What is a Cipher?

In cryptography, a cipher (or cypher) is an algorithm for performing encryption and decryption — a series of well-defined steps that can be followed as a procedure. An alternative term is encipherment. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes. Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, “UQJHSE” could be the code for “Proceed to the following coordinates”. When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it; it should resemble random gibberish to those not intended to read it.

The operation of a cipher usually depends on a piece of auxiliary information, called a key or, in traditional NSA parlance, a cryptovariable. The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be difficult, if not nearly impossible, to decrypt the resulting cipher into readable plaintext.

Most modern ciphers can be categorized in several ways:

* By whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).
* By whether the same key is used for both encryption and decryption (symmetric key algorithms), or if a different key is used for each (asymmetric key algorithms). If the algorithm is symmetric, the key must be known to the recipient and to no one else. If the algorithm is an asymmetric one, the encyphering key is different from, but closely related to, the decyphering key. If one key cannot be deduced from the other, the asymmetric key algorithm has the public/private key property and one of the keys may be made public without loss of confidentiality. The Feistel cipher uses a combination of substitution and transposition techniques. Most block cipher algorithms are based on this structure.

Cipher” (Middle French as cifre and Medieval Latin as cifra, from the Arabic sifr = zero) is alternatively spelled “cypher” (however, this variant is now uncommon); similarly “ciphertext” and “cyphertext”, and so forth.

The word “cipher” in former times meant “zero” and had the same origin (see Zero — Etymology), and later was used for any decimal digit, even any number. There are these theories about how the word “cipher” may have come to mean encoding:

* Encoding often involved numbers.
* The Roman number system was very cumbersome because there was no concept of zero (or empty space). The concept of zero (which was also called “cipher”), which we all now think of as natural, was very alien in medieval Europe, so confusing and ambiguous to common Europeans that in arguments people would say “talk clearly and not so far fetched as a cipher”. Cipher came to mean concealment of clear messages or encryption.
o The French formed the word “chiffre” and adopted the Italian word “zero”.
o The English used “zero” for “0”, and “cipher” from the word “ciphering” as a means of computing.
o The Germans used the words “Ziffer” (digit, “Zahl”) and “Chiffre”.

Dr. Al-Kadi (ref-3) concluded that the Arabic word sifr, for the digit zero, developed into the European technical term for encryption.

PS : To understand more about ciphers, I will give some posts about it.

In Conclusion : A cipher is the art of Encryption and Decryption. Breaking a cipher is way more harder then breaking a password. A Password which takes 3 days to crack, a Cipher would take more than days to crack, depending on the Cipher. The easiest Cipher that could be cracked is Caesar Cipher, the cipher is based on shifting words by a parameter. Caesar Cipher could be cracked easily and normally it takes up to 10 minutes to crack a Caesar Cipher Ciphertext on a computer. A Plaintext could be assumed as the 'Text to encrypt', while Ciphertext means the Plaintext which has been encrypted or ciphered. For example, in Caesar Cipher, Plaintext "the quick brown fox jumps over the lazy dog" in parameter 3 would be: "WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ" on the Ciphertext.

This way, Cipher or Encryption and Decryption is harder or easier to crack (most of them is harder).