting application data.
• Message integrity, by using a message authentication code (MAC).

Transport Layer security (TSL)
• One of the most widely used
security services
• TLS is an Internet standard that
evolved from a commercial
protocol known as Secure Sockets
Layer (SSL)
• TLS is a general-purpose service
implemented as a set of protocols
that rely on TCP
• TLS could be provided as part of the
underlying protocol suite and
therefore be transparent to
applications
• Alternatively, TLS can be embedded
in specific packages

Transport Layer security (TSL)
• One of the most widely used
security services
• TLS is an Internet standard that
evolved from a commercial
protocol known as Secure Sockets
Layer (SSL)
• TLS is a general-purpose service
implemented as a set of protocols
that rely on TCP
• TLS could be provided as part of the
underlying protocol suite and
therefore be transparent to
applications
• Alternatively, TLS can be embedded
in specific packages

TLS Architecture
• Two important TLS concepts are the TLS session and the TLS connection.

TLS
connection

TLS session

• A transport that provides a suitable type of service
• For TLS such connections are peer-to-peer relationships
• Connections are transient
• Every connection is associated with one session

• An association between a client and a server
• Created by the Handshake Protocol
• Define a set of cryptographic security parameters which can be shared
among multiple connections
• Are used to avoid the expensive negotiation of new security
parameters for each connection

A session state is defined by the following
parameters:
Session identifier

An arbitrary byte sequence chosen by the server to
identify an active or resumable session state

Peer certificate

An X509.v3 certificate of the peer; this element of the
state may be null

Compression method

The algorithm used to compress data prior to encryption

Cipher spec

Specifies the bulk data encryption algorithm and a hash
algorithm used for MAC calculation; also defines
cryptographic attributes such as the hash_size

Master secret

48-byte secret shared between the client and the server

Is resumable

A flag indicating whether the session can be used to
initiate new connections

A connection state is defined by the
following parameters:
Server and
client random

• Byte sequences that are
chosen by the server and
client for each connection

Server write
MAC secret

• The secret key used in MAC
operations on data sent by the
server

Client write
MAC secret

• The secret key used in MAC
operations on data sent by the
client

Server write
key

• The secret encryption key for
data encrypted by the server and
decrypted by the client

Client write
key

• The symmetric encryption key
for data encrypted by the client
and decrypted by the server

Initialization
vectors

• When a block cipher in CBC mode
is used, an initialization vector (IV)
is maintained for each key
• This field is first initialized by the
SSL Handshake Protocol
• The final ciphertext block from
each record is preserved for use
as the IV with the following record

Sequence
numbers

• Each party maintains separate
sequence numbers for
transmitted and received
messages for each connection
• When a party sends or receives a
change cipher spec message, the
appropriate sequence number is
set to zero
• Sequence numbers may not
exceed 264 – 1

TLS Record Protocol
• Two important TLS concepts are the TLS session and the TLS connection.
The TLS Record
Protocol provides
two services for TLS
connections

Confidentiality

Message integrity

The Handshake Protocol
defines a shared secret key
that is used for
conventional encryption of
TLS payloads

The Handshake Protocol
also defines a shared
secret key that is used to
form a message
authentication code (MAC)

TLS Record Protocol Operation
• Two important TLS concepts are the TLS session and the TLS connection.

TLS Record Format

TLS Record Protocol Payload

Handshake Protocol Action

SSL/TLS Attacks
• Attack categories
• Attacks on the handshake protocol
• Attacks on the record and application data protocols
• Attacks on the PKI
• Other attacks

HTTPS (HTTP over SSL)
• Refers to the combination of HTTP and SSL to implement secure
communication between a Web browser and a Web server
• The HTTPS capability is built into all modern Web browsers
• A user of a Web browser will see URL addresses that begin with https://
rather than http://
• If HTTPS is specified, port 443 is used, which invokes SSL
• Documented in RFC 2818, HTTP Over TLS
• There is no fundamental change in using HTTP over either SSL or TLS and both
implementations are referred to as HTTPS

