Cryptography: An Introduction

CryptographyIn previous blog postings, I have discussed how the open source community has created powerful packet sniffing tools, and how they can be used either to administer your network or to attack it. Because these sniffing tools are open source, and because it is relatively easy to place a Linux host on your company network, you need to consider ways to minimize improper use of packet capturing tools. Encryption solutions, such as Secure Shell (SSH) and Kerberos, are two common solutions to this problem.

Algorithms are the underlying foundation of cryptography. Thus, we will look at the basics of algorithms first, starting with symmetric and asymmetric encryption.

Cryptography Defined

Cryptography predates the computer era; as long as people have been writing down information, there has been a need to keep some information secret, either by hiding its existence or changing its meaning. Encryption, a type of cryptography, refers to the process of scrambling information so that the casual observer cannot read it. An algorithm is a set of instructions for mixing and rearranging an original message (called plaintext), with a message key to create a scrambled message, referred to as ciphertext. Similarly, a cryptographic key is a piece of data used to encrypt plaintext to ciphertext, and ciphertext to plaintext, or both, depending on the type of encryption.

The word crypto has its origins in the Greek word kruptos, which means hidden. The objective of cryptography is to hide information so that only the intended recipients can read it. In crypto terms, the hiding of information is called encryption, and when information becomes readable, it is called decryption. A cipher is used to accomplish the encryption and decryption. The information that is being hidden is called plaintext; once it has been encrypted, it is called ciphertext. The ciphertext is transported to the intended recipient or recipients, where it is decrypted back into plaintext.

Finally, there are two different subclasses of algorithms: block ciphers and stream ciphers. Block ciphers work on blocks or chunks of text in a series. In contrast a stream cipher operates on each individual unit, either letters or bits, of a message.

There are many different encryption algorithms, and in each case, there are tradeoffs between security, speed, and ease of implementation. Here, security indicates the likelihood of an algorithm to stand up to current and future attacks, speed refers to the processing prower and time required to stand up to current and future attacks, speed refers to the processing power and time required to encrypt and decrypt a message, and ease of implementation refers to an algorithm’s predisposition (if any) to hardware or software usage. Each algorithm has different strengths and drawbacks and none of them are ideal in every way. The key algorithms fall into three main categories:

  • Symmetric cryptography
  • Asymmetric cryptography
  • Hashing algorithms

In the next few articles, we will review each of these categories.

External Links:

Cryptography at Wikipedia

Cryptography I – enroll in a free 6-week course in cryptography at

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