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Attacks on Network Communications

Question 1:

  • Eavesdropping is one of the potential attacks against network communications performed by an active attacker.
  • An off-path attacker can be able to perform a fabrication attack on a communication network.
  • Modification cannot be performed by a passive attacker as it requires alteration of data.
  • Eavesdropping:The appropriate cryptographic countermeasures for eavesdropping include the use of authentication mechanisms that do not allow for the transmission of the password over the network. A Kerberos protocol may be effective in this. Similarly, users should ensure that passwords are encrypted if they must be transmitted over the network or an encryption communication channel such as SSL may be used.

Modification: Cryptographic countermeasures to this attack include use of data hashing, signing and digital signatures

: This attacks can be prevented by authenticating users using the communication channel.

    • DNS plays an important role in address resolution.  It enables a host in learning the IP address for a given domain name.
    • Modification is one the attacks that is relevant to DNS. Sometimes referred to DNS hijacking in which DNS records are modified to point to a rogue DNS server or domain.
    • IP spoofing was widespread in 1990’s 
  • IP spoofing refers to attackers inserting an IP address as the source address of an IP datagram and therefore allowing them to impersonate any host. The recipient has no guarantee about the true identity of the entity that sent an IP datagram. Ingress filtering is responsible for identifying and dropping packets that have been spoofed by inspecting the source address.  This helps identifying and preventing spoof attacks.
  • To prevent against IP spoofing, users are provided with a mechanism to verify the origin of the IP datagram through the linking of the datagram to an IP address. By encrypting part of the datagram using a key known to the sender and the receiver can help prevent IP spoofing.

Question 2:

  • A TCP handshake is method used by TCP/IP networks in creating connection between local host/client to a server. It’s meant to create a TCP connection.
  • Denial-of-service attacks exploit an implementation characteristic of TCP and can be used to make the server incapable of answering a legitimate client’s application request for new TCP connections. The services that binds and listens to the TCP socket are potentially vulnerable to DOS attacks.
  • SYN cookies are used for the prevention of SYN flood attacks which the TCP handshake is vulnerable to. They allow the server to avoid dropping connections when the SYN queue fills up.
  • The Diffie-Hellman Protocol:
  • Both the sender and receiver agree on shared parameters
  • A private value is chosen from the protocol participants and then a public value is computed using the shared parameters and its private value
  • Public values are exchanged by the protocol participants over a public network
  • Each protocol participant uses their private value and the other participant’s public value to compute a shared secret.
  • A correct protocol is a protocol that always achieves the desired objective at the end of the protocol run. A secure protocol is a protocol that will achieve the stated aims of the protocol when honest parties adhere to the protocol even in the presence of attackers. Diffie-Hellman protocol is not secure because an active attacker can be able to establish keys with both participant A and B and thus gain control of the communication channel.  There is no way of confirming the message sent by one of the participants actually came from the participants.
  • To make Diffie-Hellman protocol secure needs message origin authentication thereby requiring either of the participants to share a secret key or both participants should have a public-private key pair. In the first case, both participants can compute a MAC over each message and in the second case, the first participant can compute a digital signature over the first message while the second participant can compute a digital signature over the second message. 

Question 3:

  • Using the same challenge gives the attacker an opportunity to replay a message from a previous protocol run by participants A and B. This will allow the attacker to pretend to be either A or B.
    • The hashing function would be inappropriate as the response is a randomized variable already that is harder for attackers to guess. The variable changes from time to time.
    • There is no need for 64-bit string chosen by claimant as the random value chosen would be harder to guess
    • Using bitwise XOR and a key would waste valuable networking resources since the random key value for the protocol is enough
    • Since authentication would require the respondent to acquire the claimants key, the 64-bit string chosen would play a minimal role in transmission of the messages.
    • This method would be inappropriate because the random value generated would be hard for attackers to guess.
  • An offline attack requires work from the attacker only and requires little or no communication with the server or system under attack. An online attack requires considerable communication with the system under attack.

An offline brute force attack is more dangerous it easier to perform than an online attack and stands a better chance of going undetected.

