ch 4 cryptography

User's Guide to Cryptography and Standards by Alexander W. Dent and Chris J. Mitchell Chapter 4: Encryption Overview The term "cryptography" is often mistakenly used as a synonym for "encryption" and vice versa. Cryptography is the study of algorithms that provide a security service, and includes, among others, the study of algorithms that protect the integrity of data, algorithms that guarantee the authenticity of the source of data, and algorithms that provide confidentiality for data (encryption algorithms). Encryption schemes, on the other hand, are only concerned with the provision of confidentiality. In other words encryption algorithms transform data, sometimes called messages or plaintext, into ciphertext, which can be transmitted over a public network and from which no unauthorized entity can determine any information about the message (except, possibly, its length). When the ciphertext reaches its intended recipient, he or she can undo the encryption operation and recover the message from the ciphertext. It is easy to see that, in order to do this, the recipient must have some kind of extra, secret information that is not known to any unauthorized person. This is known as a key. It is important to understand that, in general, an encryption scheme only provides confidentiality. It does not make any claim that t he message that arrives will not have been altered in some way (integrity) or has come from the source that claims to have sent it (data origin authentication), only that the message that arrives has not been read by anybody else along the way. (Although some encryption algorithms do provide some measure of integrity protection, this is considered a bonus rather than an objective of the scheme.) Unauthorized entities that try to find out information about a message from its ciphertext are known as attackers or eavesdroppers. It should be noted that encryption schemes are not the only method of providing confidentiality. For more information about which security mechanisms can be used to provide security services, see Chapter 3. 4.1 Definitions and Basic Properties As we have already mentioned, for an encryption scheme to work, the intended recipients must know some secret key that will allow them to recover messages from ciphertexts. We can now split encryption schemes into two types. If the message sender knows this secret key then the scheme is said to be symmetric. If the message sender does not know the secret key then the scheme is said to be asymmetric or public-key. The relationships between the various types of ciphers are shown in Figure 4.1.

ch 4 cryptography

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