• When HTTPS is used, the following elements of the communication are
encrypted:
• URL of the requested document
• Contents of the document
• Contents of browser forms
• Cookies sent from browser to server and from server to browser
• Contents of HTTP header

Connection Initiation

For HTTPS, the agent acting as the HTTP
client also acts as the TLS client
• The client initiates a connection to the server on the
appropriate port and then sends the TLS ClientHello to
begin the TLS handshake
• When the TLS handshake has finished, the client may
then initiate the first HTTP request
• All HTTP data is to be sent as TLS application data

There are three levels of awareness of a
connection in HTTPS:
• At the HTTP level, an HTTP client requests a connection
to an HTTP server by sending a connection request to the
next lowest layer
•Typically the next lowest layer is TCP, but it may also be
TLS/SSL
• At the level of TLS, a session is established between a TLS
client and a TLS server
•This session can support one or more connections at
any time
• A TLS request to establish a connection begins with the
establishment of a TCP connection between the TCP
entity on the client side and the TCP entity on the server
side

Connection Closure
• An HTTP client or server can indicate the closing of a connection by
including the line Connection: close in an HTTP record
• The closure of an HTTPS connection requires that TLS close the connection
with the peer TLS entity on the remote side, which will involve closing the
underlying TCP connection
• TLS implementations must initiate an exchange of closure alerts before
closing a connection
• A TLS implementation may, after sending a closure alert, close the
connection without waiting for the peer to send its closure alert,
generating an “incomplete close”
• An unannounced TCP closure could be evidence of some sort of attack so
the HTTPS client should issue some sort of security warning when this
occurs

Secure Shell (SSH)
SSH client and server
applications are widely
available for most
operating systems

• Has become the
method of choice for
remote login and X
tunneling
• Is rapidly becoming
one of the most
pervasive applications
for encryption
technology outside of
embedded systems
SSH2 fixes a number of
security flaws in the
original scheme
• Is documented as a
proposed standard in
IETF RFCs 4250
through 4256

A protocol for secure
network communications
designed to be relatively
simple and inexpensive to
implement
The initial version, SSH1
was focused on
providing a secure
remote logon facility to
replace TELNET and
other remote logon
schemes that provided
no security

SSH also provides a
more general
client/server capability
and can be used for
such network functions
as file transfer and email

SSH Protocol Stack

Transport Layer Protocol
• Server authentication occurs at the transport layer, based on the server
possessing a public/private key pair
• A server may have multiple host keys using multiple different asymmetric
encryption algorithms
• Multiple hosts may share the same host key
• The server host key is used during key exchange to authenticate the
identity of the host
• RFC 4251 dictates two alternative trust models:
• The client has a local database that associates each host name with the
corresponding public host key
• The host name-to-key association is certified by a trusted certification
authority (CA); the client only knows the CA root key and can verify the
validity of all host keys certified by accepted CAs

SSH Transport Layer Protocol Packet
Exchanges

SSH Transport Layer Protocol Packet
Formation

Authentication Methods
Publickey
• The client sends a message to the server that contains the client’s public key, with
the message signed by the client’s private key
• When the server receives this message, it checks whether the supplied key is
acceptable for authentication and, if so, it checks whether the signature is correct
Password
• The client sends a message containing a plaintext password, which is protected by
encryption by the Transport Layer Protocol
Hostbased
• Authentication is performed on the client’s host rather than the client itself
• This method works by having the client send a signature created with the private
key of the client host
• Rather than directly verifying the user’s identity, the SSH server verifies the
identity of the client host

