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Note for Cryptography And Network Security - CNS by Ganga Dhar

  • Cryptography And Network Security - CNS
  • Note
  • ANDHARA UNIVERSITY - AU
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UNIT-1 INTRODUCTION THE OSI SECURITY ARCHITECTURE: To assess effectively the security needs of an organization and to evaluate and choose various security products and policies, the manager responsible for security needs some systematic way of defining the requirements for security and characterizing the approaches to satisfying those requirements. The OSI security architecture was developed in the context of the OSI protocol architecture, which is described in Appendix H. However, for our purposes in this chapter, an understanding of the OSI protocol architecture is not required. For our purposes, the OSI security architecture provides a useful, if abstract, overview of many of the concepts.. The OSI security architecture focuses on security attacks, mechanisms, and services. These can be defined briefly as follows: 1. Security attack – Any action that compromises the security of information owned by an organization. 2. Security mechanism – A mechanism that is designed to detect, prevent or recover from a security attack. 3. Security service – A service that enhances the security of the data processing systems and the information transfers of an organization. The services are intended to counter security attacks and they make use of one or more security mechanisms to provide the service. 1.SECURITY ATTACKS: Attacks on Cryptosystems: Attacks are typically categorized based on the action performed by the attacker. An attack, thus, can be passive or active. Passive Attacks: The main goal of a passive attack is to obtain unauthorized access to the information. For example, actions such as intercepting and eavesdropping on the communication channel can be regarded as passive attack. These actions are passive in nature, as they neither affect information nor disrupt the communication channel. A passive attack is often seen as stealing information. The only difference in stealing physical goods and stealing information is that theft of data still leaves the owner in possession of that data. Passive information attack is thus more dangerous than stealing of goods, as information theft may go unnoticed by the owner.

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Active Attacks: An active attack involves changing the information in some way by conducting some process on the information. For example,  Modifying the information in an unauthorized manner.  Initiating unintended or unauthorized transmission of information.  Alteration of authentication data such as originator name or timestamp associated with information  Unauthorized deletion of data.  Denial of access to information for legitimate users (denial of service). Cryptography provides many tools and techniques for implementing cryptosystems capable of preventing most of the attacks described above. Assumptions of Attacker: Let us see the prevailing environment around cryptosystems followed by the types of attacks employed to break these systems Cryptographic Attacks: The basic intention of an attacker is to break a cryptosystem and to find the plaintext from the ciphertext. To obtain the plaintext, the attacker only needs to find out the secret decryption key, as the algorithm is already in public domain. Hence, he applies maximum effort towards finding out the secret key used in the cryptosystem. Once the attacker is able to determine the key, the attacked system is considered as broken or compromised. Based on the methodology used, attacks on cryptosystems are categorized as follows −  Ciphertext Only Attacks (COA) − In this method, the attacker has access to a set of ciphertext(s). He does not have access to corresponding plaintext. COA is said to be successful when the corresponding plaintext can be determined from a given set of ciphertext. Occasionally, the encryption key can be determined from this attack. Modern cryptosystems are guarded against ciphertext-only attacks.  Known Plaintext Attack (KPA) − In this method, the attacker knows the plaintext for some parts of the ciphertext. The task is to decrypt the rest of the ciphertext using this information. This may be done by determining the key or via some other method. The best example of this attack is linear cryptanalysis against block ciphers.  Chosen Plaintext Attack (CPA) − In this method, the attacker has the text of his choice encrypted. So he has the ciphertext-plaintext pair of his choice. This simplifies his task of determining the encryption key. An example of this attack is differential

