Classic Cryptography
Introduction

Detailed Discussion
Key-Based Encryption
Introduction
Detailed Discussion
Glossary
Basic Concepts in Data Encryption
An Introduction

There are generally two broad classification of classical cryptographic methods:

The use of masking leads to substitution. Often message is masked in such a way that the resulting message that goes out in an open communication channel, seems harmless and inconspicuous.
• Veiling
The use of veiling leads to transposition, as veiled messages are usually not masked at all, but simply combined within other items regularly in such a way that resulting message takes form of yet another message, called acrostics.

Classic Cryptography

The oldest, simplest and the most primitive of all of the methods, the 'translation table', has been used long since, before the first computer was invented.
Each 'chunk' of data (usually 1 byte) is used as an offset within a 'translation table', and the resulting 'translated' value from within the table is then written into the output stream. The encryption and decryption programs would each use a table that translates to and from the encrypted data.
While this method is very simple and fast, the down side is that once the translation table is known, the code is broken. Further, such a method is relatively straightforward for code breakers to decipher. Still, for general "unreadability" of encoded data, without adverse effects on performance, the 'translation table' method lends itself well.

More on classical methods

Key-based Encryption

Keys are the fundamental essential element in generating modern ciphertext. A string of bits used widely in cryptography, allowing people to encrypt and decrypt data; a key can be used to perform other mathematical operations as well. Given a cipher, a key determines the mapping of the plaintext to the ciphertext.

The key to a particular cryptosystem is some value which, when applied to ciphertext, allows the encrypted message to be decrypted and when applied to plaintext encrypts it. It is important to note that in the study of cryptography one talks about the lengths of keys in terms of bits. Messages, too, can be encrypted in 'n-bit blocks'. The longer a key is the more difficult it is to break the encrypted message.

The most common method of breaking ciphers is by a brute-force attack. This sort of attack involves running through possible combinations of keys and applying them to the cryptosystem until the message is decrypted. It is the same as trying to guess a PIN for someone's ATM card by going through all the possible combinations of numbers. It is said that any University computer science major would have enough computer power available to be able to break most 56-bit key cryptosystems in less than one week.

The following table, from B. Schneier. Applied Cryptography, 2e. John Wiley & Sons. 1996, is for symmetric key ciphers. It offers some guidelines on choosing appropriate key lengths.

 Nature of Data Lifetime Minimum [Symmetric] Key Length Tactical military information minutes/hours 56-64 bits Product announcements, mergers days/weeks 64 bits Long-term business plans years 64 bits Trade secrets decades 112 bits H-bomb secrets >40 years 128 bits Identities of spies >50 years 128 bits Personal affairs >50 years 128 bits Diplomatic embarrassments >65 years at least 128 bits U.S. Census data 100 years at least 128 bits

More on key-based algorithm

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