  • i). The challenge-response protocol requires private key from the participants. The attacker doesn’t require to engage the server in order to access the messages and keys of the participants

ii). A dictionary attack is an attempt to gain unauthorized access to a computer system by the use of a very large set of words in the generation of potential passwords. Offline dictionary attacks can happen when the attacker selects passwords from the dictionary and tries to generate responses that matches the recorded one. This process is repeated over and over until a successful attempt is made.

  • EKE (encrypted key exchange)  ensures that successful password can only be done online rather than offline. The protocol is designed to use:
    • Asymmetric cryptography to generate fresh, shared secret k
    • The long-term key kcv (password-based) to preserve the confidentiality of k
    • k to run a challenge-response protocol.

Question 4

    • Both UDP and TCP are used to send bits of data (packets) over the Internet.
    • A reliable channel is a channel that has a higher percentage of availability for use in a specified period of scheduled availability. Reliable channels are necessary in packet-switched digital networks as they allow for fast network functioning and scalability.
    • The Application layer protocol may sometimes use the UDP as the transport layer protocol rather than TCP where the requests and responses made are both lightweight and thus would require the use of a single packet.
    • A transport layer protocol that identifies errors introduced by the physical layer can be constructed by using the concept of ports supporting flow control and duplicate data suppression.
    • A transport layer protocol that is reliable can be constructed by using the concept of ports supporting reliable data delivery.

  • Application layer PDU’s are vulnerable to on-path attacks as they are used for lightweight single-packets that are less secure. For these reasons, detection of on-path attacks is difficult.
  • The order in which PDU’s are received is loosely attached to the application layer thereby making it hard for the detection and sequence of on-path attacks.
  • IPSec with Authentication Header provides authentication, integrity and anti-replay for an entire packet thereby making it hard for an on-path attacker to modify or re-order the application layer PDUs exchanged in a TCP session.
  • TLS is the successor protocol of SSL. They are both responsible for securing TCP sessions and safeguarding sensitive data sent between two systems thereby reducing the chances on on-path attacker to infiltrate the TCP session.
  • TCP authentication protocol ensures specifies stronger authentication codes to protect against replay attacks for long lived connections like BGP sessions. They protect against on-path attacks.

Question 5:

  • Private IPv4 was introduced so that it could help in conservation of address space (public IPv4 address space was scarce) and ensuring security of the core network. Private IPv4 has led to an expansion of the address space and made core networks more secure.
    • Network address and port translation involves a technique where by port numbers and private internet protocol addresses are mapped to one public IP address from multiple internal hosts. Network address and port translation extends the capabilities of NAT (network address translation) by allowing for translation and mapping of  port numbers, in addition to the IP address, when communicating with an external network.
    • IPsec in tunnel mode is the default mode and in this mode, the entire original IP packet is protected by IPSec. It is commonly used between gateways or at an end-station to a gateway. The Gateway here acts as a proxy for the hosts behind it.
  • When IPSec module is enabled, both inbound and outbound traffic passes through the IPSec module. The Security Policy Database is responsible for controlling how packets are processed. For an outgoing packet, outbound security policy entries are used to match the packet fields chosen as the selector. The SP entry that requires security processing, the search for corresponding Security Association (SA) is created in the Security Association Database (SAD) whenever there are no matching entries found. The packets are then processed according to the security specification given by the SA.
  • There are three message exchanges in the Internet Key Exchange Protocol: They are message 1: responsible for proposing policies by sending one or more security associations proposals; message 2: is a response from the responder to the packet sent from the initiator; and message 3: is a message that contains Diffie-Hellman Key exchange payload and nonce payload from initiator. 

Paper 2

1 (a) (i) an active on-path attacker-modification, fabrication, sinkhole, Sybil, spoofing

(ii) A passive on-path attacker-Eavesdropping, traffic analysis

(iii) An active off-path attacker- modification, fabrication, denial of services, wormhole

(iii) A passive off-path attacker-Eavesdropping, traffic analysis. 

1(b) (i) IP and UDP;

Both are transport layer protocols and they allow for application to application delivery




IP and UDP

-Both are stateless, connectionless protocols.