Connection Protocol
• The SSH Connection Protocol runs on top of the SSH Transport Layer
Protocol and assumes that a secure authentication connection is in use
• The secure authentication connection, referred to as a tunnel, is used
by the Connection Protocol to multiplex a number of logical channels
• Channel mechanism
• All types of communication using SSH are supported using separate
channels
• Either side may open a channel
• For each channel, each side associates a unique channel number
• Channels are flow controlled using a window mechanism
• No data may be sent to a channel until a message is received to indicate
that window space is available
• The life of a channel progresses through three stages: opening a
channel, data transfer, and closing a channel

Example of SSH Connection Protocol
Message Exchange

Channel Types
• Four channel types are recognized in the SSH Connection Protocol
specification
Session
• The remote execution of a program
• The program may be a shell, an application such as file transfer or e-mail, a system
command, or some built-in subsystem
• Once a session channel is opened, subsequent requests are used to start the remote
program
X11
• Refers to the X Window System, a computer software system and network protocol that
provides a graphical user interface (GUI) for networked computers
• X allows applications to run on a network server but to be displayed on a desktop machine
Forwarded-tcpip
• Remote port forwarding
Direct-tcpip
• Local port forwarding

Port Forwarding
• One of the most useful features of SSH
• Provides the ability to convert any insecure TCP connection into a secure
SSH connection (also referred to as SSH tunneling)
• Incoming TCP traffic is delivered to the appropriate application on the basis
of the port number (a port is an identifier of a user of TCP)
• An application may employ multiple port numbers

SSH Transport Layer Packet Exchanges

Thank You

College of Computing and Informatics
Network Security

Network Security
Week 9
Wireless Network Security

Contents
• Wireless network security
• Network threats
• Security measures
• Mobile device security
• Security threats
• Security strategy
• IEEE 802.11 wireless LAN
overview
• Wi-Fi Alliance
• IEEE 802 protocol
architecture • IEEE 802.11
network components and
architectural model
• IEEE 802.11 services

• IEEE 802.11i wireless LAN
security
• IEEE 802.11i services
• IEEE 802.11i phases of
operation
• Discovery phase
• Authentication phase
• Key management
phase
• Protected data transfer
phase
• The IEEE 802.11i
pseudorandom
function

Weekly Learning Outcomes
1. Describe the security threats and countermeasures for
wireless networks.
2. Discuss the types of security threats posed by the use of
mobile devices and mobile device security strategy.
3. Discuss the elements of the IEEE 802.11 wireless LAN
Standard and security architecture.

Wireless Security
• Wireless network security is the process of designing,
implementing and ensuring security on a wireless computer
network. It is a subset of network security that adds protection
for a wireless computer network. Wireless network security is
also known as wireless security.
• include the following:
• Channel
• Mobility
• Resources
• Accessibility

Wireless Networking Components
• Wireless Endpoint: WIFI-enabled laptop/tablet, cell phone, Bluetooth
device.
• Access point: Cell towers, WIFI hotspots, wireless routers
• Transmission medium: carries signals

Wireless Network Threats
1. Accidental association
2. Malicious association
3. Ad hoc networks
4. Nontraditional networks
5. Identity theft (MAC spoofing)
6. Man-in-the middle attacks
7. Denial of service (DoS)
8. Network injection

Securing Wireless Transmissions
• The principal threats to wireless transmission are eavesdropping, altering or
inserting messages, and disruption
• To deal with eavesdropping, two types of countermeasures are appropriate:
• Signal-hiding techniques
• Turn off SSID broadcasting by wireless access points
• Assign cryptic names to SSIDs
• Reduce signal strength to the lowest level that still provides requisite
coverage
• Locate wireless access points in the interior of the building, away from
windows and exterior walls
• Encryption
• Is effective against eavesdropping to the extent that the encryption
keys are secured

Securing Wireless Access Points
The main threat involving wireless access points is unauthorized
access to the network
• The principal approach for preventing such access is the IEEE
802.1x standard for port-based network access control
• The standard provides an authentication mechanism for devices
wishing to attach to a LAN or wireless network
• The use of 802.1x can prevent rogue access points and other
unauthorized devices from becoming insecure backdoors
•

