Course Work On Cryptography

Published: 2021-06-22 00:19:23
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Security systems are undergoing development with the development of cryptographic protocols that help to secure these communications channels. There have been extensive researches that have been carried out to come up with better security mechanisms that will secure information systems. A cryptographic protocol is a protocol which makes use of cryptography so that they are able to achieve their goals, these goals could include sending private or public keys over the network. There are protocols that are used for securing security systems. This paper will focus on two protocols, NRL analyzer and Bellare-Rogaway model (Tanenbaum, 2003).

NRL analyzer

How it works

It is a verification tool that is written in Prolog and is used to analyze cryptographic protocols that are used to undertake authentication of services. They are also used to distribute keys in a network. The protocol works by filtering states which are unreachable. The states which remain after the search are searched exhaustively (Fourazan, 2008).

The basis of this model is that of term-rewriting model of Dolev and Yao. In the Dolev and Yao model there is an assumption that is made that there is an intruder in the network that is in a position to read all message traffic, do a modification of message traffic, and undertake any operation on the message traffic. This operation could be encryption of decryption. All these capabilities that the intruder is able to perform are available to the legitimate user of the protocol. There is an assumption, however, that there are some words that the intruder still does not have. These words could be the keys, or messages which have already known. The objective of the intruder is to find those words. Given the fact that any message that is sent by legitimate senders of messages could be sent by intruders, the protocol can be regarded as a system of algebra that is being manipulated by the intruder. The main goal of the intruder is to undertake the manipulation of the message so that the secret word can be achieved (Diffie, 1976).

The words that have been produced by the algebraic system will be required to follow some basic principles. An example of the principle is that of the same key used for both the encryption and decryption using the private-kept algorithm is required to be self-cancelling. We can therefore think the intruder as striving to find a solution of word problem in s system that is re-writing. With the use of this knowledge, Dolev and Yao, and after some time, Dolev, Even and Karp came up with algorithms that could be used to prove the security of some limited classes of algorithms (Tipton, & Krause, 2007).

In the NRL protocol analyzer, the specification of protocols is undertaken as a way of transition of state machines. Each rule gets specified by making use of the following terms:

- It is a principle that before the rule can fire, there should be words that must have been input by the intruder.
- Values which should be held by local variables before the rule gets fired
- The words that are output, which are later learned by the intruder, should take place after the rule fires.
- New values are taken on by the local variables only after the rule has fired.
The above transition rules can also be used to describe actions that are undertaken by an intruder who will produce new messages by way of performing some operations like decryption and encryption (Dhillon, 2007).

Strengths NRL analyzer

NRL analyzer can be used to prove the security properties of cryptographic protocols and at the same time be used to locate where flaws are located. It has been particularly useful in finding flaws which were not initially known when Simmons Selective Broadcast Protocol was used (Butler et al., 2007).

The NRL has also nondeterministic feature. This is where there are sets of actions that have been predetermined and fixed.

Another strength is that a message that is regarded as secret does not have atomic term that is representing the message. In other words, it is idealized. The analysis methods that are used in this tool are regarded to be successful collection of tools and techniques.
In the word problems, each step that is being taken is a function that is symbolic from the message (input) to the output message. In this sense, the cancellation law that is followed is dkekx=x


This model is more complicated in terms of the composition of the adversary. There is no simple feature of the actions of computation. The messages are also found to be more complicated. The scheduler that is used in this model is different. There is no way that the model can be distinguished from power of induction (Stallings, 2003).

Bellare-Rogaway model

This is a model that is used for distributing entity authentication key. It is the first protocol that can be said to be safe and secure in key distribution and entity identification. Entity authentication can be described to be the process where an agent in a system which is distributed has confidence and prove enough that the communication partner with which they are communicating in the network is what it is proved in the network. Distribution partners in the network will give the session partners the key that will be used to achieve confidentiality, integrity and authentication. This is an improvement of other works like Needham-Schroeder, Encryption-decryption paradigm, confidentiality vs. data integrity issues (Benantar, 2006).

How the Bellare-Rogaway model works

In this system, all communication that exists between parties is under control of the intruder who is able to read, modify, or replay messages. In this system, the intruder is in a position of initiating new sessions anytime. Each party in the communication can be modified by an oracle in which the intruder can run. The oracles cannot interact with themselves. This model can be said to be safe because the only way in which the intruder can make the party to accept is through faithfully relaying of messages. This is referred to as benign adversary (Boyd, 2005).

In this model the players in the system are modeled by a function

In this function, 1k, is the security parameter, i is the initiator identity. J is the identity of the responder. The secret information is identified by a, while k is used to represent the conversion that has taken place so far.

Strengths of Bellare-Rogaway model

The strength of this model, there is provable security. The gains that are gained from this is that there is solutions for encryption problems, signatures, and proofs that are zero-knowledge. This comes because of choosing appropriate function h.

Another strength is found in the nature of attacker actions. The attacker actions in this model use any probabilistic poly-time computation.
It also has fine-grained security features. This means that the secret message means that there is no partial information regarding bitstring representation (Allen, 2003).


The analysis methods that are used by this method are regarded to be handproofs which are difficult and not prone to errors. Also, there is no automation.

Comparison of the two tools

The two tools seem to differ a lot. Bellare-Rogaway uses cryptographic reductions in the process of protocol analysis. NRL protocol analyzer, on the other hand, makes use of symbolic methods. In symbolic methods, messages are presented in algebraic terms. In this method, also the adversary (which is the intruder) has the ability to observe, store and take control of all the communication process. The adversary is also non-deterministic. It has the ability to decrypt, encrypt and sign the messages on condition that it has the key. Unlike Bellare-Rogaway, NRL analyzer sends message derivable from the parts that have been stored. The two models can be said to be undecidable when it comes to the use of nonses (Buchmann, 2004).

Another difference between the two models is that the number of unbounded sessions differ between those with nonses and those without nonses. Symbolic model has nonses which are bound while Bellare-Rogaway does not have nonses. The model with nonses has undecidable bounded length of message. The model without nonses, on the other hand, has time-complete (DEXP) bounded length of the message.


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Benantar, M. (2006). Access control systems: Security, identity management and trust models . New York: Springer.
Boyd, C. (2005). Cryptography and coding. Springer.
Buchmann, J. (2004). Introduction to cryptography. New York: Springer.
Butler, C., Rogers, R., Ferratt, M., Miles, G., Fuller, E., Hurley, C.(2007). IT security interviews: Why defense in depth should be implemented. Michigan: University of Michigan.
Diffie, W. (1976). New directions in cryptography. New Jersey ,(NJ): Pearson Prentice Hall
Dhillon, G. (2007). Information systems security and principles. New York: John Wiley & Sons.
Fourazan, B. (2008). Cryptography and computer security. New York: Cengage Learning.
Tanenbaum, A. (2003). Computer networks. Prentice Hall.
Tipton, H., & Krause, M. (2007). Information security management handbook. CRC Press.
Stallings, W, (2003). Cryptography and network security. Bells Pearson Prentice Hall.

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