-neither of both IP and UDP provides a reliable channel

-UDP resides on the transport layer while IP resides on internet Layer


- Both are transport layer protocols and they allow for application to application delivery

-Both TCP and UDP headers includes source and destination port numbers

-Both TCP and UDP encapsulate transport layer PDUs

-TCP provides a reliable Service while UDP does not

-TCP has error checking functionality  while UDP does not have

-TCP is connection oriented while UDP is connectionless


-In WEP data integrity is provided through CRC-32 while WPA it is provided through message integrity code

-In WEP encryption is achieved through Rivest Cipher 4(RC4) while in WAP Temporal Key Integrity Protocol(TKIP) is used for data privacy(Encryption)

WPA and WPA2

-They both use the 4 handshaking mechanism which is used for providing the key management.

-WAP is possible in deploying to both current and previous hardware’s while WAP2 is only possible to be deployed in newer hardware’s and those with NIC that is not older than 2006

-data integrity is provided through message integrity code while WAP2 is provided through Cipher block Chaining Message Authentication Code(CBS-MAC)

TCP Authentication Option and IPsec Authentication Header

-They are both derived master key tuple (MTP).

-TCP-AO uses the TCP options in providing the message authentication while AH is used for providing integrity services.

IPsec Authentication Header and IPsec Encapsulating Security Payload

- Both provides data origin authentication and data integrity services

AH protocol uses sequence numbers to detect replays while ESP uses symmetric encryption and MACs based on secret keys that are shared between endpoints.

DNS is the Domain Name System that is used for providing a structured way that is used in naming internet resources, also another purpose is maintaining a map between the internet protocol addresses and the domain names assigned and Lastly is that the browser may use DNS in determining the IP address of specific web server for a given Uniform resource Locator. The sender may use DNS in determining the IP address of the recipient’s name server from the receiver email address.

DNS query-response protocol – DNS offer provision of a request response protocol that enables the hosts in requesting the ip addresses for any known domain name. This Response protocol must have an identifier which will enable the requester to match the responses to the required requests. NB. It’s good to note that DNS requests and the responses are always encapsulated in the UDP datagrams

The Resolvers are used to run the protocols. 

Fabrication attack (Cache Poisoning)

(ii)  if attacker is successful to alter the mapping domain name to a chosen IP address this will mean a successful cache poisoning where the stub resolver is considered to use a corrupted recursive resolver and the IP datagrams that were constructed by the stub resolver will now have to be sent to IP addresses that the attacker want or had chosen. This will mean that an attacker is on the path and therefore termed to be very much more powerful 

Preventing Network Communication Problems

2(a) as we all know that the public internet is largely based on IPV4 whose addresses are 32 bits long. If we consider an example of Classless Inter-Domain Routing (CIDR) block to have the form a.b.c.d/n of which a, b, c and d are decimal numbers between 0 & 255 and n is a two digit decimal number between 0 and 32.

(b)  (i) the difference between the private IPV4 addresses and Public IPV4 addresses is that the private IP-range cannot be routed over the internet while in the public IPV4 ISPs allows routing on the public internet

(ii) Private IPv4 was introduced in RFC 1597 for the purpose of address allocation for private internets.

(iii) This is because it will be impossible in distinguishing hosts with same IP address 

(c)  Network Address and Port Translation (NAPT) makes use of port numbers to distinguish between hosts with private IP addresses where in minds we know all the TCP and UDP datagrams always have a port number. There is mapping between the source private address and the source port number to a virtual port number with the use NAT device. This will the incoming datagram, to be matched to a private IP address and port number using the destination port number that is virtual in look-up table.

The technique is an IP tunneling that modifies IP datagrams as they try to pass through a gateway. Since IP tunneling works by encapsulating where the gateway at the edge of the source network encapsulating the IP datagram in an outer datagram. In providing the confidentiality the gateways must encrypt and decrypt the encapsulated IP packets where in this manner we can provide the confidentiality service and the integrity of the data built on a virtual private network. This is often made possible by implementation of IPsec and IKE by gateways in negotiating security associations and controlling which datagrams are to be processed using IPsec. 

(d) IP spoofing is the underlying cause of a significant proportion of the security problems associated `with the Internet because all transport and application layer messages are encapsulated in IP datagrams.

