Page 223 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes Self Assessment

Fill in the blanks:
1. In secret key encryption, the secret key is used for ...................... .
2. In the public key encryption, the public key is used for ...................... of the massage.

3. Encryption and decryption normally takes care of ...................... of a network.
4. In public key encryption the private is used to ...................... the message to the plaintext.
5. ...................... involves the interception of data packets by a computer successfully pretending
to be a trusted server/resource.
14.5 Cryptography

Substitution and transposition ciphers are two categories of ciphers used in classical cryptography.
Substitution and transposition differ in how chunks of the message are handled by the encryption
process.

14.5.1 Substitution Ciphers

In cryptography, a substitution cipher is a method of encryption by which units of plaintext are
replaced with ciphertext according to a regular system; the “units” may be single letters (the
most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The
receiver deciphers the text by performing an inverse substitution.
Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the
units of the plaintext are rearranged in a different and usually quite complex order, but the units
themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are
retained in the same sequence in the ciphertext, but the units themselves are altered.
There are a number of different types of substitution cipher. If the cipher operates on single
letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters
is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire message,
whereas a polyalphabetic cipher uses a number of substitutions at different times in the message,
where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and
vice-versa.

Simple Substitution

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