It is used to fulfill the following functions:
1) Confidentiality (the information cannot be understood by anyone for whom it was unintended)
2) Integrity (the information cannot be altered in storage or transit between sender and intended receiver without the alteration being detected)
3) Non-repudiation (the creator/sender of the information cannot deny at a later stage his or her intentions in the creation or transmission of the information)
4) Authentication (the sender and receiver can confirm each other?s identity and the origin/destination of the information)
Types of ciphers:
- Monoalphabetic Substitution
The monoalphabetic substitution takes a letter of an alphabet and substitutes it with another letter, this way a ciphertext is generated. The way of converting is fixed. A character of the plaintext will be replaced by the same ciphertext character, during the entire ciphertext. There is no additional key. The only way of security is to keep the substitution-table secret.
A popular example for the monoalphabetic substitution is the caesar cipher.
The first step is to write down the plaintext alphabet. It includes all characters needed for the message. In this example only upper case letters will be used.
The second step is to build an additional random alphabet:
An A becomes /. B is replaced by Q. C replaced by W, etc.
The word „EXAMPLE“ would be encoded by: „RV/GSHR“.
2. Polyalphabetic cipher
A cipher is polyalphabetic if a given letter of the alphabet will not always enciphered by the same ciphertext letter, and, as a consequence, cannot be described by a single set of ciphertext alphabet corresponding to a single set of plaintext alphabet.
The simplest way to produce a polyalphabetic cipher is to combine different monoalphabetic ciphers.
One of the problems with monoalphabetic ciphers is that the letters occur with certain frequency in a language. This frequency can be graphed for both plaintext letters and the ciphertext letters of the enciphered message, and, after some analysis, the cipher is relatively easily broken.
The homophonic cipher changes the situation by equalizing the frequencies of the ciphertext letters. It uses the following rules:
- In order to make the deciphering unique, the sets belonging to distinct plaintext letters must be disjoint.
- The number of ciphertext symbols assigned to a plaintext letter is determined by the frequency of that letter.
|a||05, 18, 26, 38, 45, 54, 62, 84||n||20, 36, 53, 65, 97, 98|
|b||10||o||22, 30, 34, 60, 64, 67, 72|
|c||28, 06, 80||p||04, 39|
|d||24, 46, 85, 88||q||59|
|e||15, 16, 23, 31, 44, 61, 69, 77, 83, 87, 91, 95||r||08, 56, 71, 79, 92|
|f||02, 32||s||21, 42, 49, 63, 70, 94|
|g||17, 52||t||12, 50, 51, 55, 75, 78, 86, 93, 96, 99|
|h||03, 09, 33, 76, 82, 89||u||29, 01, 58|
|i||27, 47, 66, 73, 74, 81, 90||v||14|
|l||19, 37, 48, 68||y||40|