Information Security By:-H.M.Patel

Information security There are three aspects of information security Security service Security mechanism Security attack Security service: The security service is something that enhances the security of data processing systems and information transfers of an organization. It is used to counter security attacks and it uses many security mechanisms to do so

1. Authentication: Authentication refers to the authenticity of the contents of the messages being exchanged as well as that of the communicating entities. 2. Access control: Access control is the ability to limit and control the access to host systems and applications via communication links. To achieve this control, each entity trying to gain access must first be identified, or authenticated, so that access rights can be provided to the individual. 3. Data confidentiality: The contents of the message being transferred across the insecure medium should be readable to only those whom it is intended for and to no other entity. Connection confidentiality: The protection of all user data on a connection. Connectionless confidentiality: all user data in a single data block. Selective field confidentiality: the user data on a connection or in a single data block. Traffic flow confidentiality: The protection of the information that might be derived from observation of traffic flows.

4. Data integrity: Integrity can apply to a stream of messages a single message or selected fields within a message. The contents of the message should not get modified during transit and even if the message is modified, it should be detected at the receiving end. Modification causes loss of message integrity. Connection integrity with recovery, Connection integrity without recovery, Selective field connection integrity, Connectionless integrity, Selective field connectionless integrity. 5. Non repudiation: Repudiation disputes arise when one entity denies sending or receiving any message. The security mechanism should provide means to resolve such disputes.

Security mechanism: A security mechanism is a mechanism designed to detect, prevent and recover from a security attack. No single mechanism supports all the functions required to provide complete security and therefore many mechanisms work together Security attack: A security attack is any action which compromises the security of information of an organization. It is an assault on the system derived from a threat

Two types of security attacks: Passive attacks Active attacks Passive attacks: In a passive attack the attack monitors the transmissions to obtain message content or monitors traffic flows, but does not modify the message Active attacks: In an active attack the attacker acquires the message and modifies the contents of the message to obtain unauthorized effects. Types of active attacks

Modification of messages in transit: In such type of a part of the message is altered or the message is delayed to produce an unauthorized effect. Masquerade: In masquerade one entity pretends to be another entity to produce an unauthorized effect. For example: Authentication sequences can be captured and replayed after a valid authentication sequence has taken place, thus enabling an authorized entity with few privileges to obtain extra privileges by impersonating an entity that has those privileges. Interruption attacks are called as masquerade attacks. Replay: In replay attack a message sequence is captured and then retransmitted to produce an unauthorized effect. Denial of service: Denial of service attack prevents or inhibits the normal use and management of communication facilities

Conventional Encryption Model. A symmetric encryption model has five ingredients. 1. Plaintext 2. Encryption algorithm 3. Secret Key 4. Cipher text 5. Decryption algorithm

Plaintext : It is the original message or data that is fed into the algorithm as input. Encryption algorithm : It performs various substitutions and transformations on the plaintext. Secret Key : It is a value independent of the plaintext and of the algorithm. The exact substitutions and transformations performed by the algorithm depend on the key. Cipher text : It is the scrambled message produced as output. It depends on the plaintext and the secret key. Decryption algorithm : It takes the cipher text and the secret key and produces the original plaintext. Encryption process: An algorithm - produce a different output depending on the specific key being used at the time. A key – a value independent of plaintext, shared by sender and recipient. The cipher text can be transformed back to the original plaintext by using a decryption algorithm and the same key that was used for encryption. The security of conventional encryption depends on the secrecy of the key, not the secrecy of the algorithm. It is impractical to decrypt a message based on the cipher text plus knowledge of the encryption/decryption algorithm. The principal security problem is maintaining the secrecy of the key

Substitution Techniques Substitution: one letter is exchanged for another. Substitution cipher: In substitution cipher each element in the plain text is mapped into (replaced by) another element to generate the cipher text. A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers or symbols.  If the plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with cipher text bit patterns

1. Caesar Cipher: It was used by Julius Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter standing three places further down the alphabet For example Plain: meet me after the toga party Cipher: PHHW PH DIWHU WKH WRJD SDUWB

Transformation is made using the following mapping: Plain: a b c d e f g h i j k l m n o p q r s t u v w x y z Cipher: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C Let us assign a numerical equivalent to each letter from 0 to 25. Then the algorithm may be expressed as follows. For each plaintext letter p, substitute the cipher text letter C: C = E (3, p) = (p+3) mod 26 A shift may be of any amount, so that general Caesar algorithm is C = E (k,p) = (p+k) mod 26, where k takes on a value in the range 1 to 25. The decryption algorithm is simply p = D (k,C) = (C-k) mod 26 If it is known that a given cipher text is a Caesar cipher, then a brute-force cryptanalysis is easily performed: simply try all possible 25 keys. Three important characteristics of this problem enable us to use brute-force cryptanalysis: 1. The encryption and decryption algorithms are known 2. There are only 25 keys to try 3. The language of the plaintext is known and easily recognizable