In symmetric ciphers, the same key is used to encrypt and decrypt a message. Here’s how it can be done: Shift the starting point of the alphabet by three posi- tions—the encryption key is now K=3.
Standard Alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cryptographic Alphabet: DEFGHIJKLMNOPQRSTUVWXYZABC
For example:
Plaintext: WIRELESS SECURITY Ciphertext: ZLUHOHVV VHFXULWB
The weakness of the system lies in the fact that statistical analysis is based on greater use of some letters in the language than others. Julius Caesar was the first to use a symmetric cipher to secure his communications to his commanders.The key he used consisted of shifting the starting point of the alphabet a certain number of positions and substituting the letters making up a message with the corresponding letter in the cipher alphabet.
The main weakness of this type of encryption is that it is open to statistical analysis. Some languages (like the English language) use some letters more often than others, and as a result cryptanalysts have a starting point from which they can attempt to decrypt a message.
This standard form of symmetric encryption remained relatively unchanged until the 16th century. At this time, Blaise de Vigenere was tasked by Henry the III to extend the Caesar cipher and provide enhanced security.What he proposed was the simultaneous use of several different cryptographic alphabets to encrypt a message.The selection of which alphabet to use for which letter would be deter- mined though the use of a key word. Each letter of the keyword represented one of the cryptographic substitution alphabets. For example:
Standard Alphabet ABCDEFGHIJKLMNOPQRSTUVWXYZ Substitution set “A” ABCDEFGHIJKLMNOPQRSTUVWXYZ Substitution set “B” BCDEFGHIJKLMNOPQRSTUVWXYZA Substitution set “C” CDEFGHIJKLMNOPQRSTUVWXYZAB
…
Substitution set “Z” ZABCDEFGHIJKLMNOPQRSTUVWXY
If the keyword were airwave, you would develop the cipher text as follows:
Plaintext: wire less secu rity Key Word: airw avea irwa veai Ciphertext: wqia lzws avyu mmtg
The main benefit of the Vigenere cipher is that instead of having a one-to- one relationship between each letter of the original message and its substitute, there is a one-to-many relationship, which makes statistical analysis all but impos- sible.While other ciphers were devised, the Vigenere-based letter substitution scheme variants remained at the heart of most encryption systems up until the mid-twentieth century.
The main difference with modern cryptography and classical cryptography is that it leverages the computing power available within devices to build ciphers
that perform binary operations on blocks of data at a time, instead of individual letters.The advances in computing power also provide a means of supporting larger key spaces required to successfully secure data using public key ciphers. When using binary cryptography, a key is represented as a string of bits or numbers with 2n keys.That is, for every bit that is added to a key size, the key
space is doubled.The binary key space equivalents illustrated in Table 1.1 show how large the key space can be for modern algorithms and how difficult it can be to “break” a key.
Table 1.1 Binary Key Space
Binary Key Length Key Space
1 bit 21 = 2 keys 2 bit 22 = 4 keys 3 bit 23 = 8 keys 16 bit 216 = 65,536 keys 56 bit 256 = 72,057,594,037,927,936 keys
The task of discovering the one key used, based on a 56-bit key space is akin to finding one red golf ball in a channel filled with white golf balls that is 30 miles wide, 500 feet tall and which runs the distance between L.A. to San Francisco. A 57-bit key would involve finding the one red golf ball in two of these channels sitting side-by-side. A 58-bit key would be four of these channels side-by-side, and so on!
Another advantage of using binary operations is that the encryption and decryption operations can be simplified to use bit-based operations such as XOR, shifts and substitutions, and binary arithmetic operations such as additions, subtractions, multiplications, divisions, and raising to a power.
In addition, several blocks of data, each say 64 bits in length, can be operated on all at once, where portions of the data is combined and substituted with other portions.This can be repeated many times, using a different combination or sub- stitution key. Each repetition is referred to as a round.The resultant ciphertext is now a function of several plaintext bits and several subkeys. Examples of modern symmetric encryption ciphers include 56-bit DES,Triple DES using keys of roughly 120 bits, RC2 using 40-bit and 1280-bit keys, CAST using 40-, 64-, 80-, 128- and 256-bit keys, and IDEA using 128-bit keys among others.
Some of the main drawbacks to symmetric algorithms are that they only provide a means to encrypt data. Furthermore, they are only as secure as the
transmission method used to exchange the secret keys between the party encrypting the data, and the party that is decrypting it. As the number of users increases, so does the number of individual keys required to ensure the privacy of the data. As Figure 1.18 illustrates, the number of symmetric keys required
becomes exponential.
The more a symmetric key is used, the greater the statistical data that is gen- erated which can be used to launch brute force and other encryption attacks.The best way to minimize these risks is to perform frequent symmetric key change- overs. Manual key exchanges have always been bulky and expensive to perform.