Cryptography and Network Security

Does security provide some very basic protections that we are naive to believe that we don't need? During this time when the Internet provides essential communication between tens of millions of people and is being increasingly used as a tool for commerce, security becomes a tremendously important issue to deal with. There are many aspects to security and many applications, Ranging from secure commerce and payments to private Communications and protecting passwords. One essential aspect for Secure communications is that of cryptography.

Cryptography is the science of writing in secret code and is an ancient art. The first documented use of cryptography in writing dates back to circa 1900 B.C. when an Egyptian scribe used non-standard hieroglyphs in an inscription.

In data and telecommunications,cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet.Within the context of any application-to-application communication, there are some specific security requirements, including:

Authentication: The process of proving one's identity. (The primary forms of host-to-host authentication on the Internet today are name-based or address-based, both of which are notoriously weak.)

· prrivacy/confidentiality: Ensuring that no one can read the message except the intended receiver.

· Integrity: Assuring the receiver that the received message has not been altered in any way from the original.

· Non-repudiation: A mechanism to prove that the sender really sent this message. Cryptography, then, not only protects data from theft or alteration, but can also be used for user authentication.

· The three types of cryptographic algorithms that will be discussed are (Figure 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

1. Secret Key Cryptography

With secret key cryptography, a single key is used for both encryption and decryption.

As shown in Figure the sender uses the key (or some set of rules) to encrypt the plain text and sends the cipher text to the receiver. The receiver applies the same key (or rule set) to decrypt the message and recover the plain text. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption.

With this form of cryptography, it is obvious that the key must be known to both the sender and the receiver; that, in fact, is the secret. The biggest difficulty with this approach, of course, is the distribution of the key.Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers.

Stream ciphers operate on a single bit (byte or computer word) at a time and implement some form of feedback mechanism so that the key is constantly changing. A block cipher is so- called because the scheme encrypts one block of data at a time using the same key on each block. In general, the same plain text block will always encrypt to the same cipher text when using the same key in a block cipher whereas the same plaintext will encrypt to different cipher text in a stream cipher.

2. Public key cryptography

Modern PKC was first described publicly by Stanford University professor Martin Hellman and graduate student Whitfield Diffie in 1976. Their paper described a two-key crypto system in which two parties could engage in a secure communication over a non-secure communications channel without having to share a secret key. Generic PKC employs two keys that are mathematically related although knowledge of one key does not allow someone to easily determine the other key. One key is used to encrypt the plaintext and the other key is used to decrypt the cipher text. The important point here is that it does not matter which key is applied first, but that both keys are required for the process to work (Figure 1B). Because a pair of keys are required, this approach is also called asymmetric cryptography

3. Hash Functions

Hash functions, also called message digests and one-way encryption, are algorithms that, in some sense, use no key (Figure 1C). Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Hash algorithms are typically used to provide a digital fingerprint of a file's contents often used to ensure that the file has not been altered by an intruder or virus. Hash functions are also commonly employed by many operating systems to encrypt passwords. Hash functions, then, help preserve the integrity of a file.