B030710154

Cryptography Extended

In this lab we learn that cryptography algorithm can be classified into two categories, symmetric and asymmetric. Symmetric is using same key to encrypted and decrypted the plain text whereas asymmetric using different key to encrypted and decrypted plain text.

Symmetric can divide to:

1. Substitute encryption

· In substitute encryption, character in plain text being substitute with another character. Each character can be substitute with one character or multiple characters. Caesar cipher is example of substitute with one character and Vigenere cipher is example of substitute with multiple characters. Caesar cipher is easy to break by using brute force attack; an attacker can easily try every combinations of character to break the code as the numbers of possibility is 26. Vigenere cipher is an improvement from Caesar cipher.

2. Transposition encryption

· This method is change the location of characters or reordering the character in plain text. The first character in plain text might be placed on fifth position and fifth character might be placed in another location in plain text.

Asymmetric encryption involves two key in encryption and decryption. The encryption key is called public key and decryption key called private or secret key. These algorithms allow the public key to be publicized, which means people with the public key can encrypt the plain text and the proper recipient with the private key can decrypt the plain text. To produce this to key used RSA algorithm. RSA is founded by Rivest, Shamir and Adleman of MIT in 1977.

OPERATING SYSTEM

Memory protection is preventing one process from corrupting the memory. There are methods for protecting memory:

· Fence is preventing faulty user program from destroying part of the resident portion of the operating system. There are two types of fence:

o Fixed fence( predefined memory access)

§ A method to confine user to one side of the boundary.

o Register fence (used hardware register)

§ Contain address of the end of the operating system.

• Relocation is a process of changing all address to reflect actual address which the program is located.
• Base / Bound Register is useful in knowing how much space is available and checking overflows
• Tagged Architecture is used to solve problem in contiguous nature and all or nothing situation for sharing.
• Segmentation is divide process into pieces with different rights.

o Benefits of segmentation

§ Each address reference checked for protection

§ User cannot access to unpermitted segment

§ User can share access with different rights.

o Problem in segmentation

§ Segments cause fragmentation of main memory because they are varying sizes.

§ If swapping is used then additional memory management techniques must be employed

§ The operating system’s lookup of the name in the table can be slow

• Paging is divide program into equal size; frame is divide memory into equal size.

o Advantages of paging

§ Each page is the same size thus fragmentation is reduced

§ Addressing beyond a page is not a real problem since a carry just refers to the next page.

o Disadvantages of paging

§ Loss of individual access rights since there is not necessarily a relationship between lines of code in a page (unlike program segmentation)

• Combine paging with segmentation used to break each segment into equal sized pages.

o Advantages of combining segmentation with paging

§ Retained the logical unity of the segment

§ Permitted differentiated protection for the segments

o Disadvantage

§ Added an additional layer of translation for each address

Password is a character of string that used to authenticate identity of user.

Authentication is verification of someone identity who generated some data.