Mobile Device Security
• Mobile devices have become an essential element for
organizations as part of the overall network infrastructure
• Prior to the widespread use of smartphones, network security
was based upon clearly defined perimeters that separated
trusted internal networks from the untrusted Internet
• Due to massive changes, an organization’s networks must now
accommodate:
• Growing use of new devices
• Cloud-based applications
• De-perimeterization
• External business requirements

Security Threats

• Major security concerns for mobile devices:
• Lack of physical security control
• Use of untrusted mobile devices
• Use of untrusted networks
• Use of apps created by unknown parties
•Interaction with other systems (e.g., cloudbased data sync)
• Use of untrusted contents

Mobile devices Security Elements

IEEE 802.11 Wireless LAN Overview
• IEEE 802 is a committee that has developed standards for a
wide range of local area networks (LANs)
• In 1990 the IEEE 802 Committee formed a new working group,
IEEE 802.11, with a charter to develop a protocol and
transmission specifications for wireless LANs (WLANs)
• Since that time, the demand for WLANs at different frequencies
and data rates has exploded

IEEE 802.11 Terminology
•

•
•
•
•

•

•
•

Access point (AP)
• Any entity that has station functionality and provides access to the distribution system via the
wireless medium for associated stations.
Basic service set (BSS)
• A set of stations controlled by a single coordination function.
Coordination function
• The logical function that determines when a station operating within a BSS is permitted to
transmit and may be able to receive PDUs.
Distribution system (DS)
• A system used to interconnect a set of BSSs and integrated LANs to create an ESS.
Extended service set (ESS)
• A set of one or more interconnected BSSs and integrated LANs that appear as a single BSS to
the LLC layer at any station associated with one of these BSSs.
MAC protocol data unit (MPDU)
• The unit of data exchanged between two peer MAC entities using the services of the physical
layer.
MAC service data unit (MSDU)
• Information that is delivered as a unit between MAC users.
Station
• Any device that contains an IEEE 802.11 conformant MAC and physical layer.

Wi-Fi Alliance
•
•

•
•

The first 802.11 standard to gain broad industry acceptance was 802.11b
Wireless Ethernet Compatibility Alliance (WECA)
• An industry consortium formed in 1999
• Subsequently renamed the Wi-Fi (Wireless Fidelity) Alliance
• Created a test suite to certify interoperability for 802.11 products
Wi-Fi
• The term used for certified 802.11b products
• Has been extended to 802.11g products
Wi-Fi5
• A certification process for 802.11a products that was developed by the
Wi-Fi Alliance
• Recently the Wi-Fi Alliance has developed certification procedures for
IEEE 802.11 security standards
• Referred to as Wi-Fi Protected Access (WPA)

IEEE 802.11 Protocol Stack

IEEE 802.11 Extended Service Set

Table 7.2 IEEE 802.11 Services
Service
Association

Provider
Distribution system

Used to suppor
MSDU delivery

Authentication

Station

LAN access and security

Deauthentication
Disassociation
Distribution
Integration
MSDU delivery
Privacy

Station
Distribution system
Distribution system
Distribution system
Station
Station
Distribution system

LAN access and security
MSDU delivery
MSDU delivery
MSDU delivery
MSDU delivery
LAN access and security
MSDU delivery

Reassociation

Association-Related Services
• To deliver a message within a DS, the distribution service needs to know
the identity of the AP to which the message should be delivered in order
for that message to reach the destination station
• Three services relate to a station maintaining an association with the AP
within its current BSS:
1. Association
• Establishes an initial association between a station and an AP
2. Reassociation
• Enables an established association to be transferred from one AP to
another, allowing a mobile station to move from one BSS to another
3. Disassociation
• A notification from either a station or an AP that an existing association
is terminated

