Linux Security: Kinds of Encryption

This is an excerpt from “A Practical Guide to Red Hat Linux 8“. An interview with the author is available here.


Security is a major part of the foundation of any system that is not totally cut off from other machines and users. Some aspects of security have a place even on isolated machines. Examples are periodic system backups, BIOS or power-on passwords, and self-locking screensavers.

A system that is connected to the outside world requires other mechanisms to secure it: tools to check files (tripwire), audit tools (tiger/cops), secure access methods (kerberos/ssh), services that monitor logs and machine states (swatch/watcher), packet-filtering and routing tools (ipfwadm/iptables/ipchains), and more.

System security has many dimensions. The security of your system as a whole depends on the security of individual components, such as your e-mail, files, network, login and remote access policies, as well as the physical security of the host itself. These dimensions frequently overlap, and their borders are not always static or clear. For instance, e-mail security is affected by the security of files and your network. If the medium (the network) over which you send and receive your e-mail is not secure, you must take extra steps to ensure the security of your messages. If you save your secure e-mail into a file on your local system, you rely on the filesystem and host access policies for file security. A failure in any one of these areas can start a domino effect, diminishing reliability and integrity in other areas and potentially compromising system security as a whole.

This short appendix cannot cover all the facets of system security but does provide an overview of the complexity of setting up and maintaining a secure system. This appendix provides some specifics, concepts, guidelines to consider, and many pointers to security resources.

Other Sources of System Security Information

Depending on how important system security is to you, you may want to purchase one or more of the books dedicated to system security, read from some of the Internet sites that are dedicated to security, or hire someone who is an expert in the field. Do not rely on this appendix as your sole source of information on system security.


One of the building blocks of security is encryption, which provides a means of scrambling data for secure transmission to other parties. In cryptographic terms, the data or message to be encrypted is referred to as plaintext, and the resulting encrypted block of text as ciphertext. A number of processes exist for converting plaintext into ciphertext through the use of keys, which are essentially random numbers of a specified length used to lock and unlock data. This conversion is achieved by applying the keys to the plaintext by following a set of mathematical instructions, referred to as the encryption algorithm.

Developing and analyzing strong encryption software is extremely difficult. There are many nuances and standards governing encryption algorithms, and a background in mathematics is requisite. Also, unless an algorithm has undergone public scrutiny for a significant period of time, it is generally not considered secure; it is often impossible to know that an algorithm is completely secure but possible to know that one is not secure. Time is the best test of an algorithm. Also, a solid algorithm does not guarantee an effective encryption mechanism, as the fallibility of an encryption scheme frequently lies in problems with implementation and distribution.

An encryption algorithm uses a key that is a certain number of bits long. Each bit you add to the length of a key effectively doubles the key space (the number of combinations allowed by the number of bits in the key-2 to the power of the length of the key in bits) [a 2-bit key would have a key space of 4 (2^2), a 3-bit key would have a key space of 8 (2^3), and so on.] and means that it will take twice as long for an attacker to decrypt your message (assuming that there are no inherent weaknesses or vulnerabilities to exploit in the scheme). However, it is a mistake to compare algorithms based only on the number of bits used. An algorithm that uses a 64-bit key can be more secure than an algorithm that uses a 128-bit key.

The two primary classifications of encryption schemes are public key encryption and symmetric key encryption. Public key encryption, also called asymmetric encryption, uses two keys: a public key and a private key; these keys are uniquely associated with a specific individual user. Symmetric key encryption, also called symmetric encryption, or secret key encryption, uses one key that you and the person you are communicating with (hereafter, referred to as your friend ) share as a secret. Public key algorithm keys typically have a length of 512 bits to 2,048 bits, whereas symmetric key algorithms use keys in the range of 64 bits to 512 bits.

When you are choosing an encryption scheme, realize that security comes at a price. There is usually a trade-off between resilience of the cryptosystem and ease of administration.

Hard to Break? Hard to Use!

The more difficult an algorithm is to crack, the more difficult it is to maintain and to get people to use properly. The paramount limitations of most respectable cryptosystems lie not in weak algorithms but rather in users’ failure to transmit and store keys in a secure manner.

The practicality of a security solution is a far greater factor in encryption, and in security in general, than most people realize. With enough time and effort, nearly every algorithm can be broken. In fact, you can often unearth the mathematical instructions for a widely used algorithm by flipping through a cryptography book, reviewing a vendor’s product specifications, or performing a quick search on the Internet. The challenge is to ensure that the effort required to follow the twists and turns taken by an encryption algorithm and its resulting encryption solution outweighs the worth of the information it is protecting.

How Much Time and Money Should You Spend on Encryption?

When the cost of obtaining the information exceeds the value realized by its possession, the solution is an effective one.

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