(e) The aim of smurf attack is to exhaust processing resources of any victim by forcing it to processing the ICMP messages. The following example shows the working of a smurf attack.

The attack encapsulates echo requests in a spoofed IP datagram. The attacker M sends ICMP requests to B that appear to come from the victim’s IP address By doing this repeatedly, M can cause A to be flooded by ICMP echo reply messages causing a denial of service 

Reliable Communication Protocols

(f)  Ingress filtering is known as RFC 2827 attempts reducing of IP spoofing by identifying and dropping packets that have been spoofed by inspecting the source address.

In egress filtering prevention of IP spoofing which is known as RFC 3013 the IP datagrams sent to a customer should be filtered in that the destination address in the customer’s range of IP addresses. In this case a datagram should be dropped if that source address belongs to the customer’s range of IP addresses. Lastly is the Reverse Path Forwarding which is known as RFC 3704 and is termed to be a more complex form of ingress filtering? Routes between two Internet hosts are not necessarily symmetrical. This is particularly true for hosts in multi-homed ASes. In this case, RPF examines each possible interface on which a datagram could be forwarded The datagram is dropped if none of these interfaces match the one on which the datagram was received 

3 (a) These are techniques that can be used to prevent and detect problems that arise in network communications.  

(b)  The procedure below explains how the stop-and-wait protocol in constructing communications channels that are reliable.

The stop-and-wait protocol provides a reliable of a one-way channel where the sender transmits an acknowledgement if the message has arrived and the checksum has been verified. The sender must wait for an acknowledgement before he or she sends the next package or message. We note that no acknowledgement will be received if the message is not received. If there is no acknowledgement is received during the time given then the sender will have to re-transmit the message again. The stop-and-wait protocol guarantees delivery (and the messages are received in order because they are sent one at a time).

(c) The sender transmits a synchronization message to the receiver with an initial sequence number. The receiver returns an acknowledgement which includes the initial sequence number

The acknowledgement has the effect of confirming that both sender and receiver know the initial sequence number the initial sequence number is an example of shared state once the sender receives the acknowledgement it will send the first message containing application data using the next sequence number

(d) The poor design and implementations of important protocols have led to many security breaches where Lack of origin authentication in IP has led to denial-of-service attacks against innocent hosts 

(e) The use of SYN cookies means that some information that would normally held in the half-open connections table is lost. The table would record other parameters used in a TCP connection, such as window size. Hence there are some (minor) limitations on the type of TCP connections that can be established using SYN cookies However, SYN cookies do mean that a server can continue to operate even when there is a SYN flooding attack. TCP Cookies Transactions is an extension of SYN cookies (RFC 6013).TCPCT avoids the limitations of SYN cookies but it requires the initiator and responder to support TCPCT

SYN cookies are used in the IKE handshake to render certain kinds of denial-of-service attacks ineffective roughly speaking, the first two messages in an IKE handshake are stateless. The responder only commits resources to an IKE handshake once an appropriate response to the SYN cookie has been received

4 (a) The entity authentication requires the data origin authentication to confirm the identity that is provided by the MAC where freshness prevents replays of old authentication messages (provided by using a different ISN for each run of the protocol. In this context of entity authentication, the ISN is usually called a nonce (a number used once). This entity authentication protocols are often known as challenge-response protocols. The verifier sends a nonce to the c12 

(b) If an attacker M wants to impersonate A to B, M first runs the protocol with B in order to obtain the current challenge, n say, from which M deduces the next challenge, n + 1. The attacker then runs the protocol a second time, this time with A and pretending to be B. During this protocol run, M chooses n + 1 as his challenge to A. A, an honest protocol participant will encrypt n + 1 using the key she shares with B. Finally, M runs the protocol a third time, this time with B and pretending to be A. When B sends the expected challenge, n + 1, M replays the value received from A in the previous run of the protocol. Hence, M is able to trick B into believing that he is communicating with A. Note that this attack works whenever B uses a predictable method of generating n.

The trouble with an 128-bit challenge is that, even if it is chosen at random, it will repeat rather quickly, thus giving B the opportunity to replay the response to an earlier challenge. 