IEEE 802.11I Wireless LAN Security •
For privacy and security, 802.11 defined the
• Wired Equivalent Privacy (WEP) algorithm.
The privacy portion of the 802.11 standard contained major weaknesses. Subsequent to the
development of WEP, the 802.11i task group has developed a set of capabilities to address
the WLAN security issues. In order to accelerate the introduction of strong security into
WLANs,
• Wi-Fi Protected Access (WPA)
The Wi-Fi Alliance promulgated Wi-Fi Protected Access (WPA) as a Wi-Fi standard. WPA is a
set of security mechanisms that eliminates most 802.11 security issues and was based on the
current state of the 802.11i standard.
• Robust Security Network (RSN).
The final form of the 802.11i standard is referred to as Robust Security Network (RSN). The
Wi-Fi Alliance certifies vendors in compliance with the full 802.11i specification under the
WPA2 program.

IEEE 802.11I Wireless LAN SecurityServices and protocol

IEEE 802.11I Wireless LAN SecurityPhases of Operation

IEEE 802.11i Phases of Operation: Capability
Discovery, Authentication, and Association

Pairwise Keys
•

•
•

•

•

Used for communication between a pair of devices, typically between a STA and an AP
• These keys form a hierarchy beginning with a master key from which other keys are derived
dynamically and used for a limited period of time
Pre-shared key (PSK)
• A secret key shared by the AP and a STA and installed in some fashion outside the scope of IEEE
802.11i
Master session key (MSK)
• Also known as the AAAK, and is generated using the IEEE 802.1X protocol during the authentication
phase
Pairwise master key (PMK)
• Derived from the master key
• If a PSK is used, then the PSK is used as the PMK; if a MSK is used, then the PMK is derived from the
MSK by truncation
Pairwise transient key (PTK)
• Consists of three keys to be used for communication between a STA and AP after they have been
mutually authenticated
• Using the STA and AP addresses in the generation of the PTK provides protection against session
hijacking and impersonation; using nonces provides additional random keying material

PTK Parts
The three parts of the PTK are as follows.
■EAP Over LAN (EAPOL) Key Confirmation Key (EAPOL-KCK): Supports
the integrity and data origin authenticity of STA-to-AP control frames
during
operational setup of an RSN. It also performs an access control function:
proof-of-possession of the PMK. An entity that possesses the PMK is
authorized to use the link.
■EAPOL Key Encryption Key (EAPOL-KEK): Protects the confidentiality of
keys and other data during some RSN association procedures.
■ Temporal Key (TK): Provides the actual protection for user traffic

Group Keys
• Group keys are used for multicast communication in which one STA
sends MPDUs to multiple STAs
• Group master key (GMK)
• Key-generating key used with other inputs to derive the GTK
• Group temporal key (GTK)
• Generated by the AP and transmitted to its associated STAs
• IEEE 802.11i requires that its value is computationally
indistinguishable from random
• Distributed securely using the pairwise keys that are already
established
• Is changed every time a device leaves the network

Group Keys

IEEE 802.11iPseudorandom Function (PRF)

Used at a number of places in the IEEE 802.11i scheme (to
generate nonces, to expand pairwise keys, to generate the
GTK)
• Best security practice dictates that different pseudorandom
number streams be used for these different purposes
• Built on the use of HMAC-SHA-1 to generate a pseudorandom
bit stream
•

IEEE 802.11iPseudorandom Function (PRF)

Summary
• Wireless network security
• Network threats
• Security measures
• Mobile device security
• Security threats
• Security strategy
• IEEE 802.11 wireless LAN
overview
• Wi-Fi Alliance
• IEEE 802 protocol
architecture • IEEE 802.11
network components and
architectural model
• IEEE 802.11 services

• IEEE 802.11i wireless LAN
security
• IEEE 802.11i services
• IEEE 802.11i phases of
operation
• Discovery phase
• Authentication phase
• Key management
phase
• Protected data transfer
phase
• The IEEE 802.11i
pseudorandom
function