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cryptanalysis applied against block ciphers as well as hash functions. A popular public key cryptosystem, RSA is also vulnerable to chosen-plaintext attacks.  Dictionary Attack − This attack has many variants, all of which involve compiling a ‘dictionary’. In simplest method of this attack, attacker builds a dictionary of ciphertexts and corresponding plaintexts that he has learnt over a period of time. In future, when an attacker gets the ciphertext, he refers the dictionary to find the corresponding plaintext.  Brute Force Attack (BFA) − In this method, the attacker tries to determine the key by attempting all possible keys. If the key is 8 bits long, then the number of possible keys is 28 = 256. The attacker knows the ciphertext and the algorithm, now he attempts all the 256 keys one by one for decryption. The time to complete the attack would be very high if the key is long.  Birthday Attack − This attack is a variant of brute-force technique. It is used against the cryptographic hash function. When students in a class are asked about their birthdays, the answer is one of the possible 365 dates. Let us assume the first student's birthdate is 3rd Aug. Then to find the next student whose birthdate is 3rd Aug, we need to enquire 1.25*•√365 ≈ 25 students. Similarly, if the hash function produces 64 bit hash values, the possible hash values are 1.8x1019. By repeatedly evaluating the function for different inputs, the same output is expected to be obtained after about 5.1x109 random inputs. If the attacker is able to find two different inputs that give the same hash value, it is a collision and that hash function is said to be broken.  Man in Middle Attack (MIM) − The targets of this attack are mostly public key cryptosystems where key exchange is involved before communication takes place. o Host A wants to communicate to host B, hence requests public key of B. o An attacker intercepts this request and sends his public key instead. o Thus, whatever host A sends to host B, the attacker is able to read. o In order to maintain communication, the attacker re-encrypts the data after reading with his public key and sends to B. o The attacker sends his public key as A’s public key so that B takes it as if it is taking it from A.  Side Channel Attack (SCA) − This type of attack is not against any particular type of cryptosystem or algorithm. Instead, it is launched to exploit the weakness in physical implementation of the cryptosystem.  Timing Attacks − They exploit the fact that different computations take different times to compute on processor. By measuring such timings, it is be possible to know about a particular computation the processor is carrying out. For example, if the encryption takes a longer time, it indicates that the secret key is long.  Power Analysis Attacks − These attacks are similar to timing attacks except that the amount of power consumption is used to obtain information about the nature of the underlying computations.  Fault analysis Attacks − In these attacks, errors are induced in the cryptosystem and the attacker studies the resulting output for useful information. 2. SECURITY SERVICES : The primary objective of using cryptography is to provide the following four fundamental information security services. Let us now see the possible goals intended to be fulfilled by cryptography. A. Confidentiality: Confidentiality is the fundamental security service provided by cryptography. It is a security service that keeps the information from an unauthorized person. It is sometimes referred to as privacy or secrecy.

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Confidentiality can be achieved through numerous means starting from physical securing to the use of mathematical algorithms for data encryption. B. Data Integrity: It is security service that deals with identifying any alteration to the data. The data may get modified by an unauthorized entity intentionally or accidently. Integrity service confirms that whether data is intact or not since it was last created, transmitted, or stored by an authorized user. Data integrity cannot prevent the alteration of data, but provides a means for detecting whether data has been manipulated in an unauthorized manner. C. Authentication: Authentication provides the identification of the originator. It confirms to the receiver that the data received has been sent only by an identified and verified sender. Authentication service has two variants  Message authentication identifies the originator of the message without any regard router or system that has sent the message.  Entity authentication is assurance that data has been received from a specific entity, say a particular website. Apart from the originator, authentication may also provide assurance about other parameters related to data such as the date and time of creation/transmission. D. Non-repudiation: It is a security service that ensures that an entity cannot refuse the ownership of a previous commitment or an action. It is an assurance that the original creator of the data cannot deny the creation or transmission of the said data to a recipient or third party. Non-repudiation is a property that is most desirable in situations where there are chances of a dispute over the exchange of data. For example, once an order is placed electronically, a purchaser cannot deny the purchase order, if non-repudiation service was enabled in this transaction. Cryptography Primitives: Cryptography primitives are nothing but the tools and techniques in Cryptography that can be selectively used to provide a set of desired security services −  Encryption  Hash functions  Message Authentication codes (MAC)  Digital Signatures The following table shows the primitives that can achieve a particular security service on their own. Note − Cryptographic primitives are intricately related and they are often combined to achieve a set of desired security services from a cryptosystem

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