Page 303 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Cryptography, then, not only protects data from theft or alteration, but can also be used for
user authentication. There are, in general, three types of cryptographic schemes typically used
to accomplish these goals—secret key (or symmetric) cryptography, public-key (or asymmetric)
cryptography, and hash functions, each of which is described below. In all cases, the initial
unencrypted data is referred to as plaintext. It is encrypted into ciphertext, which will in turn
(usually) be decrypted into usable plaintext.
In many of the descriptions below, two communicating parties will be referred to as Alice and
Bob; this is the common nomenclature in the crypto field and literature to make it easier to
identify the communicating parties. If there is a third or fourth party to the communication,
they will be referred to as Carol and Dave. Mallory is a malicious party, Eve is an eavesdropper,
and Trent is a trusted third party.
9.4.2 Types of Cryptographic Algorithms

There are several ways of classifying cryptographic algorithms. For purposes of this paper,
they will be categorized based on the number of keys that are employed for encryption and
decryption, and further defined by their application and use. The three types of algorithms that
will be discussed are (Figure 9.1):
• Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption.
• Public Key Cryptography (PKC): Uses one key for encryption and another for decryption.

• Hash Functions: Uses a mathematical transformation to irreversibly “encrypt” information.

Figure 9.1: Three Types of Cryptography—Secret-key, Public key and Hash Function

Plaintext Ciphertext Plaintext

(a) Secret key (symmetric) cryptography. SKC usesa single key for both
encryption and decryption.

Plaintext Ciphertext Plaintext

(b) Public key (asymmetric) cryptography. PKC uses to key, one for
encryption and the other for decryption.

Hash function
Plaintext Ciphertext

(c) Has function (one-way cryptography). Hash functions have no key
since the plaintext is not recoverable from the cipher text.

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