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Today's Question
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Taken from Cert-EX0-101
Related Exam: ITIL Cert-EX0-101 ITILCert: ITIL Foundation Certification in IT Service Management version 3 (2011 Update)
Item Number: Cert-EX0-101.5.7.7
Which of the following events is an example of a service request?
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Previous question and answer:
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Taken from Cert-CISSP
Related Exam: ISC2 Cert-CISSP SecurityCert: CISSP
Item Number: Cert-CISSP.5.3.7
Which cipher type replaces the original text in a message with a different text?
- A substitution cipher is an encryption method in which blocks of characters from the original text are substituted with different characters or blocks of characters. The receiver decrypts the message to its original form by reversing the substitution process. A secret key decides the sequence and order of the functions to be performed by the receiver to convert the encrypted text back to plaintext.
A block cipher divides the message into blocks of bits and then encrypts each block. A block cipher operates on a stream of fixed-size blocks of plain text and can be operated as a stream. The cipher is more suitable in the implementation of software rather than hardware. The substitution and the transposition cipher functions are carried out one block at a time. In a block cipher, diffusion spreads the influence of a plaintext character over many cipher-text characters. Diffusion is accomplished through permutation. The technique of confusion is used in block ciphers to conceal the statistical connection between ciphertext and plaintext. The technique of diffusion is used to spread the influence of a plaintext character over many ciphertext characters.
A stream cipher treats the message as a stream of bits. Mathematical functions are performed on the individual bits. This method of functioning makes the stream cipher more suitable for hardware implementations. With a stream cipher, the same equipment can be used for encryption and decryption. A stream cipher is amenable to hardware implementations that result in higher speeds. Because encryption takes place bit by bit, there is no error propagation.
A transposition cipher scatters the original text in the message instead of substituting it with another text. Permutations and combinations are used to scramble the letters in a transposition cipher. A key determines the position of the letters moved in the original text.
There are two types of substitution ciphers: simple and polygraphic. A simple substitution cipher operates on single letters while a polygraphic substitution cipher operates on larger groups of letters. A mono-alphabetic cipher in simple substitution deploys fixed substitution over the complete message, while a polyalphabetic cipher, such as a Vigenere polyalphabetic cipher, in polygraphic substitution deploys multiple substitutions at different times in the message.
An example of the substitution cipher is the Caesar cipher, which is a mono-alphabetic cipher. In Caesar cipher, each letter in the original text is replaced with the alphabet three places beyond it in the alphabet sequence.
A cryptographic algorithm is also known as a cipher.
References:
- CISSP Certification All-in-One Exam Guide, Chapter 8: Cryptography, Types of Ciphers, pp. 683-686
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