Thank You

College of Computing and Informatics
Network Security

Network Security

Week 10
Electronic Mail Security

Contents
1. Secure Email

2. PGP
3. S/MIME
4. Domain-keys Identified Email
5. Security services
6. Security mechanisms

Weekly Learning Outcomes
1. Explain the basic functionality of SMTP, POP3, and IMAP.
2. Understand the functionality of S/MIME and the security threats
it addresses
3. Understand the basic mechanisms of DMARC and its role in e-mail
security.
4. Understand the basic mechanisms of DKIM and its role in e-mail
security.

Email Security
Email is one of the most widely used and regarded network
services
Currently message contents are not secure

• May be inspected either in transit
• Or by suitably privileged users on destination system

Email Security Enhancements
Confidentiality
• Protection from disclosure
Authentication
• Of sender of message
Message integrity
• Protection from modification
Non-repudiation of origin
• Protection from denial by sender

Pretty Good Privacy (PGP)
• Widely used de facto secure email
• Developed by Phil Zimmermann
• Selected best available crypto algs to use
• Integrated into a single program
• On Unix, PC, Macintosh and other systems
• Originally free, now also have commercial versions available

PGP Operation – Authentication
1. Sender creates message
2. Make sha-1160-bit hash of message
3. Attached RSA signed hash to message
4. Receiver decrypts & recovers hash code
5. Receiver verifies received message hash

PGP Operation – Confidentiality
•
•
•
•
•

Sender forms 128-bit random session key
Encrypts message with session key
Attaches session key encrypted with RSA
Receiver decrypts & recovers session key
Session key is used to decrypt message

PGP Operation – Confidentiality &
Authentication
• Can use both services on same message
• Create signature & attach to message
• Encrypt both message & signature
• Attach RSA/elgamal enc
• Rypted session key

PGP Operation – Compression
• By default PGP compresses message after signing but before encrypting
• So can store uncompressed message & signature for later verification
• & Because compression is non deterministic
• Uses ZIP compression algorithm

PGP Operation – Email Compatibility
• When using PGP will have binary data to send (encrypted message
etc)
• However email was designed only for text
• Hence PGP must encode raw binary data into printable ASCII
characters
• Uses radix-64 algorithm
• Maps 3 bytes to 4 printable chars
• Also appends a CRC
• PGP also segments messages if too big

PGP Operation – Summary

PGP Session Keys

• Need a session key for each message
• Of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit triple-des
• Generated using ANSI X12.17 mode
• Uses random inputs taken from previous uses and from keystroke
timing of user

PGP Public & Private Keys
• Since many public/private keys may be in use, need to identify
which is actually used to encrypt session key in a message
• Could send full public-key with every message
• But this is inefficient
• Rather use a key identifier based on key
• Is least significant 64-bits of the key
• Will very likely be unique
• Also use key ID in signatures

PGP Message Format

PGP Key Rings
Each PGP user has a pair of keyrings:
Public-key ring contains all the public-keys of other PGP users known
to this user, indexed by key ID
Private-key ring contains the public/private key pair(s) for this user,
indexed by key ID & encrypted keyed from a hashed passphrase
Security of private keys thus depends on the pass-phrase security.

PGP Key Rings

PGP Message Generation

PGP Key Management
• Rather than relying on certificate authorities
• In PGP every user is own CA
• Can sign keys for users they know directly
• Forms a “web of trust”
• Trust keys have signed
• Can trust keys others have signed if have a chain of
signatures to them
• Key ring includes trust indicators
• Users can also revoke their keys

PGP Trust Model Example

S/MIME (Secure/Multipurpose Internet
Mail Extensions)
• Security enhancement to MIME email
• Original internet RFC822 email was text only
• MIME provided support for varying content types and multipart messages
• With encoding of binary data to textual form
• S/MIME added security enhancements
• Have S/MIME support in many mail agents
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