(c) This is because the CRCs and steam ciphers do not mix where RC4 XORs is the keystream with the plaintext. When there is change in the plaintext means that the ICV field (computed

Using CRC-32) is incorrect. The structure of CRC-32 means that it is possible to predict the effect on the ICV of a bit changing in the plaintext. The attacker can modify the encrypted ICV for the modified plaintext, in such a way that the modified ICV is the CRC-32 value

for the modified plaintext. In other words, the attacker can fool the recipient into believing a

modified message is the one that was sent.

(d) Example

  1. c → v : Hi
  2. v → c : {n ??k}cv(encrypt challenge and key using v’s public key)
  3. c → v :‹ n ?v›cv (compute MAC using k) 

In the case of private-public key pairs, it is impractical to use asymmetric cipher systems for bulk encryption. Thus we often use asymmetric cryptography only to establish a shared secret.

(e) The idea is to ensure that successful password guessing requires the attacker to be on-line, rather than off-line. The generic design of an EKE protocol uses

  • asymmetric cryptography to generate a fresh, shared secret k
  • the long-term (password-based) key kcv to preserve the confidentiality of k
  • k to run a challenge-response protocol
  • EKE reveals a plaintext-ciphertext pair to an eavesdropper, but it is encrypted with k rather than kcv. This is in contrast to other entity authentication protocols we have seen

(f)  Suppose the attacker tries to guess the password (and hence Kir ). Then the decryption of messages 1 and 2 only reveals DH values and the attacker has no way of knowing whether these are the DH values chosen by the initiator and responder. Alternatively, the attacker can try to guess k, but this only works for a particular run of the protocol (and has a very low chance of success in any case).

Of course, the attacker can try an on-line attack by impersonating I, guessing Kir and selecting x. If the attacker guesses Kir correctly then he will be able to recover gy and compute k = gxy . However, the attacker has no way of determining whether he has guessed Kir correctly without completing the protocol run. (If he has guessed correctly, then he will be authenticated as he will be able to compute the correct response in message 4.) However, if the number of attempts to run the authentication protocol is limited to a very small number (typically less than five), then the likelihood of the attacker correctly guessing the password and completing the protocol run successfully is negligible (even for weak passwords). 

5 (a) Generating the TLS (Record) PDU

  1. The (SSL) sequence number is appended to the application layer data and the MAC value is computed. The sequence number is initialized during the TLS handshake protocol
  2. The resulting MAC is appended to the application layer data and encrypted
  3. Note that the sequence number is not part of the input to the encryption algorithm and is not part of the SSL/TSL PDU

(b) Processing the Record PDU

It’s good to note that the SSL/TLS software at the receiver side should receive Record PDUs in exactly the same order as they were produced because TCP is reliable .This means that the sender and receiver can maintain independent, synchronized copies of the (TLS) sequence number used in the MAC computation then  Hence, the recipient Decrypts the Record PDU (recovering the application data and the MAC)

  1. Reconstructs the input to the MAC algorithm, using its copy of the sequence number and the application data
  2. Recomputes the MAC and compares with the decrypted MAC

The recipient will “tear down” an SSL/TLS connection if MAC verification fails

  • If the attacker modifies the application data, then the modification will be detected by SSL/TLS (because MAC verification will fail).
  • If the attacker replays a TCP segment, then it will be dropped by TCP as a duplicate segment.
  • If the attacker modifies the sequence number without modifying the checksum in the TCP header, then checksum verification will fail during TCP processing and the segment will be dropped.
  • If the attacker modifies the sequence number and the checksum then it may not be dropped by TCP. However, MAC verification will fail because the segment will be appear in a different order to the sequence numbers used to compute the MACs.  

(d)  The idea behind this is that the HTTP and BGP will be used in specifying the application interaction with network.  It does not matter whether sequence numbers are encrypted or not. Why? Sequence numbers have no intrinsic value, so confidentiality is not required and they don’t need to be encrypted. Sequence numbers are only relevant to end devices, not intermediate devices, so they can be encrypted. Sequence numbers must be integrity protected. Why? If sequence numbers can be modified, the data chunks may be reassembled in the wrong order.

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